prr.hec.gov.pkprr.hec.gov.pk/jspui/bitstream/123456789/6699/1/Irshad_Ali_2015... · CERTIFICATE This is to certify that Mr. IRSHAD ALI has carried out experimental work in this dissertation

Study of Substituted Hexaferrites and Their

Composites for High Frequency Applications

Ph.D. Thesis

Irshad Ali

Session (2009-2012)

DEPARTMENT OF PHYSICS

BAHAUDDIN ZAKARIYA UNIVERSITY

MULTAN – PAKISTAN

Study of Substituted Hexaferrites and Their

Composites for High Frequency Applications

Ph.D. Thesis

Irshad ali

Session 2009-2012

A thesis submitted in partial fulfillment of the

requirement for the degree of

Doctor of Philosophy in Physics

DEPARTMENT OF PHYSICS

BAHAUDDIN ZAKARIYA UNIVERSITY

MULTAN – PAKISTAN

DEDICATED TO

MY

PARENTS

Whose prayers made me ever successful

and

My respected brother

Malik Nasrullah

CERTIFICATE

This is to certify that Mr. IRSHAD ALI has carried out experimental work in this dissertation

under our supervision in the Department of Physics, Bahauddin Zakariya, University, Multan,

Pakistan. This work is accepted in its present form by the Department of Physics Bahauddin

Zakariya, University, Multan as satisfying the dissertation requirement for the award of degree of

Doctor of Philosophy (Ph.D.) in Physics.

Submitted through:

1 Supervisor ______________

Dr. Misbah-ul-Islam

Department of Physics

B. Z. U Multan

2 supervisor______________

Dr. Muhammad Naeem Ashiq

Institute of chemical sciences

B. Z. U Multan

Declaration

I hereby declare that I have not submitted this research work titled “Study of Substituted

hexaferrites and their composites for high frequency applications” leading to the degree of

Ph.D. in Physics to any other university with in the country or outside Pakistan.

Irshad Ali

DEPARTMENT OF PHYSICS, BAHAUDDIN ZAKARYIA

UNIVERSITY MULTAN, PAKISTAN.

We, the supervisiory committee certify that the contents and form of the thesis submitted

by Mr. Irshad Ali have been found satisfactory and recommend that it be proceeded for the

award of Ph.D. (physics) degree

Supervisory Committee

1- Internal Examiner -------------------------

2- Internal Examiner -------------------------

2- External Examiner -------------------------

3- Chairman ---------------------------

ACKNOWLEDGEMENTS

Completing my PhD degree is probably the most challenging activity of my life. Foremost, I

would say Thanks (Thousands time) to my ALLAH ALMIGHTY who is the creator of all and

who makes me able to pass the long but fulfilling journey with HIS mercy and blessings from the

start till the end. I will present the humblest and most passionate thanks to HIS last Prophet

MUHAMMAD (P.B.U.H) who is a continuous source of guidance for all the humanity. My first

debt of gratitude goes to my father Allah Ditta Khan (May Allah rests his soul in peace in

heaven), as I lost him during my study tenure. I want to tell him how much I love and need him.

Special thanks to my mother, who always been very kind and caring from my birth till now. I

wish to thank my brother Malik Nasrullah, who always been very kind and loving. The long and

tiring journey of doctoral degree was impossible without his support and encouragement. I have

no words to acknowledge him for all he did…… It has been a great privilege to spend several

years in the Department of physics (BZU). My huge debts of gratitude must go to my supervisor,

Dr. Misbah-ul-Islam for the continuous support during my Ph.D study and research. He always

patiently provided me the vision, encouragement, motivation and advice necessary for me to

proceed through the doctoral program and complete my dissertation. His guidance and immense

knowledge helped me in all the times of research.

I would like to thank Dr. Muhammad Naeem Ashiq (Co-supervisor), who always been very

kind. He has always given me great freedom to pursue independent work. I could not have

imagined having a better advisor and mentor for my Ph.D study. Beside my supervisors I would

thank the Dr. Abdul shakoor Khan, who always been very kind and helping in many problems

related to the synthesis of composites. I would say thanks, Chairman, department of Physics

Prof. Dr. Ejaz Ahmed for providing all the facilities and comfortable environment at the

department. Prof. Dr. Ejaz Ahmed always encouraged me to be the certified user of all the

characterization techniques at department. He always supported me in all aspects, guiding me

how to go through the different situations.

I wish to express my deepest gratitude to Dr. Mazhar Uddin Rana, Dr. M.Y. Nadeem, Dr. Tariq

Bhatti, Dr. Amer Basher Ziya, Dr. Javeed Ahmad, Dr. Ishtiaq Ahmad Soomro, Dr. Muhammad

Ishaque, Dr. M. Azhar Khan and Dr. G. Murtaza for their inspiring guidance and consistent

encouragement for the completion of this work. Their valuable suggestions helped me in

comprehending the intricacies involved in this work.

I am thankful to Prof. Dr. Shahzad Naseem for providing magnetic properties measurement

facilities on VSM at the centre for solid state Physics, punjab University, Lahore.

I am thankful to Prof. Dr. M. S. Awan for providing facilities of SEM and EDX at facilities at

comsats Islamabad.

Thanks are due to Sir Anwar Manzoor Rana, Abdul Aziz (Ph.D. Scholar), Sajjad Ahmed

Khan(Ph.D. Scholar),Muhammad Ishfaq(Ph.D. Scholar), M. Hasan Khan (Ph.D. Scholar),

Muhammad Ramzan(Ph.D. Scholar), M. Wasiq (Ph.D. Scholar), Mukhtar Ahmed (Ph.D.

Scholar), Hafiz Tahir (Ph.D. Scholar), Muhammad Ismail (Ph.D. Scholar), M. Saeed (Ph.D.

Scholar), M. Irfan(Ph.D. Scholar), Aisha Iftikhar (Ph.D. Scholar), and Atta ullah Khan Khosa

for their valuable help from start to end.

I pay special gratitude and thanks to Nazia Karamat Goraya and Col Muhammad asif Iqbal

who have always been extending their cooperation the course of my assignment

Finally, I want to especially thank to the Higher Education Commission (HEC) of Pakistan for

the financial assistance under HEC Indigenous Ph.D scheme batch IV

Here it will be highly unjustified if I do not acknowledge the moral assistance extended by my

wife Dr Salva irshad (medical doctor)). Undoubtedly her prayers and marathon encouragement

remained source of inspiration for the project. I AM GREATFUL FOR YOUR KIND

CONCERN PLEASE

Abstract The work presented in this thesis describes the synthesis and characterization of cobalt based Y-

type hexaferrites and their composites. Three series of Tb–Mn, Eu-Ni and Sm-Ni substituted

Sr2Co2Fe12O22Y-type hexaferrites prepared by norrmal microemulsion technique have been

investigated thoroughly. Two series of composites are prepared (a) Composite thick films of

Sr1.8Sm0.2Co2 Ni1.50 Fe12O22, Y- type hexaferrite and Polystyrene with different ferrite ratio 1:0,

1:0.25, 1:0.50, 1:0.75 and 1:1(b) a composite of Co2Sr2Fe12O22 ferrite with conducting polymer

PPy-DBSA. All the samples under study are characterized by X-ray Diffraction, Scanning

Electron Microscopy, Eenergy Dispersive X-ray spectroscopy, Resistivity & dielectric

measurement, and Vibrating Sample Magnetometery. Structural analysis for all the samples is

performed using X-ray diffraction. X’pert highscore software is used to index the XRD patterns.

The indexing of each pattern reveales the formation of well defined Y-type single phase

materials. Enhancment in the intensity of peaks shows improved crystallinity suggesting that the

dopents in the nominated substitution range are entirely dissolved in the Sr2Co2Fe12O22 Y-type

lattice. Average crystallite size mearsured by Scherrer formula lies in the range of 30-86 nm ±2

nm for the substituted ferrite samples. Lattice parameter changes linearly in accordance with the

ionic radius of the substituted cations into the parent crystal lattice obeying the Vegard’s law.

The Y-type hexagonal ferrites under investigation exhibit slow variation in lattice parameter ‘a’

as compare to lattice parameter ‘c’. The EDX analysis suggests that the increment in substituents

and decrease in the substituted contents at systematic rate in the present samples preserved the

accurate stoichiometry. SEM micrographs show fine hexagonal plate like grains that make them

useful for microwave device applications. The DC electrical resistivity increases drastically from

106-109 Ω-cm with increasing the concentration of dopents Tb-Mn, Eu-Ni and Sm-Ni into the

host lattice of Co2Sr2Fe12O22 ferrite. The Curie temperature (TC) is estimated from the resistivity

curves of Tb-Mn, Eu-Ni and Sm-Ni substituted nano-ferrites, which decreases with increasing

doping concentration. The decrease in TC may be due to the fact that Re–Fe interactions on the

octahedral sites are weaker than Fe–Fe interaction. The hopping of electrons and jumping of

holes are responsible for conduction below Curie temperature (ferromagnetic region), whereas

above Curie temperature (paramagnetic region) it is due to polaron hopping. The resistivity of

the ferrite-PPy/DBSA composite decreases from 106 to 103 Ω-cm due to conducting nature of the

PPy/DBSA polymer. The DC-resistivity of the ferrite-PST composite ranges 1013-1010 Ω-cm due

to increase concenentration of ferrite filler. The variation of activation energy is in agreement

with the variation of room temperature resistivity for all the investigated samples. The

temperature dependent dc resistivity decreases for all the samples indicating semi-conducting

behavior. The enhancement in resistivity and low dielectric loss make these materials pre-

eminent contestant for MLCI applications. Dielectric properties generally follows Maxwell

Wagner model and Koops phenomenogical theory. The dielectric constant, complex dielectric

constant and loss tangent decreases with the increase of doping concentration. Dielectric constant

for Tb-Mn, Eu-Ni and Sm-Ni substituted nano-ferrites were found to decrease in the range of 16-

6, 16-7, and 16-6 at 1MHz respectively. The dielectric constant of the composite samples (PST+

Sr1.8Sm0.2Co2 Ni1.50 Fe12O22) FP1, FP2, FP3 and FP4 is 13.10, 14.24, 15.03 and 15.89 at 1MHz

respectively. The results are consistent with resistivity of the samples under investigation. The

complex dielectric loss decreases for all the substituted ferrites due to larger resistivity values.

The Composite sample ( PPY/DBSA+ Sr2Co2Fe12O22) exhibit larger dielectric loss = 35 at 1MHz

due to conducting nature of the polymer PPY-DBSA. This sample is more susceptible for EMI

shielding applications. The resonance peaks in tanδ(f) are observed when the external electric

field matches with the hopping frequency of charge carriers. Electrical modulus describes the

dielectric relaxation behavior for all the substituted ferrites and composites. The cole-cole plots

shows the semicircle for most of the samples to elaborate the grain and grain boundary

contribution towards the dielectric relaxation phenomena. It is observed that substitution makes

comparatively smaller difference on the grain resistance, but leads to a remarkable rise of grain

boundary resistance. The AC conductivity increases with increasing frequency of the applied

field for all the investigated samples. The frequency dependent AC conductivity follows power

law with large value of exponent, n that shows the polaron hopping is the likely conduction

mechanism. The magnetic properties of the Tb-Mn, Eu-Ni and Sm-Ni substituted samples and

composites have been measured using Vibrating Sample Magnetometer. The saturation

magnetization ,remanance and coercvity are measured at room temperature. The saturation

magnetization decreases from 66-16 emu/g, 66-25 emu/g and 66-30 emu/g for the three

substituted ferrite series. The corecivity inecreases from 729-3190 Oe, 729-1919 Oe and 729-

1356 Oe for the substituted samples may be due to increase in the hinderence to the domain wall

pinning at the grain boundaries and other defects like porosity. The highest value of coercivity,

~3200 Oe is observed for the sample Sr2Co1Mn 1.0 Tb0.1Fe11.90O22 Y-type hexagonal ferrite.

Higher values of coercivity ensure the use of present samples as in perpendicular recording

media (PRM). Whereas, the magnetization and coercivity increase with increasing weight ratio

of the magnetic filler i.e Sr1.8Sm0.2Co2 Ni1.50 Fe12O22 from 0.25 to 1 in PST matrix. The increase

in saturation magnetization and coercivity is attributed to the increase in the concentration of

magnetic content of the filler. The composite sample Co2Sr2Fe12O22/PPy-DBSA executes low

magnetization and high coercivity with respect to ferrite filler, which is an interesting result with

an added advantage of flexibility of the composite material.

Table of Contents

Chapter 1 Introduction 1-24

1.1 Magnetism and Magnetic Materials...................................................................................... 1

1.1.1 Fundamental Magnetic Quantities ..................................................................................... 4

1.2. Introduction to Ferrites ............................................................................................................ 5

1.2.1 Soft Ferrites ....................................................................................................................... 6

1.2.2 Hard Ferrites ...................................................................................................................... 7

1.3 Promising Applications of Hexa Ferrites ................................................................................ 11

1.4 Advantages of Hexaferrites Over Spinel Ferrites .................................................................. 12

1.5 Introduction to Polymers......................................................................................................... 12

1.5.1 Polymer Classification Based Upon Structure ................................................................. 13

1.5.1.1 Linear Polymers. ....................................................................................................... 13

1.5.1.2 Branched Chain Polymers......................................................................................... 13

1.5.1.3 Cross Linked Polymers. ............................................................................................ 13

1.5.1.4 Network polymers. .................................................................................................... 13

1.5.2 Classification Based Upon Molecular Forces .................................................................. 13

1.5.2.1 Elastomers. ................................................................................................................ 13

1.5.2.1 Fibres......................................................................................................................... 14

1.5.2.2 Thermoplastics. ......................................................................................................... 14

1.5.2.3 Thermosetting Polymers ........................................................................................... 14

1.6 Chain Length ........................................................................................................................... 14

1.7 Polystyrene .............................................................................................................................. 15

1.7.1 Polymerization ................................................................................................................. 16

1.7.2 Syndiotactic Polystyrene .................................................................................................. 17

1.7.3 Atactic Polystyrene .......................................................................................................... 17

1.8 Polymer Additive .................................................................................................................... 17

1.8.1 Fillers ............................................................................................................................... 17

1.8.2 Plasticizers ....................................................................................................................... 18

1.8.3 Stabilizer .......................................................................................................................... 18

1.8.4 Colorant............................................................................................................................ 19

1.8.5 Flame Retardant ............................................................................................................... 19

1.9 Polypyrrole .............................................................................................................................. 19

1.9.1 Blends of Conducting Polymers with DBSA................................................................... 20

1.10 Ferrites and Composites ........................................................................................................ 20

1.11 Application of Ferrite/polymer composite material ............................................................. 21

1.12 Focus and Objectives of the Present Study ........................................................................... 22

References ..................................................................................................................................... 23

CHAPTER 2 LITERATURE REVIEW 25-34

References ..................................................................................................................................... 33

CHAPTER 3 EXPERIMENTAL SETUP AND

METHODS OF ANALYSIS

35-62

3.1 Preperation of Tb-Mn substituted Y-type Hexaferrite Sr2Co2-x Mnx TbyFe12-y O22 ............... 35

3.1.1 Materials .......................................................................................................................... 35

3.1.2 Synthesis Procedure ......................................................................................................... 35

3.2 Preparation of Eu-Ni substituted Y-type hexaferrite Sr2Co2-x Nix EuyFe12-y O22 .................... 36

3.2.1 Materials .......................................................................................................................... 36

3.2.2 Synthesis procedure ......................................................................................................... 36

3.3 Preparation of Sm-Ni substituted Y-type hexaferrite Sr(2-x)Sm(x)Co2NiyFe(12-y)O22 ............... 37

3.3.1 Materials .......................................................................................................................... 37


3.4 Preparation of Sr1.8 Sm0.2 Co2Ni1.5 Fe10.5 O22/ PST Composites .............................................. 38

3.4.1Chemicals .......................................................................................................................... 38


3.5 Preparation of Co2Sr2Fe12O22 with Ppy-DBSA Composite .................................................... 38

3.5.1 Synthesis of PPY-DBSA ................................................................................................. 38

3.5.2 Synthesis of Ferrite. ......................................................................................................... 39

3.5.3 Ferrites-Polymer Composite. ........................................................................................... 39

3.6 Characterization Techniques ................................................................................................... 39

3.7 X-ray Diffraction .................................................................................................................... 40

3.7.1 Principle of X-rays Diffraction ....................................................................................... 40

3.7.2 Diffraction Methods ......................................................................................................... 41

3.8 Scanning Electron Microscopy (SEM) ................................................................................... 43

3.8.1. Working Principle ........................................................................................................... 44

3.9 Energy Dispersive X-ray Fluorescence Spectrometer (ED-XRF) .......................................... 45

3.10 DC Electrical Resistivity....................................................................................................... 46

3.11 AC Response ......................................................................................................................... 48

3.11.1 Complex Dielectric Permittivity .................................................................................... 48

3.11.2 Interfacial, Space Charge or Maxwell-Wagner type of polarization ............................. 50

3.11.3 Dielectric Losses ............................................................................................................ 51

3.11.4 Dielectric Loss Tangent ................................................................................................. 53

3.11.5 AC Conductivity ............................................................................................................ 54

3.11.6 Complex Iimpedance (Z*) ............................................................................................. 55

3.11.7. Complex Electric Modulus (M*) .................................................................................. 56

3.12 Measurement of Magnetic Properties by VSM .................................................................... 57

References ..................................................................................................................................... 61

CHAPTER 4 RESULTS AND DISCUSSION 63-202

4.1 Tb-Mn Substituted Y-type Hexaferrite ................................................................................... 63

4.1.1 Structural Analysis ........................................................................................................... 63

4.1.2 EDX Analysis .................................................................................................................. 66

4.1.3 Scanning Electron Microscopy (SEM) ............................................................................ 69

4.1.4 Electrical Properties ......................................................................................................... 71

4.1.4.1 DC Resistivity ........................................................................................................... 71

4.1.4.2 Activation Energy ..................................................................................................... 73

4.1.4.3 Drift Mobility ............................................................................................................ 74

4.1.5 Dielectric Properties......................................................................................................... 75

4.1.5.1 AC Conductivity ....................................................................................................... 77

4.1.5.2 Impedance Analysis ................................................................................................. 82

4.1.5.3 Quality Factor ........................................................................................................... 87

4.1.6 Magnetic Properties ......................................................................................................... 87

4.1.6.1 Hysteresis Loops ....................................................................................................... 87

4.1.6.2 Saturation Magnetization (Ms) ................................................................................. 88

4.1.6.3 Coercivity Hc ............................................................................................................ 92

4.1.6.4 Squareness Ratio ..................................................................................................... 95

4.2 Eu-Ni Substituted Y-type Hexaferrite .................................................................................... 96

4.2.1 Structural Analysis ........................................................................................................... 96

4.2.2 EDX Analysis .................................................................................................................. 99

4.2.3 Scanning Electron Microscopy ...................................................................................... 102

4.2.4 Electrical Properties ....................................................................................................... 103

4.2.4.1 DC Resistivity ......................................................................................................... 103

4.2.4.2 Activation Energy ................................................................................................... 104

4.2.4.3 Drift Mobility .......................................................................................................... 108

4.2.5 Dielectric Properties....................................................................................................... 108

4.2.5.1 AC Conductivity ..................................................................................................... 112

4.2.5.2 Impedance Analysis ................................................................................................ 115

4.2.5.3 Quality Factor ......................................................................................................... 121

4.2.6 Magnetic Properties ....................................................................................................... 121

4.2.6.1 Hysteresis Loops ..................................................................................................... 121

4.2.6.2 Saturation Magnetization (Ms) ............................................................................... 122

4.2.6.3 Coercivity Hc .......................................................................................................... 126


4.3 Sm-Ni Substituted Y-type Hexaferrites. ............................................................................... 129

4.3.1 Structural Analysis ......................................................................................................... 129

4.3.2 EDX Analysis ................................................................................................................ 132


4.3.4.1 DC Resistivity ......................................................................................................... 135

4.3.4.2 Activation Energy ................................................................................................... 136

4.3.4.3 Drift Mobility .......................................................................................................... 139


4.3.5.1 AC Conductivity ..................................................................................................... 144

4.3.5.2 Impedance analysis ................................................................................................. 147

4.3.5.3 Quality Factor ......................................................................................................... 152


4.3.6.1 Hysterious Loop ...................................................................................................... 152

4.3.6.2 Saturation Magnetization (Ms) ............................................................................. 153

4.3.6.3 Coericivity............................................................................................................... 158


4.4 Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22 /PST Composite Samples................................................. 160

4.4.1 Structural Analysis ..................................................................................................... 160

4.4.2 Scanning Electron Microscopy ...................................................................................... 161


4.4.3.1 DC Resistivity ......................................................................................................... 163


4.4.4 1 Concentration Dependent Dielectric Constant ........................................................ 165

4.4.4.2 AC Conductivity ..................................................................................................... 168

4.4.4 3 Frequency-Dependent Complex Electric Modulus. ................................................ 170

4.4.4.4 Quality Factor ......................................................................................................... 172


4.4.5.1 Hysteresis Loop ...................................................................................................... 173

4.4.5.2 Saturation Magnetization (Ms) ............................................................................... 176

4.5 Composite of Co2Sr2Fe12O22 with Ppy-DBSA ...................................................................... 178

4.5.1 Structural Anaylsis. ........................................................................................................ 178

4.5.2 Scanning Electron Microscopy (SEM) .......................................................................... 180


4.5.3.1 DC Resistivity ......................................................................................................... 181


4.5.4.1 AC conductivity ...................................................................................................... 186

4.5.4.2 Impedance Analysis ................................................................................................ 188

4.5.4.3 Quality Factor ......................................................................................................... 191


4.5.5.1 Hysteresis Loop ...................................................................................................... 193


References ................................................................................................................................... 197

Thesis Summary and Conclusions .............................................................................................. 203

List of Figures

Figure No. Figure Caption Page No.

Figure 1.1 Classification of magnetic materials (a) Diamagnetic (b)

Paramagnetic (c)Ferromagnetic (d) Anti-ferromagnetic (e)

Ferrimagnetic

3

Figure 1.2 Typical Hysteresis Curve showing different magnetic parameters 5

Figure 1.3 Typical Hysteresis Curve showing different magnetic parameters 7

Figure 1.4 Four types of hexagonal ferrites M, W, Y and Z . 9

Figure 1.5 ( a–c) The (110) cross-section views of M-type [(Ba,Sr)Fe12O19] (a), Y-

type [(Ba,Sr)2Met2Fe12O22] (b) and Z-type [(Ba,Sr)3Met2Fe24O41](c)

structures with the hexagonal c axis vertical.

10

Figure 1.6 Various polymer architectures of Polymers

Figure 1.7 Syndiotactic and atactic polystyrene 18

Figure 1.8 Structures of Polypyrrole2 20

Figure 3.1 Schematic of X-ray diffraction as per Braggs law 40

Figure 3.2 Geometrical representation of the constructive interference 41

Figure 3.3 Schematic diagram of X-Ray Diffractrometer 42

Figure 3.4 Schematic diagram of Scanning Electron Microscopy (SEM). 44

Figure 3.5 Block diagram for energy dispersive X-ray fluorescence (EDX) 46

Figure 3.6 Sample Holder For Resistivity measurements (b) apparatus for

Resistivity Measurement by two probe method.

47

Figure 3.7 Types of polarization on the application of AC field. 50

Figure 3.8 Typical behavior of dielectric dispersion in different frequency

regions.

51

Figure 3.9 Fig.3. 1: Real and Imaginary parts of dielectric permittivity with

frequency for a pure dielectric material.

52

Figure 3.10 Dielectric loss tangent (Ic and IR). 54

Figure 3.11 Real and Imaginary parts of absolute Impedance Z| 55

Figure 3.12 Schematic of Vibrating Sample Magnetometer.. 57

Figure 3.13 Schematic diagram of Vibrating Sample Magnetometer. 58

Figure 4.1 XRD analysis of Tb-Mn substituted hexaferrites, Sr2Co(2-x)

MnxTbyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

63

Figure 4.2 Variation of lattice parameters for Tb-Mn substituted hexa

ferrites,Sr2Co(2-x)MnxTbyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

64

Figure 4.3 Variation of crystalline size for Tb-Mn substituted hexaferrites,

Sr2Co(2-x)MnxTbyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

66

Figure 4.4 (a-f) EDX spectra for Tb-Mn substituted Co2Sr2Fe12O22. 68

Figure 4.5 (a-f) SEM images for Tb-Mn substituted Co2Sr2Fe12O22. 70

Figure 4.6 Temperature dependent resistivity of Tb-Mn substituted

hexaferrites, Sr2Co(2-x)MnxTbyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–

0.10).

72

Figure 4.7 Variation of Curie Temperature (Tc) for Tb-Mn substituted


0.10).

73

Figure 4.8 Change in Drift mobility with temperature for (Tb-Mn) substituted

Co2Sr2Fe12O22 hexa ferrites.

75

Figure 4.9 Fig.4. 1: Dielectric constant of Tb-Mn substituted hexaferrites,

Sr2Co(2-x)MnxTbyFe(12-y)O22.

78

Figure 4.10 Dielectric loss of Tb-Mn substituted hexaferrites, Sr2Co(2-

x)MnxTbyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

78

Figure 4.11 Dielectric loss Factor of Tb-Mn substituted hexaferrites, Sr2Co(2-

x)MnxTbyFe(12-y)O22.

79

Figure 4.12 Comparison of dielectric constant and resistivity of Tb-Mn

substituted hexaferrites, Sr2Co(2-x)MnxTbyFe(12-y)O22, (x = 0.00–

1.00; y = 0.00–0.10).

79

Figure 4.13 Variation in AC Conductivity Vs frequency of (Tb-Mn) substituted

Co2Sr2Fe12O22 hexa ferrites at room temperature.

80

Figure 4.14 Variation in logσAC with logω of (Tb-Mn) substituted


80

Figure 4.15 Variation of impedance with frequency of (Tb-Mn) substituted 83


Figure 4.16 Variation in Real part of electric Modulus with frequency of (Tb-

Mn) substituted Co2Sr2Fe12O22 hexaferrites at room temperature.

84

Figure 4.17 Variation in imaginary part of electric Modulus with frequency of

(Tb-Mn) substituted Co2Sr2Fe12O22 hexa ferrites at room

temperature.

85

Figure 4.18 Cole–Cole plots of (Tb-Mn) substituted Co2Sr2Fe12O22 hexa ferrites

at room temperature

86

Figure 4.19 Variation of Q values with frequency of (Tb-Mn) substituted


87

Figure 4.20 In-plane MH-loop of Tb-Mn substituted Co2Sr2Fe12O22. 89

Figure 4.21 Out-plane MH-loop of Tb-Mn substituted Co2Sr2Fe12O22. 90

Figure 4.22 In-plane and out-of-plane saturation magnetization versus (Tb-Mn)

concentration for Sr2Co(2-x)MnxTbyFe(12-y)O22 ferrites.

91

Figure 4.23 In-plane and out-of-plane Remanence versus (Tb-Mn)

concentration for Sr2Co(2-x)MnxTbyFe(12-y)O22 ferrites.

91

Figure 4.24 In-plane and out-of-plane coercivity versus (Tb-Mn) concentration

for Sr2Co(2-x)MnxTbyFe(12-y)O22 ferrites.

92

Figure 4.25 (a-f) Fitted curve of Ms for (Tb-Mn) substituted hexaferrites, calculated

by law of approach to saturation.

93

Figure 4.26 XRD analysis of (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

97

Figure 4.27 Variation of lattice parameters for (Eu-Ni) substituted hexaferrites,

Sr2Co(2-x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

98

Figure 4.28 Variation of crystallite size for (Eu-Ni) substituted hexaferrites,


98

Figure 4.29 (a-f) EDX spectra for (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

100

Figure 4.30 SEM images for (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

102

Figure 4.31 Temperature dependent resistivity of (Eu-Ni) substituted 106

hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–

0.10).

Figure 4.32 Variation of Curie temperature (Tc) for (Eu-Ni) substituted


0.10).

106

Figure 4.33 Change in Drift mobility with temperature for (Eu-Ni) substituted


0.10).

107

Figure 4.34 Dielectric constant of Eu-Ni substituted, Sr2Co(2-x)NixEuyFe(12-

y)O22,(x = 0.00–1.00; y = 0.00–0.10) hexaferrites.

109

Figure 4.35 Dielectric loss of Eu-Ni substituted, Sr2Co(2-x)NixEuyFe(12-y)O22,(x =

0.00–1.00; y = 0.00–0.10) hexaferrites.

110

Figure 4.36 Dielectric loss Factor of Eu-Ni substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

111

Figure 4.37 Comparison of dielectric constant and resistivity of Eu-Ni

substituted hexaferrites,Sr2Co(2-x)NixEuyFe(12-y)O22, (x = 0.00–1.00;

y = 0.00–0.10).

112

Figure 4.38 Variation in AC Conductivity with frequency of (Eu-Ni) substituted


0.10)

113

Figure 4.39 Variation in logσ with logω of (Eu-Ni) substituted hexaferrites,


114

Figure 4.40 Variation in impedance with frequency of (Eu-Ni) substituted


0.10).

116

Figure 4.41 Variation in Real part of electric Modulus with frequency of (Eu-Ni)

substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x = 0.00–1.00;

y = 0.00–0.10) at room temperature.

119

Figure 4.42 Variation in imaginary part of electric Modulus with frequency of

(Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x =

0.00–1.00; y = 0.00–0.10) at room temperature.

119

Figure 4.43 Cole–Cole plots of (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

120

Figure 4.44 Variation of Q values with frequency of (Eu-Ni) substituted


0.10).

121

Figure 4.45 In-plane MH-loop of (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

123

Figure 4.46 Out-plane MH-loop of (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

123

Figure 4.47 In-plane and out-of-plane saturation magnetization versus (Eu-Ni)


y = 0.00–0.10).

124

Figure 4.48 In-plane and out-of-plane Remanence versus (Eu-Ni) concentration

for Sr2Co(2-x)NixEuyFe(12-y)O22 ferrites.

125

Figure 4.49 In-plane and out-of-plane coercivity of (Eu-Ni) substituted


0.10).

125

Figure 4.50(a-f) Fitted curve of Ms for (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10) calculated by

law of approach to saturation.

127

Figure 4.51 XRD patterns of (Sm-Ni) substituted hexaferrites, Sr(2-

x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–1.25),

hexaferrites.

131

Figure 4.52 EDX spectra for Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y =

0.00–1.25), hexaferrites.

133

Figure 4.53(a-f) SEM images for Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x= 0.00–0.10; y =

0.00–1.25).

134

Figure 4.54 Temperature dependent resistivity of Sm-Ni substituted hexaferrites,

Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–1.25).

136

Figure 4.55 Variation of curie Temperature (Tc) for Sm-Ni substituted

hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y =

139

0.00–1.25)

Figure 4.56 Change in Drift mobility Vs temperature for Sr(2-x)Sm(x)Co2NiyFe(12-

y)O22, (x = 0.00–0.10; y = 0.00–1.25), hexa ferrites.

140

Figure 4.57 Dielectric constant of (Sm-Ni) substituted Sr(2-x)Sm(x)Co2NiyFe(12-

y)O22, (x = 0.00–0.10; y = 0.00–1.25), hexaferrites.

141

Figure 4.58 Dielectric loss of Sm-Ni substituted hexaferrites, Sr(2-

x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–1.25)

142

Figure 4.59 Dielectric loss Factor of (Sm-Ni) substituted hexaferrites, Sr(2-

x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–1.25).

143

Figure 4.60 Comparison of dielectric constant and DC resistivity of (Sm-Ni)

substituted , Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–

1.25), hexaferrites at room temperature.

144

Figure 4.61 Variation in AC Conductivity with frequency of (Sm-Ni) substituted

Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–1.25),

hexaferrites at room temperature.

146

Figure 4.62 Variation in logσ with logω of (Sm-Ni) substituted Co2Sr2Fe12O22

hexa ferrites.

147

Figure 4.63 Variation of impedance with frequency of (Sm-Ni) substituted


148

Figure 4.64 Variation in Real part of electric Modulus with frequency of (Sm-

Ni) substituted hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–

0.10, y = 0.00–1.25).

149

Figure 4.65 Variation in Imaginary part of electric Modulus with frequency of

(Sm-Ni) substituted hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x =

0.00–0.10, y = 0.00–1.25; ).:

150

Figure 4.66 Cole–Cole plots of (Sm-Ni) substituted hexaferrites, Sr(2-

x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10, y = 0.00–1.25; ).

151

Figure 4.67 Variation of Q values with frequency of (Sm-Ni) substituted

hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10, y = 0.00–

1.25 )

152

Figure 4.68 In-plane MH-loop of (Sm-Ni) substituted hexaferrites, Sr(2- 153

x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10, y = 0.00–1.25 ).

Figure 4.69 Out-plane MH-loop of (Sm-Ni) substituted hexaferrites, Sr(2-


154

Figure 4.70 In-plane and out-of-plane saturation magnetization versus (Sm-Ni)

concentration for Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y =

0.00–1.25), ferrites.

155

Figure 4.71 In-plane and out-of-plane Remanence versus (Sm-Ni) concentration

for Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–1.25),

ferrites.

155

Figure 4.72 (a-f) 156

Figure 4.73 In-plane and out-of-plane coercivity versus (Sm-Ni) substituted

hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10, y = 0.00–

1.25; )

159

Figure 4.74 X-ray Diffraction Patterns of PST, FP1, FP2,FP3, FP4 and Y-type

hexaferrite(Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22)

160

Figure 4.75(a-f) SEM Image of PST, FP1, FP2,FP3, FP4 and Y-type

hexaferrite(Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22).

162

Figure 4.76 Arrhenius plot of DC resistivityof PST, FP1, FP2,FP3, FP4 and Y-

type hexaferrite(Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22).

164

Figure 4.77 The variation of dielectric constant versus applied field frequency of

PST, FP1, FP2,FP3, FP4 and Y-type hexaferrite (Sr1.8Sm0.2Co2

Ni1.50 Fe10.50O22).

166

Figure 4.78 The variation of dielectric loss versus applied field frequencyof

PST, FP1, FP2, FP3, FP4 and Y-type hexaferrite (Sr1.8Sm0.2Co2

Ni1.50 Fe10.50O22).

167

Figure 4.79 Variation of dielectric tangent loss versus applied field frequency of

PST, FP1, FP2,FP3, FP4 and Y-type hexaferrite (Sr1.8Sm0.2Co2

Ni1.50 Fe10.50O22).

168

Figure 4.80 The variation of AC conductivity versus applied field frequencyof

PST, FP1, FP2,FP3, FP4 and Y-type hexaferrite(Sr1.8Sm0.2Co2 Ni1.50

Fe10.50O22).

169

Figure 4.81 Log-Log variation of AC conductivity versus applied field

frequencyof PST, FP1, FP2, FP3, FP4 and Y-type


170

Figure 4.82 Variation of real part of eletric modulus (M΄) versus applied field

frequencyof PST, FP1, FP2,FP3, FP4 and Y-type


171

Figure 4.83 Variation of imaginary part of electric modulus (M΄) versus applied

field frequencyof PST, FP1, FP2, FP3, FP4 and Y-type hexaferrite

(Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22)

172

Figure 4.84 Variations of Q values versus applied field frequency of PST, FP1,

FP2,FP3, FP4 and Y-type hexaferrite(Sr1.8Sm0.2Co2Ni1.50 Fe10.50O22).

173

Figure 4.85 In-plane MH-loop of FP1, FP2,FP3, FP4 and Y-type

hexaferrite(Sr1.8Sm0.2Co2 Ni1.50Fe10.50O22).

174

Figure 4.86 Out-plane MH-loop of FP1, FP2,FP3, FP4 and Y-type

hexaferrite(Sr1.8Sm0.2Co2 Ni1.50Fe10.50O22).

176

Figure 4.87 (a-e) Fitted curve of FP1, FP2,FP3, FP4 and Y-type

hexaferrite(Sr1.8Sm0.2Co2Ni1.50Fe10.50O22.calculated by law of

approach to saturation.

177

Figure 4.88 XRD patterns of (a) Y-type hexaferrite, Sr2Co2Fe12O22, (b)

composite (Sr2Co2Fe12O22 +PPy-DBSA) and (c) polymer PPy-

DBSA.

179

Figure 4.89 SEM graphs for (a) Y-type hexaferrite Sr2Co2Fe12O22, (b) composite

(Sr2Co2Fe12O22 +PPy-DBSA) and (c) polymer PPy-DBSA

180

Figure 4.90 Temperature dependent resistivity for (a) y-type hexaferrite

Sr2Co2Fe12O22, (b) composite (Sr2Co2Fe12O22 +PPy-DBSA) and (c)

polymer PPy-DBSA.

182

Figure 4.91 Dielectric constant of (a) Y-type hexaferrite Sr2Co2Fe12O22, (b)


DBSA.

184

Figure 4.92 Dielectric loss Factor of (a) Y-type hexaferrite Sr2Co2Fe12O22, (b)


184

DBSA.

Figure 4.93 Dielectric loss Factor of (a) Y-type hexaferrite Sr2Co2Fe12O22, (b)


DBSA.

185

Figure 4.94 Polt of AC Conductivity Vs frequency of (a) Y-type hexaferrite


polymer PPy-DBSA.

187

Figure 4.95 Variation in logσ with logω of (a) Y-type hexaferrite


polymer PPy-DBSA.

188

Figure 4.96 Polt of impedance with frequency of (a) Y-type hexaferrite


polymer PPy-DBSA.

189

Figure 4.97 Variation in Real part of electric Modulus with frequency of (a) Y-

type hexaferrite Sr2Co2Fe12O22, (b) composite (Sr2Co2Fe12O22 +PPy-

DBSA) and (c) polymer PPy-DBSA.

190

Figure 4.98 Variation in imaginary parts of electric Modulus with frequency of

(a) f Y-type hexaferrite Sr2Co2Fe12O22, (b) composite

(Sr2Co2Fe12O22 +PPy-DBSA) and (c) polymer PPy-DBSA.

191

Figure 4.99 Cole–Cole plots of electric Modulus with frequency of (a) Y-type

hexaferrite Sr2Co2Fe12O22, (b) composite (Sr2Co2Fe12O22 +PPy-


192

Figure 4.100 Variation of quality factor (Q) values with frequency of (a) Y-type

hexaferrite Sr2Co2Fe12O22, (b) composite (Sr2Co2Fe12O22 +PPy-


192

Figure 4.101 M–H loops for (a) Y-type hexaferrite Sr2Co2Fe12O22 and (b)

(Sr2Co2Fe12O22 +PPyDBSA) composite.

193

Figure 4.102 Fitted curve for Ms of (Co2Sr2Fe12O22 +PPy-DBSA) calculated by

law of approach to saturation.

195

Figure 4.103 Fitted curve for Ms of Co2Sr2Fe12O22 calculated by law of approach

to saturation.

195

List of Tables

Table No. Table Caption Page

No.

Table 1.1 Number of ions per unit formula, coordination and spin orientation

for the various metallic sublattices of Y-type structure

11

Table 4.1 C/a, volume of cell, Bulk density, X-ray density, percentage porosity

and room temperature DC resistivity of (Tb-Mn) substituted


0.10).

65

Table 4.2 Comparison of the Observed and Theoretical Weight Percents and

Content Determined by EDX Analysis of the (Tb-Mn) substituted

Co2Sr2Fe12O22.

67

Table 4.3 Slops and activation energies of ferrimagnetic and paramagnetic

regions of Tb-Mn substituted hexaferrites, Sr2Co(2-x)MnxTbyFe(12-

y)O22, (x = 0.00–1.00; y = 0.00–0.10).

74

Table 4.4 Compresses the Mobility, AC conductivityof Tb-Mn substituted


0.10).

76

Table 4.5 Real and imaginary parts of electric modulus and impedance,at

1MHz and DC activation energy, exponential n and AC activation

energy of Tb-Mn substituted hexaferrites, Sr2Co(2-x)MnxTbyFe(12-

y)O22, (x = 0.00–1.00; y = 0.00–0.10).

82

Table 4.6 Number of ions per unit formula, coordination and spin orientation

for the various metallic sublattices of Y-structure.

90

Table 4.7 Estimated saturation magnetization Ms, Anisotropy constant( K),

Magnetic moments (nB), Squareness Ratio and Grain size of Tb-

Mn substituted Co2Sr2Fe12O22.

95

Table 4.8 c/a , cell volume (Vcell), bulk density (db) X ray density (dx -

ray),P(%) percentage porosity and Room temperature DC resistivity

99

of (Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x =

0.00–1.00; y = 0.00–0.10).

Table 4.9 Comparison of the Observed and Theoretical Weight Percents and

Content Determined by EDX Analysis of the (Eu-Ni) substituted

Co2Sr2Fe12O22.

101

Table 4.10 M1 (slope of ferrimagnetic region), M2 (slope of paramegnetic

region), E1 (Activation energy of ferrimagnetic region) and E2

(Activation energy of paramegnetic region) of (Eu-Ni) substituted

hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y

105

Table 4.11 Grain size, Drift mobility, Dielectric loss, Tangent Loss, AC

conductivity (at 1MHz) of (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10)

115

Table 4.12 DC activation energy, exponential factor n, AC activation energy,

real and imaginary parts of electric modulus and impedance at

frequency of 1MHz of (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

116

Table 4.13 Estimated saturation magnetization (Ms), Anisotropy constant( K),

Magnetic moments (nB) and Squareness Ratio of (Eu-Ni)


y = 0.00–0.10).

128

Table 4.14 Compresses the compositional formula, Lattice parameters a and

c,C/a, volume of cell, Bulk density, X-ray density and percentage

porosityof Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–

1.25).

130

Table 4.15 Elemental analysis of Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10;

y = 0.00–1.25), hexaferrites. Obtained from EDX.

132

Table 4.16 Values of Grain size, Resistivity, Mobility, Dielectric Loss, Tangent

loss and AC conductivity at 1MHZ of Sm-Ni substituted

hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x= 0.00–0.10; y = 0.00–

1.25).

135

Table 4.17 Slopes and activation energies of farrimagnetic and paramagnetic 137

regions of Sm-Ni substituted hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-

y)O22, (x = 0.00–0.10; y = 0.00–1.25).

Table 4.18 Compresses the DC activation energy, exponent n , AC activation

energy, real and imaginary parts of electric modulus and impedance,

at 1MHz of Sm-Ni substituted hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-

y)O22, (x = 0.00–0.10, y = 0.00–1.25; )

146

Table 4.19 Estimated saturation magnetization (Ms), Anisotropy constant( K),

Magnetic moments (nB), Squareness Ratio for in-plane and out-

plane orientation of (Sm-Ni) substituted hexaferrites, Sr(2-


157

Table 4.20 Crystallite size, Grain size (nm), resistivity and Activation energy of


Fe10.50O22).

163

Table 4.21 Dielectric constant, Dielectric Loss, Tangent Loss and AC

conductivity of PST, FP1, FP2,FP3, FP4 and Y-type


166

Table 4.22 Exponentail factor, AC activation energy, Real part of electric

modulus, Imaginary part of electric modulus and Impedance of


Fe10.50O22).

170

Table 4.23 Saturation Magnetization (Ms), Remenances(Mr), coercivity) (Hc),

Squreness ratios (Mr/Ms), magnetocrystalline anisotropy constant

(K) and Estimated Saturation Magnetization (Ms) of PST, FP1,

FP2, FP3, FP4 and (Sr1.8Sm0.2Co2 Ni1.50Fe10.50O22) Y-type

hexaferrites

178

Table 4.24 Parameters measured from XRD patterns for ferrite (Sr2Co2Fe12O22),

(b) composite (Sr2Co2Fe12O22 +PPy-DBSA) and polymer (PPy-

DBSA).

179

Table 4.25 Real and imaginary parts of electric modulus and impedance,at

1MHz and DC activation energy, exponential factor n and AC

activation energy of (a) Y-type hexaferrite Sr2Co2Fe12O22, (b)

186

composite (Sr2Co2Fe12O22 +PPy-DBSA) and (c) polymer PPy:

Table 4.26 Saturation magnetization (Ms), remanance (Mr), coercivity (Hc),

Squareness ratio, anisotropy constants (K) and magnetic moment

For Co2Sr2Fe12O22and composite ferrite.

196

List of Publications

Thesis is based upon the following publications

1- Irshad ali, Abdul Shakoor, M.U. Islam. Muhammad Saeed. Muhammad Naeem Ashiq,

Synthesis and characterization of hexagonal ferrite Co2Sr2Fe12O22 with Doped Polypyrrole

composite Current Applied Physics 13 (2013) 1090-1095

2- Irshad Ali, M.U. Islam, Muhammad Naeem Ashiq, M. Asif Iqbal, Hasan M. Khan, Nazia

“Effect of Tb–Mn substitution on DC and AC conductivity of Y-type hexagonal ferrite” Journal

of Alloys and Compounds 579 (2013) 576–582

3- Irshad Ali, M.U. Islam, Muhammad Naeem Ashiq, Hasan M. Khan, M. Asif Iqbal,

Muhammad Najam-Ul-Haq “Effect of Eu–Ni substitution on electrical and dielectric

properties of Co–Sr–Y-type hexagonal ferrite” Materials Research Bulletin 49 (2014) 338–344

4- Irshad Ali, M.U.Islam, Muhammad Naeem Ashiq, M. Asif Iqbal, Hasan M. Khan, G.

Murtaza “ Role of Grain boundaries in the conduction of Eu-Ni substituted Y-tpye Hexaferrites”

Journal of Magnetism and Magnetic Materials, 362( 2014) 115-121

5- Irshad Ali, M.U.Islam, Muhammad Naeem Ashiq, M. Asif Iqbal, Nazia karamat, M. S.

Awan, Shahzad Naseem,” Role of Tb-Mn substitution on the Magnetic properties of Y-type

Hexaferrites” Journal of Alloys and Compounds 599 ( 2014) 131-138

6-Irshad Ali, M.U.Islam, Muhammad Naeem Ashiq, Imran sadiq, M. Azhar Khan, Nazia

Karamat, M. Ishaque, G. Murtaza “Electrical behavior of Tb-Mn substituted Y- type hexa-

ferrites for high frequency applications” journal of Electronic Materials, 44(2015)1054-1061

7- Irshad Ali, M.U. Islam, Muhammad Naeem Ashiq, M. Asif Iqbal, Nazia Karamat, M. Azhar

Khan, Imran Sadiq, Sana Ijaz, Imran Shakir, “Synthesis and characterization of hexagonal ferrite

Sr1.8Sm0.2Co2Ni1.50Fe10.50O22/PST thin films for high frequency application” Journal of

Magnetism and Magnetic Materials, 393 (2015)352–356.

8-Irshad Ali, M.U. Islam, Muhammad Naeem Ashiq, Imran Shakir, Nazia Karamat, M.

Ishaque, Majid Niaz Akhtar, Hasan M. Khan, Muhammad Irfan, Muhammad Azhar Khan

http://link.springer.com/journal/11664

http://www.sciencedirect.com/science/journal/03048853


http://www.sciencedirect.com/science/journal/03048853/393/supp/C

http://www.sciencedirect.com/science/article/pii/S0272884215005027











“Investigation of the magnetic properties of nanometric SrSmCoNi ferrite/PST matrix” Ceramics

International 41( 2015)8748–8754

9-Irshad Ali , M.U. Islam, Imran sadiq, Nazia Karamat, Aisha Iftikhar, M. Azhar khan, Afzal

Shah, Muhammad Athar, Imran Shakir, Muhammad Naeem Ashiq, “Synthesis and magnetic

properties of (Eu–Ni) substituted Y-type hexaferrite by surfactant assisted co-precipitation

method” Journal of Magnetism and Magnetic Materials 385(2015) 386–393.

Other Publications

10- Irshad Ali, M.U.Islam, M. Ishaque, Hasan M. Khan, Muhammad Naeem Ashiq,

M.U.Rana“Structural and magnetic properties of holmium substituted cobalt ferrites synthesized

by chemical co-precipitation method” Magnetism and Magnetic Materials,324(2012)3773-3777

11-Irshad Ali, Nasira Shaheen, M.U. Islam, Muhammad Irfan, Muhammad Naeem Ashiq,

M. Asif Iqbal, Aisha Iftikhar, “Study of electrical and dielectric behavior of Tb+3

substituted Y-type hexagonal ferrite” Alloys and Compounds,617( 2014)863-868

12- Imran Sadiq, Irshad Ali, Evgeny V. Rebrov, Shahzad Naseem, M.Naeem Ashiq, and M.U.

Rana “ Influence of Nd-Co Substitution on Structural, Electrical, and Dielectric Properties of X-

Type Hexagonal Nanoferrites” Journal of Materials Engineering and Performance (2013) DOI:

10.1007/s11665-013-0758-x

13- M. Asif Iqbal, Misbah-ul Islam, Muhammad Naeem Ashiq, Irshad Ali, Aisha Iftikhar,

Hasan M. Khan, “Effect of Gd-substitiution on physical and magnetic properties of Li1.2Mg0.4

Gdx Fe(2_x)O4 ferrites” Journal of Alloys and Compounds 579 (2013) 181–186

14- Attia Aslam, M.U.Islam, Irshad Ali, M.S.Awan, Muhammad Irfan, Aisha Iftikhar “High

frequency electrical transport properties of CoFe2O4 and Sr2NiMnFe12O22 composite ferrites

Ceramics International 40 (2014) 155–162

15- Muhammad Irfan, M.U. Islam, Irshad Ali, M. Asif Iqbal, Nazia Karamat, Hasan M. Khan

“Effect of Y2O3 doping on the electrical transport properties of Sr2MnNiFe12O22 Y-type

hexaferrite” Current Applied Physics 14(2014 )112-117

16- Hasan M. Khan, M.U. Islam, Yongbing Xu, M. Asif Iqbal, Irshad Ali “Structural and
























magnetic Properties of TbZn-substituted calcium barium M-type nano-structured hexaferrites”

Journal of Alloys and Compounds 589( 2014)258-262

17- M. Asif Iqbal, M.U. Islam, Irshad Ali, Muhammad Azhar khan, Imran Sadiq, Ihsan Ali

“High frequency dielectric properties of Eu+3-substituted Li–Mgferrites synthesized by sol–gel

auto-combustion method” Journal of Alloys and Compounds 86 (2014)404-410

18- Nazia Karamat, Muhammad Naeem Ashiq, Muhammad Najam-ul-Haq, Irshad Ali, M. Asif

Iqbal, Muhammad Irfan, Yasir Abbas, Muhammad “Athar Investigation of structural and

electrical properties of vanadium substituted disordered pyrochlore-type Ho2−xVxZr2O7

nanostructure” Journal of Alloys and Compounds 593( 2014)117-122

19- Hasan Mehmood Khan, Misbah-ul-Islam, Irshad Ali, Mazhar-ud-dnRana.“Electrical

transport properties of Bi2O3 doped CoFe2O4 and CoHoFe2O4ferrites” Materials science and

applications 2 (2012)1083-1089.

20 -Hasan M. Khan, M.U. Islam, Yongbing Xu, Muhammad Naeem Ashiq, Irshad Ali, M. Asif

Iqbal, Muhammad Ishaque “Structural and magnetic properties of Pr–Ni substituted

Ca0.5Ba0.5Fe12O19 hexa-ferrites nanoparticles” Ceramics International 40 (2014)6487-6493

21- M. Asif Iqbal, Misbah-ul-Islam, Irshad Ali, Hasan M. Khan, Ghulam Mustafa, Ihsan Ali

“Study of electrical transport properties of Eu substituted MnZn-ferrites synthesized by co-

precipitation technique” Ceramics International 39 (2013) 1539–1545

22- M. Ishaq, M.U.Islam.Irshad Ali, M.Azhar Khan, I.Z.Rehman “Electrical Transport

Properties of Co- Zn-Y-Fe-O System” Ceramics International 38 ( 2012), 3337-3342

23- G. Murtaza, R. Ahmad, T. Hussain, R. Ayub, Irshad Ali, Muhammad Azhar Khan, Majid

Niaz Akhtar, “Structural and magnetic properties of Nd–Mn substituted Y-type hexaferrites

synthesized by microemulsion method” Journal of Alloys and Compounds, 602 (2014)122-129

24- M. A. Khan, S. Riaz, Irshad Ali, M. N. Akhtar, G. Murtaza, M. Ahmad, I. Shakir, M.

F. Warsi, “Structural and magnetic behavior evaluation of Mg–Tb ferrite/polypyrrole

nanocomposites Ceramics International, 41(2015)651-656









http://www.sciencedirect.com/science/article/pii/S092583881400526X




25- M. N. Ashiq, S. Shakoor, M. N. Haq, M. F. Warsi, Irshad Ali, I. Shakir, “Structural,

electrical, dielectric and magnetic properties of Gd-Sn substituted Sr-hexaferrite synthesized

by sol–gel combustion method” Magnetism and Magnetic Materials,374 (2015) 173-178

26- Muhammad Azhar Khan, Kamran Khan, Azhar Mahmood, Gulam Murtaza, Majid

Niaz Akhtar, Irshad Ali, Muhammad Shahid, Imran Shakir, Muhammad Farooq Warsi,

“Nanocrystalline La1−xSrxCo1−yFeyO3 perovskites fabricated by the micro-emulsion route

for high frequency response devices fabrications” Ceramics International, 40

(2014)13211-13216

27-Imran Sadiq, Irshad Ali, Evgeny Rebrov, Shahzad Naseem, M. Naeem Ashiq, M.U.

Rana, “Nanosized Ce–Zn substituted microwave absorber material for X-band

applications” Magnetism and Magnetic Materials, 370(2014) 25-31

28- Muhammad Azhar Khan, M. Javid ur Rehman, Khalid Mahmood, Irshad Ali, Majid

Niaz Akhtar, Ghulam Murtaza, Imran Shakir, Muhammad Farooq Warsi, Impacts of Tb

substitution at cobalt site on structural, morphological and magnetic properties of

cobalt ferrites synthesized via double sintering method Ceramics International, 41(2015)

2286–2293

29 - A. hakeem, A.shakoor, M.irfan, Irshad. Ali, M. Azhar Khan , M. Naeem Ashiq, M.

Ishaq, A. Aziz , Synthesis And Electrical Properties Of Doped Polypyrole Withhexagonal

Ferrite Journal of Ovonic Research 10 (2014) 149 – 156.

30 - M. Ishaque, Muhammad Azhar Khan , Irshad Ali, Hasan M. Khan, M. Asif Iqbal, M.U.

Islam, Muhammad Farooq Warsi “Investigations on structural, electrical and dielectric properties

of yttrium substituted Mg-ferrites” Ceramics International 41(2015)4028–4034

31- Imran Sadiq, Shahzad Naseem, M.U. Rana, Muhammad Naeem Ashiq, Irshad Ali“

Temperature dependent magnetic and microwave absorption properties of doubly substituted

nanosized material” Magnetism and Magnetic Materials 385(2015)236–24

32- Tooba Khursheed, M.U. Islam, M. Asif Iqbal, Irshad Ali, Abdul Shakoor, M.S. Awan,

Aisha Iftikhar, Muhammad Azhar Khan, Muhammad Naeem Ashiq “Synthesis and

























characterization of polyaniline-hexaferrite composites” Magnetism and Magnetic Materials 393

(2015)8–14

33- M. Ishaque, Muhammad Azhar Khan, Irshad Ali, Hasan M. Khan, M. Asif Iqbal, M.U.

Islam, Muhammad Farooq Warsi “Study on the electromagnetic behavior evaluation of

Y3+ doped cobalt nanocrystals synthesized via co-precipitation route” Magnetism and

Magnetic Materials, 372(2014) 68–73

34- M. T. FARID, I. AHMAD, S. AMAN, M. KANWAL, G. MURTAZA, I. ALI, I. AHMAD,

M. ISHFAQ, SEM, FTIR AND DIELECTRIC PROPERTIES OF COBALT SUBSTITUTED

SPINEL FERRITES. Journal of Ovonic Research 11(2015)1 - 10

35- Z.ANWAR, M. AZHAR KHAN, I. ALI, M. ASGHAR, M. SHER, I. SHAKIR, M.

SARFRAZ, M. FAROOQ WARSI, INVESTIGATION OF DIELECTRIC BEHAVIOR OF

NEW Tb3+ DOPED BiFeO3 NANOCRYSTALS SYNTHESIZED VIA MICRO-EMUSLION

ROUTE Journal of Ovonic Research,6(2014)265 - 273

36-Hasan M. Khan, M. U. Islam, Yongbing Xu, M. Asif Iqbal, Irshad Ali, Muhammad Ishaque,

Muhammad Azhar Khan, Structural, magnetic, and microwave properties of NdZn-substituted

Ca0.5Ba0.5Fe12O19 hexaferrites J Sol-Gel Sci Technol DOI 10.1007/s10971-015-3700-x

37- M. T. FARID, I. AHMAD, S. AMAN, M. KANWAL, G. MURTAZA, I. ALI, I. AHMAD,

M. ISHFQ, STRUCTURAL, ELECTRICAL AND DIELECTRIC BEHAVIOR OF NixCo1-

xNdyFe2-yO4 NANO-FERRITES SYNTHESIZED BY SOL-GEL METHOD, Digest Journal of

Nanomaterials and Biostructures 10(2015)265 – 275

38-Muhammad Irfan, N. A. Niaz, Irshad Ali, S. Nasir, Abdul Shakoor, Abdul Aziz, Nazia

Karamat, N. R. Khalid, Dielectric Behavior and Magnetic Properties of Mn-Substituted Ni–

Zn Ferrites journal of Electronic Materials 44(2015) 2369-2377

39-M. Ishaque, Muhammad Azhar Khan, Irshad Ali, Hasan M. Khan, M. Asif Iqbal, M.U.

Islam, Muhammad Farooq Warsi, “Impacts of yttrium substitution on FMR line-width and

magnetic properties of nickel spinel ferrites”Journal of Magnetism and Magnetic Materials, 382(

2015)98–103

40-Imran Sadiq, Shahzad Naseem, Muhammad Naeem Ashiq, M. Asif Iqbal, Irshad Ali, M.A.

Khan, Shanawar Niaz, M.U. Rana “Spin canting effect and microwave absorption properties of

Sm-Mn substituted nanosized material” Journal of Magnetism and Magnetic Materials,(Accepted

Manuscript)





http://link.springer.com/search?facet-creator=%22Muhammad+Irfan%22

http://link.springer.com/search?facet-creator=%22N.+A.+Niaz%22

http://link.springer.com/search?facet-creator=%22Irshad+Ali%22

http://link.springer.com/search?facet-creator=%22S.+Nasir%22

http://link.springer.com/search?facet-creator=%22Abdul+Shakoor%22

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http://www.sciencedirect.com/science/journal/aip/03048853

http://www.sciencedirect.com/science/journal/aip/03048853

CHAPTER 1 INTRODUCTION

1

1. INTRODUCTION

1.1 Magnetism and Magnetic Materials The term magnetism arises from ‘Magnesia, an area in Asia Minor where loadstone (iron

ore) was found naturlly. The spin of electric charges inside the atom plays a crusel role for the

generation of magnetic field in the magnetic materials. Moreover the applied temperature can

influence the magnetic phase (or state). A magnetic material can show more than one form of

magnetism with temperature [1]. By and large magnetism arises from two distant sources [2];

Electric currents and moving electric charges.

Several particles have nonzero magnetic moments. As every particle has a definite charge

and mass therefor, all have some magnetic moment, perhaps zero.

Rotation of electron in the atom about the nucleus may be responsible for the creation of

magnetic moment. A single electron may be considered as a tiny magnet that has magnetic

moment initiating from orbital and spin motion. The magnetization is typically induced due to

the localized magnetic moments. For each electron this particular magnetic moment is named

Bohr’s magnetron and signified by;

μB = eh/2m (1.1)

The uncoupled electron spins are mainly responsible for the formation of magnetic dipole. The

iron has a magnetic moment of 5(μB) due to the 5 uncoupled spins. Commonly the net effect of

magnetic field is minute or zero if the spins produced by one electron are cancelled by the other

electrons. However the magnetic fields in Fe, Co and Ni do not cancel all the spins, as they have

unoccupied electron shells. The small region in which alignment of dipoles takes place is termed

as the magnetic domain. Though, all magnetic materials are influenced distinctly by the

application of a magnetic field. It was found long ago that definite materials have a tendency to

orient in a specific direction. The magnetic materials can be distinguished as paramagnetic,

diamagnetic, ferromagnetic, ferrimagnetic and anti-ferromagnetic material on the bases of

alignment of magnetic dipoles with applied magnetic field. The paramagnetic are attracted to a


2

magnetic field while diamagnetism are repelled by a magnetic field. Others magnetic materials

have a considerably further complex relationship with the magnetic field for example

antiferromagnetism and spin glass behavior. Substances on which applied magnetic fields has

negligible affect are identified as non-magnetic materials. Such substances are aluminum,

copper, plastic and gases. Pure oxygen shows magnetic properties once chilled to a liquid state

[3-5]. In the Diamagnetic materials the atomic magnetic moments are almost negligible due to

the unpaired electrons. The applied field prompts a magnetic moment that resists the applied

magnetic field subsequently low magnetization is achieved as presented in Fig. 1.1.(a). Many

elements of periodic table are diamagnetic. In Paramagnetic materials Fig. 1.1.(b) the unpaired

electrons are haphazardly oriented due to the thermal fluctuations and unpaired electrons align in

the direction of magnetic field. The ferromagnetism is the main source of the magnetic field in

the Permanent magnets. This is the most familiar and strongest category of magnetism. All

unpaired electrons in the Ferromagnetic materials (Fig. 1.1. (c) aligned in specific direction as

magnetic field is applied. Whereas all unpaired electrons in anti-ferromagnetic materials aligned

in opposite direction as depicted in Fig. 1.1.(d). It is more obvious to recall the fact that unequal

magnetic moments in ferromagnetic materials have anti parallel spin arrangements due to which

complete cancellation is not possible consequently leaving a some net magnetic moment. There

is a net magnetization (M) per unit atom as presented in Fig. 1.1.(e).

Magnetic features of the material is characteristically investigated by its magnetic susceptibility

(χ) which is the ratio of magnetization (M) to magnetic field (H) and given by the relation,

M= χH, (1.2)

Magnetic materials can also be categorized by their magnetic susceptibility as diamagnetic

materials have small and negative susceptibility. Paramagnetic materials have small but positive

susceptibility. Ferromagnetic materials possesses a large and positive susceptibility whereas

anti-ferromagnetic having small but positive magnetic susceptibility [6]. Anti-ferromagnetic

magnetic materials vary from paramagnetic materials with respect to magnetic susceptibility at

higher temperature.


3

Fig. 1.1: Classification of magnetic materials (a) Diamagnetic (b) Paramagnetic

(c)Ferromagnetic (d) Anti-ferromagnetic (e) Ferrimagnetic [6]

The magnetic susceptibility for anti-ferromagnetic materials incases with the increase in the

temperature but it remains unaffected for paramagnetic magnetic materials. A shift from anti-

ferromagnetism and ferromagnetism to paramagnetic is typically perceived if adequately heated.

The temperature at which transition perceived is named Curie temperature (TC) for

ferromagneticmagnetic materials and Neel’s temperature (TN) in case of anti-ferromagnetic

magnetic materials [6].


4

1.1.1 Fundamental Magnetic Quantities Ferromagnetic and ferrimagnetic are widely used magnetic materials in the most of the electronic

applications owing to their magnetic moments even in the absence of an applied magnetic field.

Low conductivity and eddy current losses make them useful at high frequency applications [7].

Magnetization may be defined as the net magnetic moment per unit mass or per unit volume.

Induction (B) is associated with H by the relation B = μ0(H+M), whereas μ0 is the free space

permeability and M is the magnetization. To have a proper insight of magnetization behavior, M-

H loop for various magnetic materials such as ferromagnetic and ferrimagnetic are taken as

depicted in Fig. 1.2 [8].

The magnetic properties like saturation magnetization, coercivity, and remanent magnetization

were calculated from MH loop. The Coercivity (Hc) is greater for hard magnetic materials and

small for soft magnetic materials. The magnetization at zero applied field is called Remanant

magnetization (Mr). Another additional essential magnetic property is the squarness ratio

(Mr/Ms). The values of the squarness ratio actually suggest that either the applied field is aligned

properly with easy axis of magnetization or not. Magneto-crystalline anisotropy (K) is also very

significant parameter for the ferromagnetic materials. There are hard and easy axis in a

crystalline magnetic material.The magnetic moment would favorably align along the easy axis.

It is reported [9] that Nano structured magnetic materials can be used as adsorbents mainly due

to their large surface area and distinctive features of easy separation as it exposed to the applied

magnetic field. Based on these excellent features one can suggest their use in various

applications such as targeted drug delivery, magneto-optical device, information storage, solid

devices, cell separation, magnetic resonance imaging enhancement, environmental capture,

magnetic refrigeration and purification of bio molecules.

The spin-orbit coupling in most magnetic materials is very weak but in heavy rare-earth metals

the coupling is quite strong [8].So we can conclude that heavy rare-earth metals strongly resist

the re-orientation of the domains away from the easy directions, so high coercive field is needed.

Such types of magnetic materials are quite beneficial for permanent magnets and perpendicular

recording media.


5

Fig. 1.2: Typical Hysteresis Curve showing different magnetic parameters.

Temperature at which certain magnetic materials undergo a sharp change in their magnetic

properties. Higher temperatures make magnets weaker as spontaneous magnetism only occurs

below the Curie temperature. When materials are cooled below their Curie points, magnetic

atoms spontaneously realign so that the ferromagnetism, antiferromagnetism, or

ferrimagnetism revives.

1.2. Introduction to Ferrites Ferrites are chemical compounds with iron (III) oxide (Fe2O3) as their major component.

Ferrites are generally insulating ferrimagnetic ceramic compounds derived mainly from iron

oxides like hematite (Fe2O3) or magnetite (Fe3O4) as well as oxides of many other metals. Like

many other ceramics ferrites are brittle and hard. Magnetic materials are the essential part of

the many home appliances, communication equipment, electronic products, and data-

processing devices. They are technologically vital materials owing to their versatile electrical

and magnetic properties particularly at high and ultra high frequencies. The versatile use of

http://www.britannica.com/science/temperature

http://www.britannica.com/science/ferromagnetism

http://www.britannica.com/science/ferrimagnetism


6

ferrites in the radio and television, satellite communications and microwave devices has brought

many revolutionary changes in term speed and squeezing the size. The main benefit of ferrites is

that they can yield higher efficiency with lower preparation costs than that of other magnetic

materials. They are also superior than metals and many other ceramic oxides owing to the high

values of electrical resistivity at room temperature (~106_1011Ω-cm) and the semiconducting

nature with increasing temperature. Suitable results can be achieved by controlling the crystalline

size ferrites. The particle size and shape, composition, crystallinity, cation distribution, synthesis

route and easy axis of magnetization all can influence various properties of these ferrites [10].

On the basis of their magnetic properties, the various ferrites are often categorized as "soft" or

"hard", which refers to their low or high coercivity (Hc).

1.2.1 Soft Ferrites A ferrimagnetic materials having chemical formula XFe2O4, where X denotes a divalent metal

ion. Soft ferrites have comparably low coercivity ~ 1K Oe. Due to limitation of cut-off frequency

the soft ferrites can sustain up to few Giga hertz frequency but cannot with stand very high

frequency more than X-band. Amongst the soft ferrite only the Li ferrite is the one that can be

used at hyper frequency. Yet even Li-ferrite could not meet the necessities for high tech

applications.


7

Fig.1.3: Unit cell of spinel lattice showing tetrahedral (A) and octahedral (B) sublattices[11].

The simplest spinel structure is composed of close-packed array of oxides (O2−) in lattice is

linked with two kinds of sub-lattices, one site is coordinated tetrahedrally with oxygen ions is

called A site and the other site is octahedrally with oxygen ions as depicted in Fig. 1.3. The unit

cell is large, including eight formula units and comprising 64 tetrahedral and 32 octahedral sites.

Normally 8 of the A sites and 16 of the B sites are filled [11], inorder to maintain charge

neutility.

1.2.2 Hard Ferrites

The materials which are magnetized to saturation and require a relatively very strong fields

(magnetic field) is of the order of 102 -103 oersteds in reversal of polarity. It is believed that

hard-magnetic materials are categorized by high values of the magnetic energy

(BH )max, coercive force Hc and residual induction Br, in the demagnetization process (back of a

hysteresis loop).Owing to their high values of Hc and Br the hard magnetic materials after


8

magnetization remain permanent magnets. The various types hard magnetic ferrites are listed

below.

1) M – Type BaO.6Fe2O3

2) W- Type BaO.2MeO.8Fe2O3

3) Y – Type 2BaO.2MeO.6Fe2O3

4) Z – Type 3BaO.2MeO.12Fe2O3

5) x – Type 2BaO.2MeO.14Fe2O3

6) u - Type 4BaO.2MeO.16Fe2O3

The group of ferrites having hexagonal structures is named hexagonal ferrites. Four

different types of hexagonal ferrites are specified as M, Y, W and Z as presented in the

composition diagram in Fig.1.4. These hexagonal ferrites correspond to (BaO+MeO)/Fe2O3

proportions of 1:6, 3:8, 4:6 and 5:12 correspondingly. Where Me is a transition cation or a

combination of different cations such as it would happen in spinels. The magnetic crystal

structures of the various types of hexagonal magnetic ferrites are extraordinarily complex, as

shown for the M-type BaFe12O19 in Fig.1. 5. The fundamental unit cell comprises 10 oxygen

layers, successively built from 4 blocks, R (hexagonal), S (spinel), R* and S*. Both S* and R*

are rotated 180° around the c axis with respect to S and R respectively, but atomic arrangements

are comparable to S and R. Both S or S* block contains two O2- layers; however R or R* block

comprises of three O2- layers, one site of the middle oxygen layer is substituted.


9

Fig. 1.4: Four types of hexagonal ferrites M, W, Y and Z [12].

The structure of Y-type hexaferrite magnetic material has space group (R3m) and is frequently

specified as (TS)΄΄(TS)(TS)΄ (TS)΄΄(TS), whereas the prime indicates that block is rotated at 120

about the c-axis [13]. The structure consists of three formula units. It is obvious to recall that

each Y block contains two layered spinel S block and four-layered antiferromagnetic T block.

Technically 36 Fe3+ ions in the unit cell of Y- type hexaferriteare distributed amongst six distinct

sites: 3av1, 18hVI, 6cvI, 3bv1 octahedral, 6c1v, 6cIV tetrahedral. Amongst these six site, three 3av1,

18hVI and 3bv1 have spin upward howeverremaining three sites 6cvI, 6c1v and 6cIV have spin

downward direction. Crystallographic and magnetic properties of these six sites are listed in

Table 1.1


10

Fig.1.5(a–c): The (110) cross-section views of M-type [(Ba,Sr)Fe12O19] (a), Y-type

[(Ba,Sr)2Met2Fe12O22] (b) and Z-type [(Ba,Sr)3Met2Fe24O41](c) structures with the hexagonal c

axis vertical[12].


11

Table 1.1 Number of ions per unit formula, coordination and spin orientation for the various

metallic sublattices of Y-type structure [12].

1.3 Promising Applications of Hexa Ferrites During the last few decades, hexagonal ferrites have been focused due to its substantial

significance to the electronic materials industry. The hexagonal ferrites material owns

comparatively high values of saturation magnetization, coercive force and magnetic anisotropy

field additionally outstanding corrosion resistivity and chemical stability. Hard ferrites have

fascinated considerable consideration for applications in microwave devices, permanent magnets

and perpendicular magnetic recording media [14, 15]. These above mentioned industrial

applications need materials with a prim control of homogeneity, shape and particle size and

magnetic characteristics. The synthetic method powerfully defines its various chief properties.

More importantly more than 50% need of the permanent magnets market covered by hexagonal

ferrites. Due to the lower price and their chemical stability hexaferrites have vast range of

applicability and has captured the market. Hard ferrites are more suitable materials used as a

electromagnetic wave absorber.

Sublattice Coordination Block Number of

ions

Spin

6c1v Tetrahedral S 2 Down

3av1 Octahedral S 1 UP

1 8hVI Octahedral S-T 6 UP

6cvI Octahedral T 2 Down

6cIV Tetrahedral T 2 Down

3bv1 Octahedral T 1 UP


12

1.4 Advantages of Hexaferrites Over Spinel Ferrites Presently, the progress of communication and information technology has brought an excessive

mandate for chip soft-magnetic components in the range of high-frequency, such as chip

electromagnetic interference filters and multi-layer chip inductors (MLCI). The conventional

materials used for MLCI, like NiZnCu spinel soft ferrites, may not be used in high frequency

range due to the restriction of cut-off frequency [16]. Owing to the planar magnetic anisotropy,

Y-type hexagonal ferrites have high cut-off frequencies up to the many GHz frequency. About

an order of magnitude higher than those of spinel ferrites [17]. Additionally, Y-type hexagonal

ferrites show exceptional magnetic properties in high frequency range, and are excellent

contenders as soft magnetic materials for chip components in very high frequency rang [18-20].

Besides the magnetic properties of these Y-type hexaferrites materials, the dielectric

permittivity is also very vital for MLCI [21-23]. The limitation of the cut-off frequency of

inductive componentsin the multi-layer structure is mainly due to the unavoidable wiring

capacity which resonated with inductance. Low values of permittivity are likely to improve

the frequency character of inductive components.

1.5 Introduction to Polymers Polymers are composed of several very small molecules named as monomers these monomer

linked with each other to form long chains. A normal polymer may contain tens of thousands of

monomers. Owing to their huge size, polymers are identified as macromolecules. Polymers have

benefited the human being, owing to the adaptability of polymers in the form of oils, resins, tars,

and gums. In 1830s, Charles Good year was able to produce a valuable form of natural rubber by

a procedure known as "vulcanization." later on, Celluloid succeed to prepare a hard plastic

formed through nitrocellulose. Regardless of these developments, evolution in polymer science

was bit slowly until the 1930s, when materials like vinyl, polystyrene, neoprene and nylon were

industrialized.The development of these innovative materials initiated an explosion in polymer

field that is still working on today.


13

1.5.1 Polymer Classification Based Upon Structure The physical properties of the of polymer not only influenced by size and molecular weight but

also on the structure. The polymer can be categoried into the four parts on the basis of their

molecular structure.

1.5.1.1 Linear Polymers.

In such type of polymer the monomers are connected with each other to construct the linear

chain. High densities, strong intermolecular forces, well packed, high melting points and high

tenssil strength are the main features of the linear polymer. Polyester, polyethylene nylon, PAN,

PVC, etc are common examples of linear polymers.

1.5.1.2 Branched Chain Polymers.

In such type of polymers the monomers are linked to construct the branches of various lengths

or extended long chains as a side chains. The small tensile strength of these branched chain

polymers is mainly due to the irregular packing and consequently, branched chain polymers

have, low melting points, low density and low boiling point as compared to the linear polymers.

Starch, low density polythene, glycogen, etc. are common examples of these branched chain

polymers.

1.5.1.3 Cross Linked Polymers.

In such type of polymers the monomers are cross linked together to construct a network

polymers (three dimensional). Such type of polymers is brittle, hard and rigid owing to their

network structure. Formaldehyde, bakelite, melamine and resin are prime examples of this kind.

1.5.1.4 Network polymers.

Three dimensional networks is formed by trifuntional monomer having three active covalent

bonds is called Network polymer. Actually, such type of polymer are highly cross linked may be

categorized as network polymer. Network polymers exhibit versatile thermal and mechanical

properties.

1.5.2 Classification Based Upon Molecular Forces The polymers can be distinguished on the basis of the intermolecular forces into four categories.

1.5.2.1 Elastomers.

The polymers which comes to their original shapes after removing the stresses applied on it is

called elastomers polymer. The weak intermolecular forces are mainly responsible for their


14

elastic nature. Additionally owing to their weak forces, by applying slight stress the polymers

can be stretched and recovers to their original shape as the stress is taken off. The natural rubber

is most significant example of elastomers.

1.5.2.1 Fibres.

In such type of polymers very strong intermolecular forces exist amongst the different chain.

These strong intermolecular forces are mainly due to the either dipole-dipole interaction or

hydrogen bonds. The strong intermolecular forces are mainly responsible for their clos packing,

less elasticity and extraordinary tensile strength. Due to these all reasons the melting points is

very sharp. These polymers are thin, thread like and long. Dacron, silk and Nylon 66 are main

examples of these samples.

1.5.2.2 Thermoplastics.

These are the polymers which can be easily softened repeatedly when heated and hardened when

cooled with little change in their properties. The intermolecular forces in such type of polymers

are in-between those of elastomers and fibres. There is lack of cross linking among the chain.

The lack of cross linking in this polymer might be the main resson for their softening nature as

the polymer chain move more freely. When such types of polymer is melted and fluid is formed

which can be converted into the desire shape by moulting and then cooled to achieve the required

product. Polystyrene, Polythene, Teflon, PVC are the prime examples of this type.

1.5.2.3 Thermosetting Polymers

In such type of polymer permanent changes occurred due to heating. Thermosetting polymers

become infusible and hard on heating. The semifluid substances (low molecular mass) are used

to prepare this polymer. The insoluble and hard infusible products can be achieved by heating

them as they get highly cross linked. So we can easily conclude that thermosetting plastic is

cross linked and is everlastingly rigid. The melamine formaldehyde resin and bakelite are the

prime examples of these polymers.

1.6 Chain Length The number of monomers combined into the chain is called degree of polymerization. The length

and size of the polymer chain play a very deceive role to control the physical properties of a

polymer. For example, impact resistance, boiling and melting point can be enhanced by

http://en.wikipedia.org/wiki/Degree_of_polymerization


15

increasing the chain length. Moreover, the resistance to flow or viscosity of the polymer in its

melt state is also increased. The polymer viscosity and chain length Z can be related as ~

Z3.2, so that increase in length of polymer chain favors the increase in viscosity more

rapidly. Increasing chain length additionally tends to decline chain mobility. Fig.1.6. shows

different polymers architecture.

Fig. 1. 6: Various polymer architectures [24].

1.7 Polystyrene Polystyrene is a cheap, unbreakable and hard plastic and possibly only polyethylene is common

in everyday life. The outside body of the computer is possibly prepared of polystyrene. Airplanes

http://en.wikipedia.org/wiki/Viscosity

http://www.pslc.ws/mactest/pe.htm


16

and model cars are also made of polystyrene. The drinking plastic cups, molded parts inside, the

radio knobs, toys, computers, and kitchen appliances are prepared of polystyrene.

Polystyrene is most famous vinyl polymer. Structurally, it is made of a extended hydrocarbon

chain, through a phenyl group connected to every other by carbon atom. Polystyrene is made

by free radical-vinyl polymerization, from the styrene monomer shown as follows.

1.7.1 Polymerization

Polystyrene is formed when different monomers of the styrene interconnected with each other.

During such type of polymerization in the vinyl group carbon- carbon pi is broken and new

sigma (carbon-carbon single) is formed by connecting some other styrene monomer to the chain.

It is very interesting to recall the fact that broken pi bind is much weaker than newly formed

sigma bond. Thus depolymerization of polystyrene is very difficult to achieve owing to the very

strong nature sigma bond. Typically one polystyrene chain is composed of few thousand

monomers. So the molecular weight of the one chain is approximately 100,000–400,000. Each of

the backbone carbons occupies the center of a tetrahedron, and the 4 bonds of the backbone

carbons directed toward the vertices. The rotation of -C-C- bonds compels the backbone chain to

occupy plane of the diagram. It is extremely hard to conclude that at what angle phenyl group is

directed inward or outward from the plane of the diagram. If the phenyl groups are directed on

the same side of the isomer than it is termed as isotactic polystyrene.

http://www.pslc.ws/mactest/vinyl.htm

http://www.pslc.ws/mactest/radical.htm

http://en.wikipedia.org/wiki/Vinyl

http://en.wikipedia.org/wiki/Number_average_molecular_weight

http://en.wikipedia.org/wiki/Tetrahedron

http://en.wikipedia.org/wiki/Chemical_bond

http://en.wikipedia.org/wiki/Phenyl

http://en.wikipedia.org/wiki/Isomer


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1.7.2 Syndiotactic Polystyrene

Ordered syndiotactic polystyrene can be achieved by Ziegler-Nattapolymerization whereas the

phenyl groups are located on alternating sides of the hydrocarbon backbone. This type of

ordered syndiotactic polystyrene is extremely crystalline with a Tm of 518 °F (270 °C). Such type

of materials cannot be commercially manufactured owing to their slow the polymerization.

1.7.3 Atactic Polystyrene

Atactic polystyrene is commercially very important. The phenyl groups on the both sides of

polymer chain are randomly oriented. Such type random positioning of phenyl groups hinders

the chains to line up with enough predictability to realize any crystallinity. The Atactic

polystyrene has a glass transition temperature Tg of ~90 °C. Fig. 1.7. represent the Syndiotactic

and atactic polystyrene.

1.8 Polymer Additive Molecular structure can influence many properties of the polymer. However, it is very essential

to change the physical, mechanical and chemical properties up to some extent. The modification

in the properties can achieve by altering the basic molecular structure. For this motivation, few

external substances are deliberately introduced into polymer termed as additive and making the

polymer more beneficial for the practical use. These additives are of five kinds.

1.8.1 Fillers Many features of polymer like compressive strength, abrasion resistance, thermal stability,

toughness and other properties can be improved by using different fillers. The polymer which

comprises fillers is called composite materials. The main advantage of the filler is that not only it

reduced the overall cast but also improved many properties. Wood flour, Clay, talc, limestone,

CNT and even many synthetic polymers are the examples of filler.

http://en.wikipedia.org/wiki/Ziegler-Natta_polymerization

http://en.wikipedia.org/wiki/Randomness

http://en.wikipedia.org/wiki/Crystallinity

http://en.wikipedia.org/wiki/File:Polystyrene_formation.PNG


18

Fig. 1.7: Syndiotactic and atactic polystyrene[25].

1.8.2 Plasticizers These plasticizers are added to the polymers to improve the ductility, toughness and flexibility of

polymers. The reduction in stiffness and hardness is also due to the presence of plasticizers.

Normally, the plasticizers are liquid having low molecular weight and low vapor pressure. More

importantly plasticizers are responsible for lowering glass transition temperature.

1.8.3 Stabilizer Stabilizers are used directly or indirectly to avoid the several effects like chain scission,

oxidation, and uncontrolled recombinations. More importantly cross-linking reactions that are

mainly occurred by photo-oxidationare properly controlled by the stabilizer. Polymers are badly

affected due to the direct or indirect influence of ultraviolet light and heat. The efficiency of

the stabilizers mainly depends on solubility.

http://en.wikipedia.org/wiki/Bond_cleavage

http://en.wikipedia.org/wiki/Oxidation

http://en.wikipedia.org/wiki/Cross-link

http://en.wikipedia.org/wiki/Photo-oxidation_of_polymers

http://en.wikipedia.org/wiki/Polymer

http://en.wikipedia.org/wiki/Ultraviolet

http://en.wikipedia.org/wiki/Heat

http://en.wikipedia.org/wiki/Stabilizer_(chemistry)


19

1.8.4 Colorant The main purpose of colorant addition in polymer is to impact the specific. Dyes and pigments

are more common colorant used in this regard. Different molecules in the dye in fact dissolve in

polymer matrix whereas, pigments are the filler type materials which do not entirely dissolve in

the polymer matrix, but sustain its identity as an individual phase. Commonly they have very

minute particle size, and their refractive index is appropriate to the parent polymer.

1.8.5 Flame Retardant Main purpose of the addition of such type of additive is to enhance the Flammability resistance

in the polymer matrix. It is understood fact that many of the polymers in their pure form are

flammable. Exception includes only those polymers which significantly contain contents like

chlorine or fluorine, like vinyl chloride. Such retardant may work properly by interfering with

combustion processes in the gas phase or by starting a various combustion reactions which really

produces lesser heat, thereby minimizing temperature.

1.9 Polypyrrole

Pyrrole polymer is a aromatic heterocyclic organic compound, all membered ring with the

common formula C4H4NH [26]. Various substituted products are may also termed pyrroles. Such

as, C4H4NCH3 (N-methylpyrrole). Porphobilinogen is the trisubstituted pyrrole[27]. Polypyrrole

can be synthesized by oxidizing pyrrole employing various techniques, chemical polymerization

and electrochemical polymerization. The polypyrrole is electrically conducting polymers has

gained much interest as a emerging class of materials in electronic industry, because of its

exceptional optical, magnetic, electronic properties and processing benefit of polymers[28].

Polypyrrole provide remarkable technological prospective like battery electrodes [29], corrosion

protection [30] biological sensors[31], e-Textiles and artificial muscles [32] microwave

shielding[33, 34], and sensors. Generally shows limited solubility in water and all organic

solvents. Consequently processability is limited. Attempts are executed to solve these difficulties

by inserting counter-ions into the polymers matrix backbone. The basic structure of Polypyrrole

is shown in the fig.1.8.


20

Fig. 1. 8: Structures of Polypyrrole.

1.9.1 Blends of Conducting Polymers with DBSA For mixing two or more polymer with each other, it is very essential that the polymers should be

soluble in common solvent. It is understood fact that polypyrrole (PPy) is basically conducting

polymer but is not soluble in any solvent. Therefore, it is very essential to make it soluble.

Keeping in view of this problem PPy is doped with sulphonates, DBSA and sulphuric acid. The

composite of PPy-DBSA has improved thermal stability. PPy-DBSA is cheap and offer strong,

flexible and free standing process able films.

1.10 Ferrites and Composites Ferrites constantly remain one of the distinguished magnetic materials ever discovered

and cannot be replaced easily by some additional magnetic material since ferrites are stable,

cheap and have a extensive range of technological applications [35, 36]. Consequently ferrites

are quiet commonly used wherever the product cost is a key consideration over magnetic

materials performances. The ferrite materials have unique properties that make them more

appropriate for certain applications than other magnet. Frequently rising demands regarding

security, efficiency and consistency of highly-loaded components e.g. in aerospace and

automotive applications are causing in increased attention in the use of polymer-based

composites. The composites are made up of at least two phases or constituents. The exceptional

mechanical properties of composites and chiefly the versatile mixture of low-density with high-


21

density (maximum strength and rigidity), have led not only to wide research but also to

extremely advanced technology. The foremost benefit of nano-composites is the capability to

tailor materials for distinct purposes [37].

1.11 Application of Ferrite/polymer composite material In recent times, there is a growing need for multifunctional composites to encounter the

additional necessities of electronic components. The composites like Polymer-based have

attracted huge attention owing to their tunable properties, flexibility and easy processability. The

main benefits of polymer bonded magnets (PBMs) above their metallic, ferrite ceramic

counterparts include low cost, low weight, resistance to corrosion, ease of forming and

machining and ability of extraordinary production rates. Because of the outstanding features

magnetic properties and dielectric of the ferrite/polymer composites, like abruptly reduced

dielectric loss matched to the bulk ferrites, More importantly microwave absorption properties

remained unchanged due to the domination of natural ferromagnetic resonance absorption in the

loss process of materials (ferrite absorber), consequently these materials becomes quite

beneficial not only as capacitive and inductive materials but as well as microwave absorber

materials [38]. Ferrite/polymer composite are prerequisite to diminish the radiation pollution

produced by the use of electromagnetic radiation sources (e.g. TV and radio broadcasting,

mobile phones, communication, microwave ovens and radar systems, etc.). Additionally,

complex electronic devices would also be sheltered from undesirable outer electromagnetic

fields. Whereas the battle airplanes covering with coating of microwave absorbers or not detected

by radar. Typically, ferrite composites with conducting materials are used as a backing material

to attain microwave absorption [39, 40]. The absorbing features of the materials heavy depend on

the skin depth, frequency, complex permeability, and complex permittivity. The absorbing

features might be changed by adjusting the ferrite filler (volume fraction) in the composite

materials. The resonance frequency for the reflection loss could also be changed by adjusting the

layer depth of absorbing materials [41]. Consequently, it is essential to examine the performance

of these structures at several operational excitation frequencies in order to explore the varying

behavior.


22

1.12 Focus and Objectives of the Present Study Hexferrites are the class of magnetic materials that have got startling magnetic and

electric properties pertaing to the development of high frequency devices and miniaturization of

the electronic gugets. Y-type hexaferrites with magnetic properties comparable to soft ferrites are

under investigations now a days. The enhancement in the magnetic properties due to rare earth

substitution and its polymer-composites make it prime material to be used in Multilayer Chip

Inductor, EMI, and microwave devices.

Sr-Co based hexaferrite nanostructured ferrites have been an area of interest since many

decades for the technological applications in many electronic, biomedical and memory devices.

Recent advances in ferrite technology is based upon the modification and enhancement of

physical properties of these ferrites due to large structural diversity and flexibility in chemical

composition and synthesis route. The present research focuses on the enhancement of electrical

and magnetic properties of these Sr-Co Y-type hexaferrites by tailoring the composition with

rare earth substitution and prepared by microemulsion method to achieve the best possible

homogenous nanostructured materials. The rare earths Tb, Eu and Sm ions substituted in Y-type

hexaferrite have not yet been reported frequently in the literature to the best of our knowledge.

Tb, Eu and Sm is also a potential candidate for the enhancement of properties

The objectives of this work are as follows:

To synthesize the single phase Sr-Co base Y-type hexaferrite by wet chemical method

and study the structural parameters.

To study the influence of co-doping of Tb–Mn, Eu-Ni and Sm-Ni on the DC electrical

resistivity of cobalt- strontium based Y-type hexaferrites.

To study the electrical properties and conduction mechanism of the prepared samples.

To check whether the dielectric parameters (dielectric constant, dielectric loss factors)

decrease or increase for high frequency applications.

To obtain high coercivity in the substituted samples for their use in perpendicular recording

media.

To prepare the composites with conducting/nonconducting polymer susceptible for EMI

shielding applications and micro wave devices.


23

References 1. I. Zutic, J. Fabian, S. Das Sarma: Rev. Mod. Phys. 76 (2004) 323.

2. C. Kittel, Introduction to Solid State Physics,Wiley New York 1986.

3. L. Neel, Properties of magnetic ferrites; ferrimagnetism and antiferromagnetism,

Annals of physics (Paris), 3 (1948) 137.

4. D. Jiles, Introduction to m agnetism and Magnetic materials: 2nd ed.; St.

Edmunsdsbury Press: Suffolk, (1991).

5. J.P. Jakubovics, Magnetism and Magnetic Materials; Institute of Metals: London,

(1987).

6. B.D. Cullity, Introduction to Magnetic Materials: Addison-Wesley Publishing

Company: Menlo Park, (1972).

7. K. Kondo, S. Yoshida, H. Ono, M. Abe, J. Appl. Phys. 101 (2007) 09M502.

8. S. O. Kasap: Principles of Electronic Materials and Devices, 3rd edn. (McGraw-

Hill, New York 2001).

9. M. Khairy, S. A. El-Safty, M. Ismael, H. Kawarada Applied Catalysis B: Environmental

127 (2012) 1– 10

(Oxford Univ. Press, Clarendon (1997).

10. A. Aslam, M.U.Islam, I. Ali, M.S.Awan, M. Irfan, A. Iftikhar, Ceram. Int., 40 (2014)155–

162.

11. E.H. Frei, S. Shtrikman, D. Treves, Phys. Rev. 106 (1957) 446.

12. G. ALBANESE ColloqueCi, Supplement Au N° 4, Tome 38, Avril1977, Page Cl-85.

13. G.Albanese,M.Carbucicchio,A.Deriu,G.Asti,S.Rinaldi, J. Appl. Phys.7(1975)227.

14. E. Lacroix, P. Gerard, G. Marest and M. Dupuy, J. Appl. Phys. 69 (1991)4770.

15. Zaquine, H. Benazizi and J.C. Mage, J. Appl. Phys. 64 (1988) 5822.

16. Hsu J Y, Lin H C, Shen H D, and Chen C J, IEEE T Magn. 33 (1997)3325.

17. J. Smit , and Wijn H B J, Ferrites(1959)Ch. 13-16.

18. Y. Bai, J. Zhou, Z. Gui, and L. Li, J. Magn. Magn.Mater. 246 (2002)140.

19. N. C. Pramanik, T. Fujii, M. Nakanishi, J. Takada, S. Seok, Mater. Lett. 60 (2006)2718.

20. S. G. Lee, and S. J. Kwon, J. Magn. Magn.Mater. 153 (1996)279.

21. M. El-Saadawy,J. Magn. Magn.Mater. 219 (2000) 69–72.


24

22. A.M.A. El Ata, M.A. Ahmed, M.A. El Hiti, M.K. El Nimr,J. Mater. Sci. Lett. 18(1999)

563–567.

23. M.A. El Hiti, A.M.A. El Ata,J. Magn. Magn.Mater. 195 (1999) 667–678.

24. http://en.wikipedia.org/wiki/Polymer#cite_note-PC14-29.

25. http://www.pslc.ws/mactest/styrene.htm.

26. Loudon Marc G: "Chemistry of Naphthalene and the Aromatic Heterocycles."Organic

Chemistry (Fourth ed.). New York: Oxford University Press. (2002)1135–1136. .

27. Cox Michael, Lehninger Albert L, Nelson David R: (2000). Lehninger principles of

biochemistry. New York: Worth Publishers. ISBN 1-57259-153-6.

28. V.Shaktawat, K. Sharma, Saxena NS J Ovonic Res 6(2010) 239–245.

29. K. JH, A. Sharma, Y. Lee Mater Lett. 60(2006) 1697–1701.

30. G. Han, J. Yuan, G. Shi, F. Wei Thin SolidFilms 474(2005)64–69.

31. S.Walkiewicz, A.Michalska, K.MaksymiukElectroanalysis 17(2005) 1269–1278.

32. E.Hakansson, A.Kaynak, T. Lin, S.Nahavandi, T. Jones, E. Hu Synth Met 144(2004)21–

28.

33. M. Kim, H. Kim, S. Byun, S. Jeong, Y. Hong, J.Joo, K. Song, J. Kim, C. Lee, J. Lee Synth

Met 126(2002) 233–239.

34. O.Yavuz, M. Ram, M.Aldissi, P.Poddar, H.Srikanth Synth Met. 151(2005) 211–217.

35. J. Smit, H.P.J. Wijin, Ferrites, Philips Technical Library, Tokyo, Japan, 1959–1965.

36. M.R. Anantharaman, S. Sindhu, S. Jagatheesan, K.A. Malini, P.Kurian, J. Phys. D 32

(1999) 1801.

37. D.K. Hale, J. Mater. Sci. 11 (1976) 2105.

38. J. Otaigbe, H.S. Kim, J. Xiao, Polym. Comps. 20(1999) 697.

39. Y. Natio, K. Suetake, IEEE Trans. Microwave Theory Tech. Mtt.19 (1971) 65.

40. S.S. Kim, S.B. Jo, K.I. Gueon, K.K. Choi, J.M. Kim, K.S. Churn, IEEE Trans. Magn. 27

(1991) 5642.

41. D.Y. Kim, Y.C. Chung, T.W. Kang, H.C. Kim, IEEE Trans. Magn. 32 (1996) 555.

http://en.wikipedia.org/wiki/Polymer#cite_note-PC14-29

http://www.pslc.ws/mactest/styrene.htm

CHAPTR 2 LITERATURE SURVEY

25

2. LITERATURE SURVEY

In the recent years, the hexaferrites have been demonstrated to be vital materials for high

frequency applications. The electrical and magnetic properties can be enhanced either by

controlling the sintering temperature or by the addition of different types and amounts of metal-

ion substitution. These materials are able to catter the needs of new devices for modern

applications. Constantly growing demands concerning safety, efficiency and reliability of

extremely-loaded components e.g. in automotive and aerospace applications are resulting in

increased interest in the use of polymer-based composites. Particularly endless fiber-reinforced

composite materials with their outstanding, load-adapted strength and stiffness are considered to

be attractive for the design of novel lightweight components. Many researchers are engaged to

explore these materials and the review of their findings are as follows;

Albanese [1] reported the comparative study of the various type of hexagonal ferrites. Various

techniques like Mossbauer, NMR, neutron spectroscopies and VSM were employed to

characterize samples investigated. The cation distribution of different ions in the hexagonal

lattice was systematically investigated. It revealed that proper knowledge of cation distribution in

the hexagonal lattice is quite supportive to conclude the discussion of numerous magnetic

parameters.

El Hiti et. al [2] presented the systematic study of the Y-type hexaferrites prepared by the

ceramic technique. Composition and temperature dependent magnetic properties, electric,

thermoelectric and electrical properties were reported. It was found that DC electrical

conductivity, drift mobility, initial magnetic permeability and carrier concentration increased but

fermi energy showed the decreasing trend with the rise of temperature. It was concluded that

electrons might be the majority charge carriers by employing thermoelectric technique. The

strong temperature-dependence of drift velocity (µd) and its smaller values designate that

predominant mechanism at high temperatures region is hopping conduction.

A series of Y-type hexaferrites synthesized and characterized by Yang et. al hasbeen reported [3]

Y-type phase formation of the reported samples was confirmed by by X-ray diffraction and grain


26

morphology was studied by scanning electron microscopy. Impedance analyzer was used for the

magnetic permeability measurement. More interestingly, it was found that sintering temperature

decreased due to the cation deficiency. These samples sintered at low temperature have

exceptional magnetic properties for various practical application.

Bai et.al reported [4] Cu & Zn-substituted Co2Y hexaferrite synthesized by solid-state reaction.

The phase formation was confirmed by XRD and SEM study and it revealed that Zn content had

no effect on microstructure, it is chiefly influence by sintering temperature. Experimental

findings indicated that Cu variation lowers the sintering temperature, and that of Zn variation

was found to increase in initial magnetic permeability. It was summarized that substitution of Cu

& Zn has improved the high frequency magnetic properties.

El Ata et.al studied [5] the preparation of Y-type hexaferrites by the standard ceramic method.

The effect of temperature, frequency and composition on the dielectric properties was

investigated systematically. The dispersion at higher frequencies in the AC conductivity spectra

was logically explained on the basis of interfacial polarization due to the inhomogeneous

structure of the ferrite material. It was found that dielectric constant is high at low frequency and

decreases with the rise of frequency. Several dielectric parameters were elucidated on the basis

of the supposition that the process of dielectric polarization is analogous to that of the conduction

mechanism.

Salunkhe et.al [6] reported that improvement of magnetic properties of Y type hexaferrite is

mainly recognized due to the significant role of nickel ions substitution. The phase of nickel

doped Sr–Y hexaferrite prepared by solid-state reaction was confirmed by XRD and it was found

in single phase form. The SEM study of the investigated samples obviously proposed that

crystallization rate become faster with increasing Ni substitution. The susceptibility measured

appling Gouy’s method, it was found that compound exhibited ferromagnetic behavior at room

temperature. It was believed that high value of TC might be due to the existence of Fe3+ and

Ni2+ions in the investigated samples. Electrical conductivity data obviously purposed the dual

conduction mechanism in the samples.


27

Bai et.al [7] investigated the structure and magnetic properties of Y-type hexagonal ferrite,

particularly the tunable dc magnetic permeability. The SEM study revealed platelike grains

linked with the planar magnetocrystalline anisotropy of the investigated hexagonal ferrite

samples. The observe increased in the magnetic permeability and its tunability was explained

logically. The exceptional tunability of magnetic permibility had made these samples best

candidate for design of tunable multilayer chip inductors in the range wide frequency.

Safaan et.al [8] employed ceramic method to synthesize the Y-type hexagonal ferrites. The

single phase formation was confirmed by the XRD analysis. and the effect of composition on the

variation of unit cell parameters, porosity and density was elucidated systemically with

increasing dopant concentration. Three different regions were examined in the temperature

dependent permeability spectra was justified. Improved values of selectivity was obtained at

lower frequency

Bai et. al [9] presented the comparative study of Y-type hexagonal ferrites samples prepared by

two different methods, i.e, citrate sol–gel auto-combustion and solid-state reaction method. The

findings of various properties such as XRD, SEM and magnetic properties were systematically

compared. It was found that sintering temperature of the samples prepared by citrate sol–gel

auto-combustion method is quite lower than that of solid-state reaction method. Moreover, the

samples showed improved magnetic properties in high frequency than those prepared by SSRM.

The enhancement in permeability and lowering of of cut-off frequency might be due to the

increasing concentration of Zn.

Bai et.al [10] reported the dielectric and magnetic properties of Bi–Zn co doped Y-type

hexagonal ferrites prepared by solid-state reaction method. The samples were logically

characterized by XRD and SEM structural. It was observed that slight doping of Bi will not

change the phase formation. Furthermore, it was observed that doping of Bi lowers the sintering

temperature. The impedance analyzer was employed to peep through magnetic and dielectric

properties. The outstanding magnetic and dielectric properties were obtained in high frequency.

Costa et.al [11] reported that preparation conditions such as sintering temperature, chemical

composition and amount of substitutions play a vital role to control the physical properties of the

investigated samples. The complex electric modulus formalism is quite adventurous to explain


28

the numerous electrical transport parameters such as conductivity, relaxation time, ion hopping

rate in the material.

Kouril et.al [12] reported that NMR study is quite useful to understand the distribution of Zn2+

in single crystal at hexagonal cationic sites of Y-type hexaferrite. It was found that they

interpreted experimental data systematically by comparing it with NMR spectra. Moreover, it

was concluded that the value of the γ parameter can be estimated by comparing the measured

NMR spectra and simulated line shapes.

Iqbal et.al [13] reported Mn- and Cr substituted Y-type hexa ferrites prepared by the sol–gel

method and sintered at 950C˚ to achieve the pure Y-type phase.The studied samples were

characterized by employing the various techniques that is XRD, EDX, SEM and electrical

properties. More, interestingly it had been found that substitution did not affect the structural

properties. The crystallite size was calculated by using XRD data and was found in the range of

13–45 nm. The Co-substituted (Cr-Mn) samples had high resistivity, high Curie temperature and

a low dielectric constant.

Jotania et.al [14] reported that all the hexaferrite samples synthesized by chemical co-

precipitation technique and characterized by employing XRD, SEM, FTIR and dielectric

measurements. X-ray results endorse that Mn doping at the expanse of Cu did not change the Y-

type hexaferrite phase. The SEM study reflected that grain size of the prepared samples

improved noticeably with the rises of Mn and the porosity decreases. The formation of pure

hexaferrite was also confirmed by the FTIR measurement.

Elahi et.al [15] investigated a series of single phased Y-type hexagonal ferrites prepared by the

sol–gel method. The variation of structural, electrical and magnetic properties had been

inspected by replacing Mg2+ at Ni2+-sites. The investigated samples were systematically

characterized by XRD, FTIR, SEM, VSM and electrical measurement. All the ferrites showed

platelet-like microstructure of the grains which is ultimately a most appropriate shape for

microwave absorption properties. More interestingly, dielectric behavior of the investigated

samples obviously purposed their use in MLCIs.


29

Pasquale et.al [16] synthesized by mixing the ferrite powder with a phenol-type binder and cold

pressing was applied with a typical compaction pressure. It was observed that obtained values of

a magnetostriction and Young’s modulus are quite encouraging for sensing applications.

Furthermore these investigated composite samples showed high frequency characteristics along

with good chemical stability.

Slama et.al [17] reported the permeability of ferrite and magneto polymers synthesized by

mixing the ferrite ceramics with polymer. The experimental results were matched with the

predictions of magnetic circuit model associated for dynamic situations. The permeability of

magnetopolymer was approximately constant in a high frequency range than for sintered ferrite.

Kazantseva et.al [18] synthesized Mg-Zn/polyaniline - composites. The permeability of the

composites was measured up to the 3 GHz frequency. The picric acid was used to adjust the

conductivity of polyaniline coating. The shifting of resonance frequency toward higher

frequency accured at the higher polyaniline concentration in conductivity spectra. The

microwave field boundary conditions at the interface among the ferrite ceramic particle and

polyaniline matrix were responsible for variation in the magnetic properties of composite

samples.

The ferrite (BaFe12O19) had been prepared by coprecipitation by Makled et.al [19]. The fine

powders were incorporated into a rubber matrix with different loading levels. The variations in

the properties of rubber–ferrite composites (RFCs) were systematically explored as a function of

ferrite loading. It was observed that the coercivity (Hc) and the saturation magnetization were

enhanced linearly as mass fraction of the filler increased. The strain, tensile strength and

modules are enormously influenced by the shape, size and volume fraction of ferrite ceramic

particles. The present investigated composites presented a distinctive characteristic of high stored

energy (BHmax = 1.18MGOe).

Preparation and characterization of nanocomposite of nickel-ferrite and hydrophilic polymers

such as (PHPMMA) or (PVA) was presented by Sindhu et.al [20]. The composite samples were

synthesized by direct mixing of the polymer and the ferrite filler, followed by sonication.

Crystalline nature of ferrite was confirmed by X-ray diffraction. SEM analysis shows spherical

grains. Furthermore composites were also characterized by FTIR spectroscopy and EDXS. The


30

improved values of magnetization were obtained in composites with PHPMMA than that of PVA

based composites.

The permittivity, permeability and microwave absorption properties of polymer–ferrite

composites had been examined in high frequency range by Abbas et.al [21]. The polymer–ferrite

composites synthesized with varying ferrite ratios of 50%, 60%, 70% and 80% in polyurethane

matrix. It was observed that at higher frequencies, ferrite’s permeabilities reduced radically.

However, improved values the dielectric losses in the selected composition might be the

exchange of Fe+3 to Fe+2 ions (electron hopping). The reflection loss was determined by using

the different dielectric parameter. The composite sample with 80% ferrite content had shown a

smallest reflection loss. The synthesized composites samples were suggested for suppression of

EMI and stealth technology.

The microwave absorption and electromagnetic properties of synthesized the Z-type

ferrite/polymer composites were inspected by Li et.al [22]. The various properties of the multi-

phase composites samples were strongly influenced by particle size of the Z-type ferrite fillers.

Simultaneously, Microwave absorption properties were influenced due to the strong correlation

occurred amongst electromagnetic parameters and reflection loss of composites.

A self-propagating combustion method was employed to prepared the nanosized powders at

fairly low temperature by Liangchao et.al [23]. Polyaniline/ferrite nanocomposites were

synthesized by in situ polymerization of aniline in the presence of ferrites. The structural,

morphological, and magnetic properties of ferrite and nanocomposites were taken by XRD,

TEM, SEM, and VSM. The results had cleary suggested that ferrite were coated successfully by

polyaniline, which minimized the agglomeration of ferrite ceramic particles to definite degree,

and was supportive to the stabilization and decentralization of nanoparticles. The

nanocomposites had shown hysteresis loops of the ferromagnetic nature under applied field. The

composite samples had exhibited soft magnetic nature as of their coercivity was low than that of

pure ferrites.

Nickel ferrite was prepared by citrate precursor method by Ahmed et.al [24]. The structure,

particle size, the homogeneity and shape of the nanoparticles were investigated by XRD, TEM

and IR was employed to investigate the composition features of the as prepared sample. The


31

susceptibility of the samples was investigated at different magnetic field intensities and

temperatures. Dielectric constant, AC conductivity and loss tangent of composite samples

synthesized by mixing the ferrite with different ratios in polystyrene matrix had been examined.

The magnetic susceptibility of the ferrite/PSTmatrix was also examined.

Functionalized poly(N-vinylcarbazole) had been prepared by Basavaraja et.al [25] using an

oxidative polymerization embedded with magnetic nanoparticles. The composite samples had

shown both magnetic and electrical properties. Spectroscopic investigation of these prepared

composite samples had shown a fruitful functionalization of the ferrite nanoparticles into the

polymer (PVK) matrix. The magnetization data had shown an substantial hysteresis loop. The

composite samples had shown the characteristic of semi-conducting.

Ting et.al.[26] reported the composite samples of NiZn ferrite coated with polyaniline prepared

by in situ polymerization at different NiZn ferrite weight ratio. The composite samples were

studied by employing FTIR, XRD, SEM, TEM and electron spin resonance. It was observed that

addition of NiZn ferrite content in polymer (polyaniline) matrix might be the main reason for

enhancement of broader absorption frequency range.

Martins et.al [27] reported the solvent casting and melt was employed to synthesize the

composite films of polymer (vinylidene fluoride) and ferrite (CoFe2O4 and NiFe2O4). It was

observed that well-spread nanoparticles of ferrite nucleate the polymer. The ratio of ferrite

nanoparticles in the polymer matrix strongly influenced the dielectric and magnetic behavior of

the composite samples. Both dielectric constant and magnetization increased as the ferrite ratio

increased. The composite NiFe2O4/PVDF samples had shown superparamagnetic conduct, but

CoFe2O4/PVDF samples established a hysteresis cycle having small values of coercivity.

H. Sozeri et.al [28] reported that nanocomposite PANI/Co0.5Mn0.5Fe2O4 was prepared by

chemical polymerization of aniline in the presence of (APS). The structural, thermal,

morphological and magnetic measurements of the nanocomposite were examined by XRD,

FTIR, TGA, SEM and VSM. The line profile method was used to calculate the average

crystallite size of nanocomposite. The superparamagnetic behavior of nanocomposite was

observed through VSM measurements. The saturation magnetization (Ms) of the prepared


32

composite was substantially low matched to that of pure ferrite nanoparticles owing to the

surface spin disorder.

Aslam et.al [29] reported that composite ferrite materials made by mixing CoFe2O4 and

Sr2NiMnFe12O22 powders annealed for 3h at 1050C˚. The prepared nanosized samples were

characterized by empioying XRD, SEM, FTIR, dielectric measurements and electrical resistivity.

XRD study reveals that there was no new phase seen. It was observed that the intensity of Y-

phase slowly decreased by increasing spinel ferrite. Furthermore, room activation energies and

temperature resistivity increased with increasing spinel ferrite. Due to the Low values of

dielectric constant, dielectric losses and high value of resistivity these composite ferrites were

very favorable for microwave devices and electromagnetic attenuation materials.


33

References 1. G. Albanese, J. Phys. I Colloque CI, supplement au No. 4, Tome 38(1977) Cl-85-C194.

2. M.A. El Hiti, A.M. Abo El Ata, J. Magn. Magn. Mater. 195 (1999) 667-678.

3. B. Yang, Z. Ji, G. Zhilun, L. Longtu, J. Magn. Magn. Mater., 250 (2002) 364–369.

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162.

CHAPTER 3 EXPERIMENTAL SETUP AND METHODS OF ANALYSIS

35

3. EXPERIMENTAL SETUP AND METHODS OF ANALYSIS

Three series of Terbium, Europium and samarium substituted Y-type hexafrrites

(Co2Sr2Fe12O22) were prepared by microemulsion method and two series of composites were

prepared accordingly.

(1) Tb-Mn substituted Y-type hexaferrite with nominal composition Sr2Co2-xMnx TbyFe12-yO22

(x = 0.0, 0.2, 0.4, 0.6, 0.8, 1, Y =0.0, 0.02, 0.04, 0.06, 0.08, 0.10).

(2) Eu-Ni substituted Y-type hexaferrite with nominal composition Sr2Co2-xNix EuyFe12-yO22 (x

= 0.0, 0.2, 0.4, 0.6, 0.8, 1, Y =0.0, 0.02, 0.04, 0.06, 0.08, 0.10).

(3) Sm-Ni substituted Y-type hexaferrite with nominal composition Sr(2x)Sm(x)Co2NiyFe(12-y)O22,

(x = 0.0, 0.02, 0.04, 0.06, 0.08,0.10; y = 0.00, 0.25, 0.50, 0.75, 1.00, 1.25).

(4) A series of composite samples were prepared with different ferrite

(Sr(1.8)Sm(0.2)Co2Ni1.50Fe(10.50)O22 ) percentage i, e ( 0%, 25%, 50%, 75%, 100%) in the PST

matrix

(5) A composite was prepared with 1:1 ratio of ferrite Co2Sr2Fe12O22 with Ppy-DBSA

3.1 Preperation of Tb-Mn substituted Y-type

Hexaferrite Sr2Co2-x Mnx TbyFe12-y O22

3.1.1 Materials The chemicals of analytical grade were used to synthesize Y-type strontium hexa-ferrites Sr2Co2-

xMnx TbyFe12-yO22. The starting materials were Fe(NO3)3·9H2O (Riedel-de Haen, 97%),

Co(NO3)2·6H2O (Merck, >99%), MnCl2. 2H2O(Merck, >99%), Sr(No3)2 (Merck, 99%), Tb2O3

(Merck, 99%), (cetyltrimethyl ammonium bromide) CTAB (Merck, 97%) as a surfactant, NH3

(Fisher Scientific, 35%) as a precipitating agent and methanol (Merck, 99%) as washing agent.

3.1.2 Synthesis Procedure The Y-type hexaferrite samples with nominal composition Sr2Co2-xMnx TbyFe12-yO22 (x = 0.0 –

1, Y =0.0 – 0.1) were prepared by the normal microemsulsion method. The metallic salt solution

of the required molarities were prepared in deionized water and mixed in a baker. The CTAB


36

was also added in metals solutions with ratio 1: 1.5 (metals: CTAB). The solution was stirred on

the magnetic hot plate at 60C until it formed a clear solution. The ammonia solution was added

dropwise to form the precipitates. After that the precipitates were washed with deionized water

and finally with methanol. Then the precipitates were dried in an oven at 150°C and finally

annealed at 1050°C for 8 h using box furnace (Heyaius, D – 6450 Hanau, Germany).The

formation of substituted Y-type hexaferrites from the starting materials can be shown in scheme

1

Tb2O3 + HNO3 → Tb(NO3)3aq + H2O

(12-y)Fe(NO3)3(aq) +Sr(NO3) 2(aq) + 2-xCo(NO3)2 + xMnCl2(aq) + yTb(NO3)aq + NH4OH

↓

Sr(OH)2 12-yFe(OH)3 2-xCo(OH)2 xMn(OH)2 yTb(OH)3+NH4NO3+ NH4Cl + H2O

↓1050ᵒC

Sr2Co2-xMnx TbyFe12-yO22

Scheme 1: Formation of Tb-Mn substituted Y-type hexaferrites from their starting materials

3.2 Preparation of Eu-Ni substituted Y-type hexaferrite

Sr2Co2-x Nix EuyFe12-y O22

3.2.1 Materials The Y-type hexaferrite samples with nominal composition Sr2Co2-xNix EuyFe12-yO22 (x = 0.0–1, y

= 0.0–0.1) were prepared by the normal microemulsion method. The analytical regents

Fe(NO3)3·9H2O (Riedel-de Haen, 97%), Co(NO3)2·6H2O (Merck, >99%), NiCl2.6H2O (Merck,

99%), Sr(No3)2 (Merck, 99%), Eu2O3 (Merck, 99%), ( cetyltrimethyl ammonium bromide)

CTAB (Merck, 97%) as a surfactant, NH3 (Fisher Scientific, 35%) as a precipitating agent and

methanol (Merck, 99%) as washing agent were used to synthesize the Y-type hexaferrites.

3.2.2 Synthesis procedure The metal salt solutions of the required molarities were prepared in deionized water and mixed in

a beaker. Eu2O3 was first dissolved in HNO3 by heating up to 60Cº in a beaker and continuously


37

stirred by a magnetic stirrer in order to obtain Europium nitrate and then mixed with the solution.

The CTAB was also added in metals solutions with ratio 1: 1.5 (metals: CTAB). The solution

was stirred on the magnetic hot plate at 60°C until it formed a clear solution. Then the

precipitates were dried in an oven at 150°C and finally the samples were annealed in the different

ranges of temperature and different brackets of time. The sintering temperature was optimized at

1050C. The temperature 1050Cº for 8h is found most accurate to achieve the single phase Y-type

hexaferrites.

3.3 Preparation of Sm-Ni substituted Y-type hexaferrite

Sr(2-x)Sm(x)Co2NiyFe(12-y)O22

3.3.1 Materials The analytical regents were used to synthesize Fe(NO3)3·9H2O (Riedel-de Haen, 97%),

Co(NO3)2·6H2O (Merck, >99%), NiCl2.6H2O (Merck, 99%), Sr(NO3)2 (Merck, 99%),

Sm(NO3)3 (Merck, 99%), ( cetyltrimethyl ammonium bromide) CTAB (Merck, 97%) as a

surfactant, NH3 (Fisher Scientific, 35%) as a precipitating agent and methanol (Merck, 99%) as

washing agent.

3.3.2 Synthesis Procedure The Y-type hexaferrite samples with nominal composition Sr(2x)Sm(x)Co2NiyFe(12-y)O22 (x = 0.0 –

0.1, Y =0.0 – 1.25) were prepared by the normal microemulsion method. The metallic salt

solution of the required molarities were prepared in deionized water and then mixed in a baker.

The CTAB was also added in metals solutions with ratio 1: 1.5 (metals: CTAB). The

nanoparticles shape and size can be controlled by the metal to surfactant ratio. This ratio was

optimized and then used for the preparation of these samples. The solution was stirred on the

magnetic hot plate by keeping the temperature maintained at 60C until it formed a clear

solution. The 2M ammonia solution was added drop wise to form the precipitates. There after the

precipitates were washed with deionized water and finally with methanol. The precipitates were

then dried in an oven at 150°C. Final annealing was carried out at 1050°C for 8 hours, using box

furnace (Heyaius, D – 6450 Hanau, Germany).


38

3.4 Preparation of Sr1.8 Sm0.2 Co2Ni1.5 Fe10.5 O22/ PST

Composites

3.4.1Chemicals The analytical regents used to synthesize Y-type strontium hexa-ferrites Sr1.8Sm0.2Co2 Ni1.50

Fe12O22.The starting materials were Fe(NO3)3·9H2O (Riedel-de Haen, 97%), Co(NO3)2·6H2O

(Merck, >99%), NiCl2. 2H2O(Merck, >99%), Sr(No3)2 (Merck, 99%), Sm(NO3)2 (Merck, 99%), (

cetyltrimethyl ammonium bromide) CTAB (Merck, 97%) as a surfactant, NH3 (Fisher Scientific,

35%) as a precipitating agent and methanol (Merck, 99%) as washing agent.

3.4.2 Synthesis Procedure The metallic salt solution of the required molarities were prepared in deionized water and mixed

in a beaker. The CTAB was also added in metals solutions with ratio 1: 1.5 (metals: CTAB). The

solution was stirred on the magnetic hot plate at 60C until it formed a clear solution. The

ammonia solution was added dropwise to form the precipitates. After that the precipitates were

washed with deionized water and finally with methanol. Then the precipitates were dried in an

oven at 150°C and finally annealed at 1050°C for 8 h using box furnace (Heyaius, D – 6450

Hanau, Germany). Pure PST Aldrich (commercially availible) and ferrite Sr1.8Sm0.2Co2 Ni1.50

Fe12O22 were dissolved in toluene, the mixture stirred for 48 h. The contents of ferrite in the PST

matrix were 0.25: 1, 0.50:1, 0.75 and 1:1 respectively. The ferrite filler was used in PST mixture

was poured in a leveled glass Petri dish. The film thickness is optically uniform.

3.5 Preparation of Co2Sr2Fe12O22 with Ppy-DBSA

Composite Pyrrole was obtained from the Aldrich chemical and vacuum-distilled before use.

Ammonium persulfate (APS) was obtained from Fluka Ltd and hydroquinone and

poly(methyl methacrylate) supplied by Aldrich, were used as received.

3.5.1 Synthesis of PPY-DBSA 0.15 mol of DBSA was dispersed in 100 ml of distilled water, 0.3 mol of pyrrole was added to

the mixture, and the solution was kept on magnetic stirrer after 3 h, 0.15 mol of the oxidant

ammonium per sulphate (APS) dissolved in 200 ml distilled water was added drop wise under

vigorous stirring. Then after 24 h 1 liter of methanol was added in the solution and the reaction


39

mixture was kept at room temperature during 2 days. After that the suspension was filtered and

washed. The black paste of doped polypyrrole was obtained and was dried under vacuum at 90°C

for 24 h.

3.5.2 Synthesis of Ferrite. The chemicals used in the synthesis of Y-type strontium hexa-ferrites were Fe(NO3)3·9H2O

(Riedel-de Haen, 97%), Co(NO3)2·6H2O (Merck, >99%), Sr(NO3)2 (Merck, 99%), CTAB

(Merck, 97%) as a surfactant, NH3 (Fisher Scientific, 35%)as a precipitating agent and methanol

(Merck, 99%) as washing agent. The strontium hexa-ferrite sample with nominal composition

Sr2CO2Fe12O22 were prepared by co- precipitation assisted by surfactant (microemulsion). The

metallic salt solution of the of the required molarities were prepared in distilled water and mixed

in a beaker. The solution was stirred on the magnetic hot plate at 60C until it formed a clear

solution. After that the precipitates were washed with deionized water and finally with methanol.

Then the precipitates were dried in an oven at 150°C and finally annealed at 1000°C for 8 h.

3.5.3 Ferrites-Polymer Composite. The experimental preparation of composite involved mixing the doped PPY-DBSA with

Sr2Co2Fe12O22 by1:1 molar ratio. Thorough grinding was carried out by an agate mortar and

pestle. After thoroughly grinding, powder of synthesized materials were pressed into the disk

shaped pellets with the diameter 8 mm to 9 mm and thickness ranging from 3 mm to 4 mm under

the load of 25 KN by using the paul otto Weber hydralic press.

3.6 Characterization Techniques The following experimental techniques were employed to characterize the synthesized samples:

(1) X-ray Diffractrometer modal Shimadzu 5A, equipped with copper Kα radiation source was

used.

(2) Scanning Electron Microscopy

(3) EDX

(4) Resistivity measurnment

(5) Dielectric Properties

(6) Vibrating Sample Magnetometery (VSM) (Lake Shore; 7407, USA)


40

3.7 X-ray Diffraction X-ray diffraction is an excellent technique for the qualitative and quantitative investigation of the

materials, and supports to find out the crystal and structure crystallographic informations such as

Miller indices hkl, Lattice parameters, d-spacing, crystalline sizes, X-ray density, and porosity.

The production of X-rays is mainly due to the deceleration of charged particle of appropriate

kinetic energy. Commonly for the production of X-rays, electrons are used in an evacuated tube

where two electrodes along with source of electron. One of the electrodes is called anode or

target and the potential difference between the electrodes is causes to attract the electrons from

the source toward the target. Characteristic X-rays are produced due to the inner shell transition

[1]

3.7.1 Principle of X-rays Diffraction Fig.3.1 shows the basic principle of X-rays diffraction. As the crystal is exposed in front of

monochromatic beam of X-rays, the every individual electron re-radiate some of its energy

inform of spherical wave. If the electrons are settled symmetrically having a spacing d, then

these spherical wave will combine constructively only in condition where their path-length

difference equals an integral multiple of the wavelength ‘λ’ and this is recognized as Bragg’s

law.

nSind hkl 2 (3.1)

Fig.3. 1: Schematic of X-ray diffraction as per Braggs law


41

Fig.3. 2: Geometrical representation of the constructive interference[2]

As dhkl, θ, n and λ are the inter planer spacing, Braggs angle, an integer and wavelength of the X-

ray beam respectively. This equation must be fulfilled in case of constructive interference.

Geometrical illustration of the constructive interference is depicted in the Fig. 3.2. The

Schematic diagram of X-Ray Diffractrometer is shown in the Fig. 3.3.

3.7.2 Diffraction Methods Diffraction of X-rays occurs when the Bragg’s law is satisfied. There are three methods for X-

ray diffraction;

• Laue method

• Rotating Crystal method

• Powder technique

Laue method was the very first technique used for diffraction. In this particular technique a

single crystal is used at fixed Bragg’s angle for every individual set of atomic planes. Every set

of planes diffract a specific wavelength that fulfills the Bragg’s law, so that diffracted beam has

different wavelengths. In the rotating crystal method the single crystal is mounted with one of its

central crystallographic direction and cylindrical detector is placed around the axis of rotation.

Powder method is most versatile and widely used technique for X-ray diffraction of

polycrystalline crystalline materials. The solid is generally taken in fine powder form and

samples is placed on a sample holder on the rotating stage by exposing in fort a monochromatic


42

beam of X-rays[1]. The diffraction is shown by every single set of atomic planes by considering

every individual particle as a single crystal but these planes oriented in the different directions.

The crystallite size is determined by using the Full Width Half Maximum (FWHM) of the

diffracted peak from particular plane.

Fig.3. 3: Schematic diagram of X-Ray Diffractrometer [3].

Phase purity of the prepared hexaferrite powders was confirmed by recording their X-ray

diffraction patterns using an X-ray diffractometer (JDX-60PW JEOL Boston model) which

employs Cu-Kα as radiation source. The patterns were recorded in the range 20–70° with a scan

step of 0.02°. Tube voltage for XRD measurement was kept 40kV. Unit cell volume (Vcell) of

the hexagonal unit cell was calculated as follows [4];

Vcell = 0.866a2c (3.2)

Bulk density of the samples was calculated by the relation [5];

db=m/πr2h (3.3)

Where m, r and h are the mass, radius and thickness of the pellets, respectively.


43

The X-ray density, dX was calculated using the following relation [1];

dX = 3M/NAVcell (3.4)

Where, numeric factor denotes the number of formula unit in a unit cell, M is the molar mass, NA

is Avogadro number and Vcell is unit cell volume.

The porosity (P) of the samples were calculated as follows [6];

P = 1 – db/dx (3.5)

Crystallite size (D) is calculated by using Scherrer formula [1];

D = kλ/β cosθB (3.6)

Where λ is the X-ray wavelength and of Cukα 1.5418Å, β is the half peak width, θB is the Bragg

angle and k is the shape factor which is equal to 0.89 for hexagonal system.

3.8 Scanning Electron Microscopy (SEM) Scanning electron microscopy (SEM) is extensively used for high resolution imaging of

the surfaces of the samples. Electron microscopy provides a systematic approach to explore the

objects that are too problematic to examine by optical microscopes. At the start the compound

microscopes were used to inspect the different objects and their various characteristics. Latter on

that were replaced by transmission light microscopes. Subsequently due to the shorter

wavelength electromagnetic and versatile Capability of advanced spatial resolution X-ray

microscopes were used. Some serious issues related to the resolution of the imaging were

properly solved with the discovery of wave nature of electrons. Owing to the wave nature,

electrons can move into solid structure and can be diffracted by atomic planes. Due to the shorter

wavelength, electron can enter in the material up to few microns. Suitable modifications of

magnetic and electric fields are responsible for proper focusing of electrons on the specimen [7].

In Scanning Electron Microscopy, imaging achieved by the employing the electrons is fairly

analogous to the light microscope where visible light used. Still, SEM is excessive beneficial as


44

compared with light microscope. Because Scanning Electron Microscope (SEM) has greater

magnification (>100,000X) and very high depth of field up to 100 times than that of ordinary

light microscopy.

3.8.1. Working Principle In Scanning Electron Microscope (SEM), electrons are ejected from the surface of the sample as

the beam incident on the surface of the sample. These ejected electrons are recognized as

secondary electrons. More interestingly, these low energy ejected electrons (secondary electrons)

have a lesser mean free path. Consequently, the information is coming from a penetration depth

of nearly 10 nm. An image establishes by assembling secondary electrons from every point of

the sample.

Fig.3. 4: Schematic diagram of Scanning Electron Microscopy (SEM) [9].

The SEM study was established in New Jersey (at the RCA laboratories). A LaB6 or tungsten

filament can be employed as the source of electron for the SEM. The magnetic lenses (axially

symmetric) are used for the production of magnetic field. The electron probe having diameter

less than ~10 nm is characteristically employed. The electron beam is scanned in x & y plane


45

horizontally across the sample (specimen). The formation of the electron due to the inelastic

scattering are known as secondary electrons (SE) and while due to the elastic scattering are

known as back scattered electrons (BSE). SE images are naturally employed to explore the

surface properties of the specimens [8]. Composition analysis of both types of scattering suggest

that results obtained by employing the back scattered electrons (BSE) are scientifically more

reliable, as the electron penetrate very deep inside the specimen. The intensity of these

backscatter electrons improved with the rise of atomic number and therefore signal comprises

information about the chemical composition. Fig.3.4 reflect the Schematic diagram of Scanning

Electron Microscope (SEM)

3.9 Energy Dispersive X-ray Fluorescence Spectrometer

(ED-XRF) Energy Dispersive X-ray Spectroscopy is quite versatile and nondestructive methodology

with extraordinary speed to determine the elemental composition of investigated samples.

Energy Dispersive X-ray Spectroscopy provides both quantitative and qualitative investigation of

various types of elements in a widespread range of concentration. In EDX analyzer, the

generated typical X-rays are directed toward a semiconductor X-ray detector. The energy levels

in the detector are separated, in this way elements in the sample can be investigated

instantaneously. EDX analyzer is quite easy to control and can be employed as a multi-element

analyzer. The analyzer has versatile ability of to explore components in liquids, solids and

powders form at extraordinary speed and without destroying the sample. More importantly, it

needs no standard sample. It comprises of a vacuum pump, a data processing unit, an analyzing

unit, and a computer. The analyzer unit contains a power supply used for X-ray tube (B) and a

extraordinary accuracy current and voltage control circuit used for the power supply that provide

the X-rays radiations which are properly controlled by X-ray controller (A) to the specimen. The

data processing unit contained a extraordinary rate pulse processing circuit, which control the

detector (C) signals. The Window operating system is preinstalled in the computer (D) along

with some sort of software which affords very easy approach to spectrum analysis, spontaneous

quantitative and qualitative study of the sample. The block diagram of the EDX is set in Fig.3.5.


46

Fig.3. 5: Block diagram for energy dispersive X-ray fluorescence (EDX) [10].

3.10 DC Electrical Resistivity Cobalt based Y-type hexaferrites are extensively used in electronic industry owing to the

extraordinary intrinsic resistivity of the hexaferrites. It will not be out of place to recall the fact

that DC resistivity is chief property for the dielectric magnetic materials that founds application

in the devices which work at higher frequency. For proper functioning of these devices low loss

magnetic material is prerequisite having higher values of DC resistivity. Basically, DC resistivity

is the converse capability of magnetic materials to conduct charges and is predominantly

dependent upon the temperature and microstructure of the magnetic material. In case of metallic

compounds this particular capability rises with temperature increase. DC resistance depends on

Ohm’s law (V=I/R), where R is the resistance and hence resistivity may be calculated.

Two probe method was employed to measure the Dc resistivity because of the high resistivity of

these ferrites. Before starting the measurements, both sides of the samples were polished in order

to eliminate scratches from the surfaces. Furthermore polishing of the samples also play a very

vital role to eradicate oxide layer formed during quenching process. A schematic diagram of

sample holder is shown in the Fig. 3.6(a). It consists of two pressure electrodes made of copper.

A Keithly source meter model-197 was used for the said purpose. A Keithly source meter was

connected in series with a sample holder as presented in Fig.3.6 (b).


47

Fig.3. 6: (a) Sample Holder For Resistivity measurements (b) apparatus for Resistivity

Measurement by two probe method.


48

The voltage was varied and the equivalent value of current was noted. By using current voltage

plot the values of Dc resistances for each sample were calculated. By applying the equation

=RA/t [11] dc resistivity was calculated. where R is resistance, A is contact area, and t is

thickness of sample. For Y-type hexaferrites the DC resistivity drops with rise in temperature

satisfying the Arrhenius equation [12], hence showing the semiconducting behavior.

ρ = ρ0 exp ΔE/KBT (3.7)

Where “ρ0” is room temperature resistivity, KB Boltzmann constant and ΔE is the activation

energy attained from the slope of the linear plot between “ρ” (resistivity) and 1/T.

Drift mobility, µd of all the samples were calculated using the relation.

µd = σDC /ne (3.8)

Where “e” is the charge of electron, “σDC” is conductivity and “n” is the concentration of charge

carriers calculated from the well-known equation;

n = NAdbPFe/M ……………………………………… (3.9)

where NA is the Avogadro’s number, db is the measured bulk density of sample, PFe is the

number of iron atoms in the chemical formula of the ferrites and M is the molecular weight of

the samples.

3.11 AC Response Frequency dependant electrical measurement helps us to understand comprehensive

understanding of conduction mechanism [13, 14]. The AC measurement are listed below,

• Complex dielectric permittivity (ɛ*)

• Complex impedance (Z*)

• Complex electric modulus (M*)

3.11.1 Complex Dielectric Permittivity By applying voltage to the samples of ferrite material the amount of electric energy stored in the

ferrite material comparative to that stored in the vacuum is called permittivity. Permittivity is a


49

complex function. The Complex permittivity comprise of two parts, real parts of permittivity έ(f)

and imaginary parts of permittivity ɛ″(f). Both these part of permittivity represented as and

respectively and it can be represented as can be expressed follows;

ε∗ = έ − ϳε″ (3.10)

Real part of the permittivity is termed as dielectric constant έ can be defined as;

έ = cd/ Aεo (3.11)

Where C, d, A and ɛ0 is the capacitance, thickness, the cross sectional area and the permittivity

of free space respectively.

The imaginary part of the permittivity is named as the dielectric loss. By applying external

applied field the dielectric material become polarized. The formation of the dipoles is mainly due

to the action of applied external field, as the charges pile up in the direction of the field. It is

obvious to mention the well-established fact that positive charges orient themself towards the

field. Whereas, negative charges orient themself opposite to the applied external field. So in this

way, we can conclude that energy is stored during polarization process. There are various type of

polarizations experienced by the electric charges depending on the strength of the applied

external field like orientation, interfacial, ionic, and electronic polarization. Dielectric

polarizability (α) and dielectric constant can be expressed follows [15];

χe =Nα/εo (3.12)

Where as the electrical susceptibility (χe) associated to the dielectric constant as χe= ɛr-1. The

dielectric polarization is mainly due to the displacement of the electric charges like ions dipoles

and electron clouds. As the applied external AC field is increased the dipoles reorient in the

direction of the field within no time. Moreover, the dipoles align quickly back and forth with the

oscillating field as presented in Fig. 3.7. Consequently polarization becomes significant at radio

frequencies (<1010 Hz lower frequencies). All type of polarizations occur in the ceramic

materials but as the frequency enhances above the 1010 Hz (microwave region) dipoles unable to

pile up with the external AC field and polarization becomes constant. At the higher frequency

region ~1013Hz (infrared and far infrared) ionic polarization unable to respond [16]. In case of

ultraviolet frequency region electronic polarization becomes constant as depicted in Fig. 3.8. The


50

permittivity of dielectric decreases with enhances in the frequency and such phenomenon is

known as dielectric dispersion or dielectric relaxation. The dielectric dispersion of cobalt Y-type

hexaferrites and polymer composites have been examined in the frequency range of 106 Hz to

3×109 MHz by employing the Agilent impedance analyzer model E4991ARF.

Fig.3. 7: Types of polarization on the application of AC field [17].

3.11.2 Interfacial, Space Charge or Maxwell-Wagner type of polarization

Microstructure of dielectric materials is key feature that effects relaxation behavior and the

dielectric dispersion. Hindrance in the mobility of charges in the dielectric material due to the

existence of interfaces such as grain boundaries, charges at that moment reside in a double layer

establishing a capacitor at the interfaces. The net polarizations completely govern by the


51

movement of charges at the interfaces and termed as interfacial or space charge polarization of

Maxwell-Wagner type [18] This type polarization conduct is quite diverse from other types of

polarizations and is vital for elucidating the dielectric dispersion of Y-type hexaferrites

established on the jumping or hopping of electric charges in materials.

Fig.3. 8: Typical behavior of dielectric dispersion in different frequency regions [17].

3.11.3 Dielectric Losses As real part (έ) of dielectric permitivity indicate its ability to store energy, imaginary part (ɛ″) of

the dielectric permittivity indicates the losses. Dielectric losses in the dielectric material are

exactly comparable to the friction loss. Once the mobile charges align themselves on the

application of AC external electric field the resistance faced by dipoles is related with loss of

energy. Dielectric losses with varying the applied external AC field have been presented in Fig.

3.9. It is desired to minimize the dielectric losses for the applications of these materials in


52

electronic devices particularly at micro and nano level. Still extraordinary dielectric losses might

be mandatory in the some applications where heat generation is compulsory just like in the

microwave oven [19]. The alteration of external AC field and polarization of the dielectric

materials may be responsible for the variation in dielectric losses.

Fig.3.9: Real and Imaginary parts of dielectric permittivity with frequency for a pure dielectric

material [17].

For the current research scheme the goal was to attain a low dielectric loss ceramic material

having extraordinary resistivity and small dielectric constant especially in case of ferrite. Similar

to the permittivity, dielectric losses may also be influenced by the frequency of the AC external

field as depicted in the Fig. 3.9. It has been anticipated in the figure that the peaks in the

dielectric losses typically characterize the dielectric relaxation phenomena where the frequency

of dipoles becomes equal with the oscillation frequencies. A peak originates as an outcome of

resonance where ω=1/τ. And τ is the relaxation time, time needed for the dipoles to return and

readjust with the variation in the magnitude of AC electric field. The relaxation phenomenon is


53

governed by Debye equation. When ω>>1/τ dielectric losses (ɛ″) is small however and if ω<<1/τ

polarization phenomena can obey the AC field alteration.

3.11.4 Dielectric Loss Tangent Dielectric loss tangent (tanδ) is the ratio of imaginary and real parts of the permittivity and it can

written as;

tanδ = ɛ″/έ (3.13)

When a resistor is connected to voltage the sinusoidal voltage is written as;

V=V0exp (jωt) (3.14)

exp(ωt) is a complex function can be written as

exp (jωt) = cosωt + jsinωt (3.15)

The relation between the AC voltage, capacitor C and charge Q is given by;

Q = CV (3.16)

Q = CV0exp (jωt) (3.17)

This accumulation of charges can be quantified by a charging current Ic and written as;

Ic = dQ/dt = CdV/dt (3.18)

Ic = jCV0ωexp(jωt) = jωCV (3.19)

For an perfect capacitor the voltage always lags behind the charging current by exactly 900. The

current and voltage shifts in case of dielectric material can be properly explained with suitable

combination of the resistor and capacitor. Consequently whole current is sum of the loss current

and charging current. Ic every time leads the voltage (V) by 900 where as loss current is mainly

due to the energy dissipation in the dielectric material when polarization occure and is in phase

with applied voltage (V) [20]. Fig. 3.8 (b) illustrate the phenomena

Itotal= Ic+ IL (3.20)

Itotal=jCωV+GV (3.21)


54

tanδ=IL/Ic (3.22)

tanδ = ωV C0ɛr″/ωV C0έr = ɛr″/έr (3.23)

Therefore entire current is a vector sum of two different currents and permanently leads by the

voltage not by 900 however at an angle 900-δ. Whereas δ is the loss angle depicted in Fig. 3. 10.

Fig.3. 20: Dielectric loss tangent (Ic and IR)[20].

3.11.5 AC Conductivity Typically AC conductivity can be written as,

σAC = σ0(T) + σ (ω, T) (3.24 )

AC conductivity is a sum of frequency independent part σ0(T) known as DC conductivity and

frequency dependent part σ (ω, T) known as AC conductivity owing to hopping of electrons at

hexagonal octahedral site. AC conductivity can be written as;

σAC=Aω (3.25)

where A carrying the unit of conductivity and n is the slope of the graph (lnσAC) vs lnω. The AC

conductivity can be formulated by the following formula

σAC =2πԐ0f ɛ″ (3.26)


55

3.11.6 Complex Iimpedance (Z*) Impedance (Z) is basically opposition in the movement of charges through a circuit on the

application of applied external AC electric field. It’s a totally complex function having

magnitude and phase angle [21]. The magnitude of impedance Z is a proportion of voltage

and current and is generally characterized in the polar coordinates to explain the phase angle as

depicted in the Fig. 3.11.

Z=V/I (3.27)

Z*= Z/-jZ// (3.28)

Z=R+jX (3.29)

where as

R=Zcosθ and X= Zsinθ (3.30)

Fig.3. 31: Real and Imaginary parts of absolute Impedance Z| [21].


56

By connecting the resistor and capacitor parallel total impedance for dielectric is given by

(3.31)

(3.32)

(3.33)

A cole-cole or Nyquist diagram can be achieved by plotting the real and imaginary parts of

impedance. The conduction phenomena can be properly explained with the help of the cole-cole

diagram. The contribution of defects and interfaces exist in the dielectric material can easily be

elucidated with help of the of impedance spectroscopy. Also the cole-cole plots can offer the

magnitude of capacitance grain and grain boundary resistance [22].

3.11.7. Complex Electric Modulus (M*) Electric modulus is additional technique to elucidate the electrical conduction phenomena in the

dielectric medium. It is customarily believed that AC data would be determined either in

impedance plane plot or with the help of electric modulus. If the variation in relaxation process is

mainly due to the variation in capacitance, then complex impedance (cole-cole) is quite useful

however if it is owing to the change in resistance the data will be then determined by modulus

plane plots [23]. In our present experimental case the relaxation phenomena is suitably

elucidated with the help of electric modulus. The electric modulus is frequently denoted as a

function of frequency [24]. The real and imaginary parts of the electrical modulus, M΄ and

M΄΄respectively may can be calculated as fallows [25]:

M*= i/ ε* =M΄( ω) +ᵢ M΄΄( ω) (3.34)

M΄ = ε΄/( ε΄)2 +( ε΄΄)2 (3.35)

M΄΄ = ε΄΄/( ε΄)2 +( ε΄΄)2 (3.36)

Where ε΄ and ε΄΄are real and complex parts of permittivity respectively.


57

3.12 Measurement of Magnetic Properties by VSM A common technique for evaluating the magnetic properties of an wide variety of magnetic

systems is “Vibrating Sample Magnetometer" (VSM). With this technique, the magnetic moment

of a magnetic sample can be measured with a high accuracy. The Faraday's Law of

electromagnetic induction is considered as basic unit for proper functioning of vibrating sample

magnetometer (VSM) [4]. According to this law the changing magnetic field will give the

electric field. The detailed information about the changing magnetic field can be perceived by

properly measuring this electric field. So the magnetic performance of magnetic materials can

easily be determined by employing a VSM. When a sample of any magnetic material is

positioned within a magnetic field which is produced among the poles of an electromagnet,

consequently a dipole moment will be induced. If the sample initiates vibrating with sinusoidal

motion, induced sinusoidal electrical signals are produced with in the pick-up coils. The

detection coil is presented in Fig. 3.12. More importantly, it is essential to recall the fact that

induced signals have the vibrational frequency and amplitude which is proportional to that of

magnetic moment frequency and amplitude.

Fig.3. 42: Schematic of Vibrating Sample Magnetometer [18].


58

The diagram of vibrating sample magnetometer (VSM) is depicted in Fig. 3.13. The sample is

positioned to a sample holder located at the end of a sample rod mounted in an electromechanical

transducer. The transducer is driven by a power amplifier which itself is operated by an oscillator

at a frequency of 90 Hz. So, the sample vibrates along the Z axis perpendicular to the

magnetizing field. The latter induces a signal in the pick-up coil system that is fed to a

differential amplifier.

Fig.3. 13: Schematic diagram of Vibrating Sample Magnetometer.

The output of the differential amplifier is consequently connected to a tuned amplifier and an

internal lock-in amplifier that receives a reference signal supplied by the oscillator. The output of

this lock-in amplifier, or the output of the magnetometer itself, is a DC signal proportional to the


59

magnetic moment of the sample being studied. The electromechanical transducer can move

along X, Y and Z directions in order to find the saddle point. Calibration of the vibrating sample

magnetometer is done by measuring the signal of a pure Ni standard, of known saturation

magnetic moment, placed in the saddle point. The output measurement exhibits the magnetic

moment M as a function of the magnetic field H.

Magnetic parameter like saturation magnetization (Ms), coercivity (Hc) and remanence (Mr) for

studied samples were determined from the M-H loop at 300K starting from zero applied

magnetic field. These M-H loops were taken by using VSM provided by Lake Shore’s new 7400

series [5].

Lakeshore vibrating sample magnetometer (VSM) can determine the magnetic

moments of magnetic samples from 4.2K to 1273K, this particular equipment offers field to

above 3T, moreover provide a stability of 0.05% per day. Extraordinary attention is focused to

achieve constant amplitude of the oscillations over the time interval. The magnetometer (VSM)

is calibrated with nickel (Ni) with known saturation magnetization (Ms). A Hall Probe was

employed to determine the values of the applied field. By means of the computer interface, the

M-H loops were recorded on XY- plotter coupled with Lock-in amplifier of the investigated

samples by applying the maximum applied field of ( 8000G). To estimate the magnetic

properties like saturation magnetization (Ms), coercivity (Hc) and remanance (Mr) for each

investigated sample, the attained values are in milli volts (mV) from the loops. The obtained

values were then calibrated in electromagnetic units (emu) by comparing a Ni standard. The

obtained values were divided by volume or mass of the corresponding magnetic sample to

achieve the values in emu/cc or emu/g.

The MH loops were measured at room temperature using a vibrating sample magnetometer

(VSM) model Lake Shore, new 7400 series, USA. By using the law of approach to saturation,

the values of saturation magnetization (MS) were deliberated from the loops [26] and calculated

by above mentioned law at the room temperature. The magnetization M(H) is replaced by the

specific name polarization in polycrystalline sample, in order to apply this law. The data were

fitted using a least squares procedure by the following law of approach to saturation [27, 28].

M = Ms(1- A/H - B/H2) + χH (3.37)


60

Where H is the applied field, Ms is saturation magnetization supposing that the Brillion function

is equal to unity, A is in homogeneity parameter, B factor which is proportional to K2 (K is the

anisotropy constant and χ is the susceptibility. The magnetic moment (nB) is calculated using the

following formula

Magnetic moment (nB) = molecular weight ×saturation magnetization/5585 (3.38)


61

References 1. B. D. Cullity, Element of x-ray diffraction, 2nd Ed., Addison- Wesley publishing

company, Inc, (2009).

2. West, A. R. Solid State Chemistry and its Applications, John Wiley & Sons: Singapore,

1989.

3. B. D. Cullity, Element of x-ray diffraction, Addison- Wesley publishing company, Inc,

(1956).

4. B. D. Cullity Element of X-Ray Diffraction 2nd edition pp 88, 284, 502.

5. M. J. Iqbal, R. A. Khan, J. Alloys and Compounds 374(2009)286-289.

6. T.W. Lambe, Soil testing for engineers: John Wiley and sons, Inc., New York, (1951)165.

7. O. G. Wells, Scanning Electron Microscopy (McGraw-Hill, New York, 1974).

8. Ray F. Egerton, Physical Principles of Electron Microscopy, Springer Science+Business

Media, Inc (2005).

9. E. Suzuki, Journal of Microscopy. 208, 3(2002)153.

10. Cowell, M. R. Coin Analysis by Energy Dispersive X-ray Florescence Spectrometery,

Royal Numismatic Society: London, 1998.

11. S.O. Kasp, Principles of Electronic Materials and Devices. 2002. New York: McGraw

Hill.

12. R.V. Magalaraja, S. Ananthakumar, P. Manohar, F.D. Gnanam, J. Magn. Magn. Mater.

253 (2002) 56.

13. P.A. Shaikh, R.C. Kambale, A.V. Rao, Y.D. Kolekar, J. Alloy. Compd. 482

(2009) 276.

14. R. Dhanaraju , M.K. Raju , V. Brahmajirao , S. Bangarraju, Int. J. of Sci. and

Tech. 1. 5 (2012) 275.

15. M.L. Minges, Elecronic Materials Handbook. Vol. 1 Electronic Package.

Materials Park, OH: ASM. (1989).

16. J. Singh, Semiconductor Devices: basic Principles. New York: Wiley (2001).

17. G.G. Raju, Dielectrics in Electric Field. Boca Raton, FL: CRC Press (2003).

18. R.M. Mohamed, M.M. Rashad, F.A. Haraz , W. Sigmund, J .Magn. Magn. Mater. 322

(2010) 2058.

19. M. Pardavi-Horvath, Journal of Magnetism and Magnetic Materials, 215 (2000)171.


62

20. Chiang, Y. et al.: Physical Ceramics, John Wiley & Sons 1997, New York.

21. Alexander, Charles; Sadiku, Matthew (2006). Fundamentals of Electric Circuits (3,

revised ed.). McGraw-Hill. pp. 387.

22. M.G. Chourashiya, J.Y. Patil, S.H. Pawar, L.D. Jadhav, Mater. Chem. Phys. 109

(2008) 39.

23. Asma B. Afzal, M. Javed Akhtar, M. Nadeem, M.M. Hassan, J. Current Applied Phys 10

(2010) 601.

24. J. Liu, Chun-Gang Duan, Wei-Guo Yin, W.N. Mei, R.W. Smith, J.R. Hardy, J.

Chem. Phys. 119 (2003) 2812.

25. A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectric Press, London, 1983.

26. S.E. Shirsath, B.G.Toksha, K. M. Jadhav, Mater. Chem. Phys. 117 (2009) 163-168.

27. M. J. Iqbal, R. A. Khan, J. Alloys Comp. 478 (2009) 847–852.

28. A critical examination of the law of approach to saturation, R. Grossinger, phys. Stat.

sol.(a) 66, 665 (1981).

CHAPTER 4 RESULTS AND DISCUSSION

63

4. RESULTS AND DISCUSSION

4.1 Tb-Mn Substituted Y-type Hexaferrite 4.1.1 Structural Analysis.

Fig.4.1 presents powder X-ray diffraction patterns of the Tb –Mn substituted Sr2Co2 Fe12O22

sample at room temperature. All the diffraction peaks were compared with standard patterns of

Y-type hexa ferrite [card 00-019-019-0100] and corresponding Miller Indices (h k l) were

assigned to the each peak. The results show single phase formation of the Y-type hexaferrite.

20 25 30 35 40 45 50 55 60 65 70

11 0

1 0

13

1 1

9

1 1

12

1 0

19 0 0

27

2 1

19

0 0

1

8

2 0 200 1

1

4

x= 0.0, y= 0.0

x= 0.2, y= 0.02

x= 0 .4, y= 0.04

x= 0.6, y=0.06

x= 1.0, y= 0.10

x= 0.8, y= 0.08

2Ɵ (Degree)

Inte

nsi

ty (

arb

. unit

s)

Fig.4. 2: XRD analysis of Tb-Mn substituted hexaferrites, Sr2Co(2-x)MnxTbyFe(12-y)O22, (x =

0.00–1.00; y = 0.00–0.10).

Replacement of the Co and Fe ions with the doped Tb and Mn ions respectively reflects a

minor deviation in the lattice parameter “a” varies from 5.88 to 5.94 A˚, while the parameter “c”

increases from 43.37 to 43.45A˚ as shown in the Fig. 4. 2 which is attributed to the difference in


64

the radii of the substituted and the host ions. As the elements of the smaller ionic radius Co2+

(0.745A˚) and Fe3+ (0.64A˚) were replaced by element of larger ionic radius Mn2+(0.80A˚) and

Tb3+ (0.923A˚) which consequently enhanced the lattice parameters “a” and “c”. This is in

agreement with the observation by other researchers [1, 2].

0.0 0.2 0.4 0.6 0.8 1.0

0.00 0.02 0.04 0.06 0.08 0.10

5.88

5.89

5.90

5.91

5.92

5.93

5.94

B

C

Tb. Contents

a (Å)

43.36

43.38

43.40

43.42

43.44

43.46

C(Å

)

Mn. contents

Fig.4. 3: Variation of lattice parameters for Tb-Mn substituted hexa ferrites,Sr2Co(2-

x)MnxTbyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

The deviation in the lattice parameters indicates that Tb and Mn ions entirely dissolved in the Sr–

Co–Y crystal lattice. Vcell increases gradually by increasing the dopant concentration in the

crystal lattice and values are listed in Table 4.1.

The calculated values of bulk density decreased with substitution of (Tb-Mn) in Sr2Co2

Fe12O22 ferrites as listed in Table 4. 1 and is accredited to the difference in the density of the

substituted and the host ions. It is expected that the effect of smaller room temperature density of

the doped Mn2+ (7.21 g·cm−3) than that of the host Co2+ (8.90 g·cm−3) surpasses the negative

effect of Tb3+ (8.23 g·cm−3) substitution for Fe3+(7.874 g·cm−3). The X-ray density, dX is listed in

the Table 4. 1.Very small decreases of X-ray density (dx) from 5.05 to 4.96 g/cm3 with the (Tb-

Mn) substitution has been observed which is mainly due to small increase in the cell volume of a

http://en.wikipedia.org/wiki/Density



65

particular sample, as the cell volume is inversely associated to the X-ray density. The porosity of

ceramic samples commonly results from two sources, i.e intragranular or intergranular

depending on whether pores remain within the grains or pores lie in the grain boundaries. When

the grain growth rate is very fast, pores are left behind the rapidly moving grain boundaries that

are trapped within the grains. It is evident that Tb-Mn substitution do not promotes densification,

which leads to enhance the porosity as listed in the Table 4. 1. Crystallite size(D) were found in

the range of 31–45 nm. Fig.4. 3 shows that Crystallite size (D) increases with (Tb-Mn)

substitution due to grain growth at high sintering temperature.

Table 4. 1: C/a, volume of cell, Bulk density, X-ray density, percentage porosity and room

temperature DC resistivity of (Tb-Mn) substituted hexaferrites, Sr2Co(2-x)MnxTbyFe(12-x)O22, (x =

0.00–1.00; y = 0.00–0.10).

Compositional Formula c/a V(Aᵒ)3 db(g/cm3) dx(g/cm3) P(%) ρ(Ω-cm)

Sr2Co2Fe12O22 7.37 1298.56

4.92

5.05 2.57

1.23×106

Sr2Co1.8Mn0.2Tb.02Fe11.98O22 7.36 1303.88

4.89

5.03 2.78

4.21×106

Sr2Co1.6Mn0.4Tb.04Fe11.96O22 7.35 1308.61

4.85

5.02 3.39

2.92×107

Sr2Co1.4Mn0.6Tb.06Fe11.94O22 7.34 1313.65

4.8

5.00 4.00

7.80×107

Sr2Co1.2Mn0.8Tb.08Fe11.92O22 7.32 1322.87 4.78

4.9

3.82 2.08×108

Sr2Co1Mn1Tb0.1Fe11.90O22 7.31 1327.64

4.75

4.96 4.23

1.51×109


66

0.0 0.2 0.4 0.6 0.8 1.0

0.00 0.02 0.04 0.06 0.08 0.10

30

32

34

36

38

40

42

44

46

Tb. Contents

Cry

stal

lite

size

(nm

)

Mn. contents

Fig.4. 4: Variation of crystalline size for Tb-Mn substituted hexaferrites, Sr2Co(2-x)MnxTbyFe(12-

y)O22, (x = 0.00–1.00; y = 0.00–0.10).

4.1.2 EDX Analysis EDX technique was employed for the determination of the composition for present examined

samples. All possible detail discussion has been presented in below section for the suitability of

strontium, instead of barium, as a group II metal in a Y-type hexaferrites. All the calculated

metallic cations of the synthesized samples are listed in Table 4. 2; it is obvious from the analysis

that stoichiometric Tb and Mn contents were enhanced while Fe3+ and Co2+ content decreased.

Fig. 4. 4 (a-f) illustrates the EDXS spectrums of present Y-type hexaferrites.

The characteristic peaks in the EDXS spectra ensure the presence of Sr, Co, Tb, Mn and Fe.

Experimental calculation of all stoichiometric contents has close agreement with theoretical

calculation. The increment in substituents and decrease in the substituted contents at systematic

rate clearly suggest that the investigated samples preserved the accurate contents stoichiometry.

Table 4. 2. Indicate that Sr2+ contents is 1.98 in the pure Sr2Co2Fe12O22 sample, this value is

close to their theoretical values of 2. However a slight variation observed in Sr contents suggests

that Sr2+ ions have slightly lower solubility in the present pure samples. This can be elucidated

by recalling the fact that the maximum solubility of Sr2+ ions in the Y-type hexaferrite is slightly


67

Table 4. 2: Comparison of the Observed and Theoretical Weight Percents and Content

Determined by EDX Analysis of the (Tb-Mn) substituted Co2Sr2Fe12O22.

Sample Name

Elements

Fe Sr Co Mn Tb

Sr2Co2Fe12O22

Theoretical 69.57 18.19 12.23 0 0

Experimental 69.49 18.01 11.99 0 0

Content 11.98 1.98 1.96 0 0

Sr2Co1.8Mn0.2 Tb 0.02Fe11.98O22

Theoretical 69.36 18.16 10.99 1.13 0.32

Experimental 69.29 18.11 10.87 1.11 0.28

Content 11.97 1.98 1.78 0.19 0.018

Sr2Co1.6Mn 0.4 Tb 0.04Fe11.96O22

Theoretical 69.15 18.14 9.76 2.27 0.65

Experimental 69.09 18.12 9.69 2.24 0.62

Content 11.94 1.99 1.58 0.39 0.039

Sr2Co1.4Mn 0.6 Tb 0.06Fe11.94O22

Theoretical 68.95 18.12 8.53 3.40 0.98

Experimental 68.88 18.09 8.46 3.35 0.94

Content 11.93 1.99 1.38 0.59 0.057

Sr2Co1.2Mn 0.8 Tb 0.08Fe11.92O22

Theoretical 68.74 18.09 7.30 4.53 1.31

Experimental 68.53 18.11 7.19 4.44 1.22

Content 11.88 2.00 1.18 0.78 0.07

Sr2Co1Mn 1.0 Tb0.1Fe11.90O22

Theoretical 68.54 18.07 6.07 5.66 1.63

Experimental 68.26 18.14 5.02 5.51 1.57

Content 11.85 2.01 0.82 0.97 0.096


68

Fig.4. 5: (a-f) EDX spectra for Tb-Mn substituted Co2Sr2Fe12O22.


69

lower , due to the smaller size of Sr2+ (0.127 nm) as compared with Ba2+ (0.143 nm), However,

the occupation of the Sr2+ in the hexagonal lattice can be increased to 2.0 (theoretical) as shown

in Table 4.2. EDX studies is confirmatory study for the elemental analysis which suggest that

the presence of dopants (Mn2+ and Tb3+) is actually responsible for the incorporation of Sr2+ ions

in the hexagonal lattice by creating more space inside the T block of Y-type hexaferrites. As

Sr2+ ions occupy positions in the basic oxygen layers of the T block. The better approximation of

the Sr–Y formula is achieved at higher dopant level.

4.1.3 Scanning Electron Microscopy (SEM) To elucidate the surface morphology of the present investigated ferrite samples Sr2Co2-

xMnx TbyFe12-yO22 (x = 0.0 – 1, Y =0.0 – 0.1), Scanning Electron Microscope (SEM) was

employed. Fig. 4. 5(a–f) elucidates the SEM images of all the ferrite samples. The calculated

values of grain size are listed in the Table 4. 4. Very careful observation of SEM images suggest

that grains are almost well packed, crack free but grain size distribution is not uniform. The grain

morphology appears plate-like which is in accordance with reported work by many investigators

[3, 4]. For microwave absorbing purposes this typical shape is very favorable[5]. The grains

grow larger in size with increasing concentration of Tb-Mn. A close examination of these

micrographs clearly indicates that intercrystalline porosities slightly enhanced and grain

boundaries becomes almost sharp which subsequently result in increasing grain size. However

some particles agglomerates were observed. Chemical reaction during the sintering process play

very crucial role for the formation of these agglomerates. It is worth noting to mention the fact

that relatively weak Van der Waals bonds and Magnetic forces play vital role to hold together

these agglomerates [6]. Under the action of the persistent forces the uniqueness of the

agglomerates can be preserved. Average grain size calculated from the SEM images enhanced

with increasing dopant level.


70

Fig.4. 6: (a-f) SEM images for Tb-Mn substituted Co2Sr2Fe12O22.


71

4.1.4 Electrical Properties

4.1.4.1 DC Resistivity

Table 4.1. Shows that room temperature resistivity increases from a value 1.23×106 to 1.51×109

(ῼ cm) with increasing (Tb-Mn) concentration. As Tb ions prefer to occupy octahedral sites

followed by the migration of some Fe3+ ions to tetrahedral sites and converting them into Fe2+

ions in order to preserve overall electrical neutrality. As a result Fe3+ ions concentration is

lowered at octahedral sites. Conduction in ferrites is accountable due to electron transfer from

Fe2+ to Fe3+ at these sites [7]. All these factors would limit the hopping probability between Fe3+

and Fe2+ ions thereby enhancing the resistivity.

Moreover, the increase in resistivity may also be due to the fact that Tb and Mn are more

resistive (1.150x10-6 ohm-m and 1.44x10-6 ohm-m at 293k) than that of Fe and Co (9.71x10-8

ohm-m and 6.2 x10-8 ohm-m at 293k)[8]. These parameters make these materials useful for high

frequency applications, as a radar absorbing materials and electromagnetic interference

attenuation [7]. The temperature dependent DC electrical resistivity of Tb-Mnsubstituted Sr2Co2-

xMnxTbyFe12-yO22hexaferrites of different compositions has been measured from 293 to 673 K .

DC electrical resistivity as a function of temperature follows the Arrhenius equation.

ρ = ρ0 exp ΔE/KBT (4.1)

The resistivity decreases with increasing temperature, as shown in Fig. 4. 6, showing the typical

semiconducting behavior for the present ferrite materials. It is observed that the transition

temperature has occurred which is in good agreement with the Curie temperature showing that

the kink in each case has occurred at the Curie point of the corresponding ferrite [9, 10].

Alike transitions in the locality of the Curie point have also been studied by many researchers in

different ferrite systems. It was shown theoretically that on passing through the Curie point a

change must occur in the gradient of straight line [11] and the magnitude of this effect depends

on the exchange interaction between the outer and inner electrons which alters at the Curie point

[12]. The experimental observation of the transition near the Curie point in present case is in

agreement with the theory developed by Irkin and Turov[13].

As the temperature increases, thermal motion competes with the ferromagnetic tendency

for dipoles to align. When the temperature rises beyond a certain point, called the Curie


72

temperature, there is a second-order phase transition and the system can no longer maintain a

spontaneous magnetization, although it still responds paramagnetically to an external field.

1.5 2.0 2.5 3.0 3.5

10

12

14

16

18

20

22

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Log

(o

hm-c

m)

1000/T (K-1

)

Fig.4. 7: Temperature dependent resistivity of Tb-Mn substituted hexaferrites, Sr2Co(2-

x)MnxTbyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

Fig. 4. 7, elucidate the concentration dependence of Curie temperature (Tc) for the inspected

samples. From the figure it is clear that, the values of curie temperature (Tc) decreases

successively with increasing (Tb-Mn) contents.A similar decrease of the Tc (K) with the

composition was also observed by many investigators [12, 14]. It is understood fact that Fe3+–

O – Fe3+ super exchange interactions and the Fe3+–Fe3+ direct exchange interactions are the

elementery interactions in ferrimagnetic materials[15]. Suitable explanation for the decrease

Curie temperature (Tc) can be given as, incorporation (Tb-Mn) in Co-Sr- Y- type hexaferrite,

could result in diluting magnetic moment interactions or to decrease the Fe3+–O–Fe3+ super

exchange interactions. The existence of spin canting, promoting the reduction of magnetic

moment interactions, which favors the lowering of Curie temperature (Tc).RE ions containing

samples exhibit lower Curie temperatures than those of ferrite without RE [16], This is in

tremendous resemblance with our present experimental judgments. The decline of TC may also

http://en.wikipedia.org/wiki/Phase_transition


73

explained on the basis of fact that Tb–Fe interactions on the B sites are smaller than Fe–Fe

interaction[17, 18]. Moreover, The substitution of RE on Fe3+ ions causes partial disorder and

weakens Fe3+ –O–Fe3+ super exchange interactions, where the valence of the iron ion changes

from Fe3+ with a high spin state (3d5 with 5uB) to Fe2+ with a low spin state (3d6 with 4uB)[2, 19].

0.00 0.02 0.04 0.06 0.08 0.10

420

440

460

480

500

520

540

560

0.0 0.2 0.4 0.6 0.8 1.0

Tc

(K)

Tb. Contents

Mn. contents

Fig.4.8: Variation of Curie Temperature (Tc) for Tb-Mn substituted hexaferrites, Sr2Co(2-

x)MnxTbyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

4.1.4.2 Activation Energy

Experimental data for DC electrical resistivity of Tb-Mn substitutedSr–Co Y-hexaferrites are

given in Table 4. 3. The activation energies in the ferrimagnetic region and paramagnetic regions

are calculated from the slopes of logρ versus 103/T. The activation energy increases with Tb-Mn

substitution, the variation being almost similar to the compositional variation of DC resistivity.

Since resistivity is high, therefore the probability of electron transfer is obviously lowered,

thereby increasing activation energies [20-23]. It can also be observed from the table that the


74

activation energy in the paramagnetic region is higher than that in the ferrimagnetic region.

Generally the change of slope is attributed to change in conductivity mechanism. The conduction

at lower temperature below Curie temperature is due to hopping of electrons [24] between Fe2+

and Fe3+ ions, whereas at higher temperature above Curie temperature, due to hopping of

polarons[25, 26].The calculated values of activation energy in a para region E are greater than

0.40 (eV) which clearly suggest that the conduction is due to hopping of polarons.

Table 4. 3: Slops and activation energies of ferrimagnetic and paramagnetic regions of Tb-Mn

substituted hexaferrites, Sr2Co(2-x)MnxTbyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

4.1.4.3 Drift Mobility

The variation in drift mobility with the temperature for Tb-Mn substituted Co2Sr2Fe12O22 ferrite

samples is shown in the Fig. 4. 8. These samples show a bend at a specific temperature i.e. the

drift mobility increases with the increase in temperature and above the specific temperature, the

drift mobility increases abruptly with increase in the temperature. The drift mobility of all the

synthesized samples decreases with increasing Tb-Mn concentration and listed in the Table 4. 4.

The decrease drift mobility is may be due to decrease in conductivity by doping Tb-Mn

ions. The calculated values of drift mobility for the Tb-Mn doped samples are in the range 10-12 –

Compositional Formula

Slop Activation energy FM PM

Region Region

M1 M2

FM PM

Region Region

E1 (ev) E2 (ev) ∆E=E2-E1(ev)

Sr2Co2Fe12O22 1.78 2.61 0.352 0.51 0.164 Sr2Co1.8Mn0.2Tb.02Fe11.98O22 1.80 2.67 0.356 0.52 0.172

Sr2Co1.6Mn0.4Tb.04Fe11.96O22 1.81 2.90 0.358 0.57 0.215 Sr2Co1.4Mn0.6Tb.06Fe11.94O22 1.83 2.94 0.362 0.58 0.219 Sr2Co1.2Mn0.8Tb.08Fe11.92O22 1.84 2.98 0.364 0.59 0.225 Sr2Co1Mn1Tb0.1Fe11.90O22 1.85 3.07 0.366 0.60 0.241


75

10-15 cm2v-1s-1 K-1, which are slightly lower than the reported values of 10-11–10-14 cm2v-1s-1 K-1

[1]. This behavior is to be expected as the drift mobility has a direct relation with conductivity.

These results can be clarified on the basis of the electrical conductivity data of these samples.

The initial increase in the drift mobility with increase in the temperature is due to the increase in

the electrical conductivity in the temperature range which causes to increase the mobility of the

charge carriers. The increase in drift mobility above transition temperature is due to the fact that

the electrical conductivity further increases above this temperature and as a result the mobility of

charge carrier increases rapidly.

250 300 350 400 450 500 550 600 650 700

0.0

2.0x10-11

4.0x10-11

6.0x10-11

8.0x10-11

1.0x10-10

1.2x10-10

1.4x10-10

1.6x10-10

1.8x10-10

x= 0.00 y=0.0

x=0.02 y= 0.2

x=0.04 y=0.4

x=0.06 y=0.6

x=0.08 y=0.8

x=0.10 y=1.0

Mob

ility

(cm

2 v-1s-1

)

T(K)

Fig.4. 9: Change in Drift mobility with temperature for (Tb-Mn) substituted Co2Sr2Fe12O22 hexa

ferrites.

4.1.5 Dielectric Properties Fig. 4. 9 shows the variation of the dielectric constant Vs frequency for (Tb-Mn) substituted Y-

type hexa ferrite system. It is clear from the figure that the dielectric constant initially decreases

with increase in frequency and at higher frequency it decreases slowly. With increase in

frequency the decrease in dielectric constant is a common dielectric behavior of ferrites. Alike

behavior is also studied by other researchers [1, 27, 28]. The variation of dielectric constant with

frequency reveals the dispersion due to Maxwell–Wanger type interfacial polarization, which is


76

in agreement with Koop’s phenomenological theory[29]. According to this model, the dielectric

materials with heterogeneous structure can be imagined to contain well conducting grains

separated by high resistive thin layered grain boundaries. It has been reported that the hopping is

the source of polarization mechanism in ferrites. The increase in frequency decreases

polarization and then reaches a constant value due to the fact that after a definite frequency of

external field, the electron exchange between Fe2+ and Fe3+ cannot follow the external alternating

field. So the decrease in dielectric constant with frequency is normal behavior of ferrite because

of the fact that species contributing to polarization lag behind the applied field at higher

frequencies. The large value of dielectric constant at lower frequency is due to the predominance

of species like Fe2+ ions, interfacial dislocations pile ups, oxygen vacancies, grain boundary

defects,etc [30].

Table 4. 4: Compresses the Mobility, AC conductivityof Tb-Mn substituted hexaferrites, Sr2Co(2-

x)MnxTbyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

The variation of dielectric loss and tansδ as a function of frequency for all the synthesized

samples are shown in the Figs.(4.10- 4.11) Both dielectric loss and tansδ decreases with

increasing frequency. An interesting feature was noted that the relaxation peaks are observed in

both the parameters i,e. tanδ and dielectric loss. A qualitative explanation can be given for the

peaking behavior observed in dielectric loss and tanδ versus frequency curves on the basis of the

previous supposition [31, 32]. It is believed that there exist a strong correlation between the

conduction mechanism and the dielectric polarization of ferrite [31]. When hopping frequency

becomes equal to that of external applied electric field then a resonance peak observed [33]. A

Compositional Formula Grain size (nm) μd (cm2v-1s-1) Dielectric loss

Tangent Loss σAC(Ω-cm)-1

Sr2Co2Fe12O22 73 3.3×10-12 2.31 0.14 1.3×10-4

Sr2Co1.8Mn0.2Tb.02Fe11.98O22 57 4.1×10-13 2.12 0.17 1.2×10-4

Sr2Co1.6Mn0.4Tb.04Fe11.96O22 211 1.44×10-13 1.94 0.20 1.1×10-4

Sr2Co1.4Mn0.6Tb.06Fe11.94O22 223 5.4×10-14 1.73 0.21 9.7×10-5

Sr2Co1.2Mn0.8Tb.08Fe11.92O22 192 2.1×10-14 1.45 0.19 8.1×10-5

Sr2Co1Mn1Tb0.1Fe11.90O22 226 2.8×10-15 0.72 0.13 4.1×10-5


77

similar peaking behavior was also observed by several investigators [34-36] in various ferrite

systems.

Fig. 4.12 shows the comparative variation of dielectric constant and Dc resistivity. The decrease

in dielectric constant (permittivity) with increase in Tb-Mn contents is attributed to increase in

resistivity of the samples. The change in permittivity can also be ascribed by the microstructure

(porosity, grain boundary, etc.). For the less dense sample, more porosity will reduce the

permittivity dramatically [37] which is consistent with our experimental results.

4.1.5.1 AC Conductivity

The Composition dependent AC conductivity varies from 10-4to 10-5 (Ω-cm) -1.Which is slightly

higher than the already reported values 10-6 to 10-7 (Ω-cm) -1[38]. Compositional dependent AC

conductivity is listed in the table 4.4. The decrease in AC conductivity with increasing

substitution level may be attributed to increase in impedance. It is observed that AC

conductivity increases with increasing frequency of the applied field as shown in the Fig. 4. 13.

Since the increase in frequency enhances the hopping of the charge carriers between Fe2+ and

Fe3+, the conductivity increases. This behavior of ac conductivity can be explained on the basis

of Maxwell–Wagner model and Koop’s phenomenological theory. According to which the

ferrites are imagined to act as a multilayer capacitor in which the ferrite samples are

characterized by a microstructure consisting of conducting grains separated by highly resistive

thin layers (grain boundaries). According to this model our results of ac conductivity at low

frequencies describe the grain boundary behavior, while the dispersion at high frequency may be

attributed to the conductivity of grains[27, 39].

At low frequencies, the low conductivity is clearly observable which is attributed to the

blocking effects at grain boundaries [40] and moreover appearance of the plateau appearing at

low frequencies is also due to the grain boundary contribution to the total conductivity,

comparatively high values of the AC conductivity observed at higher frequencies is due to the

bulk contribution [40].


78

14 16 18 20 22

3

4

5

6

7

8

9

10

11

12

13

14

15

16 x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Die

lect

ric

Con

stan

t (

lnf (Hz)

Fig.4. 10: Dielectric constant of Tb-Mn substituted hexaferrites, Sr2Co(2-x)MnxTbyFe(12-

y)O22,

14 16 18 20 22

0.0

0.5

1.0

1.5

2.0

2.5

Die

lect

ric

Los

s

lnf (hz)

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Fig.4. 11: Dielectric loss of Tb-Mn substituted hexaferrites, Sr2Co(2-x)MnxTbyFe(12-y)O22, (x =

0.00–1.00; y = 0.00–0.10).


79

14 16 18 20 22

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

20.9 21.0 21.1 21.2 21.3 21.4 21.5 21.6 21.7

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

diel

ectri

c lo

ss fa

ctor

(tan

)

lnf (Hz)

Y A

xis

Title

X Axis Title

Fig.4. 12: Dielectric loss Factor of Tb-Mn substituted hexaferrites, Sr2Co(2-x)MnxTbyFe(12-y)O22.

0.0 0.2 0.4 0.6 0.8 1.0

0.00 0.02 0.04 0.06 0.08 0.10

-2.0x108

0.0

2.0x108

4.0x108

6.0x108

8.0x108

1.0x109

1.2x109

1.4x109

1.6x109

Tb. Contents

(o

hm-c

m)

Mn. contents

6

8

10

12

14

16

Dielectric C

onstant (

Fig.4. 13: Comparison of dielectric constant and resistivity of Tb-Mn substituted hexaferrites,



80

14 16 18 20 22

0.0

5.0x10-2

1.0x10-1

1.5x10-1

2.0x10-1

2.5x10-1

3.0x10-1

13 14 15 16 17 18 19

0.0

5.0x10-4

1.0x10-3

1.5x10-3

2.0x10-3

2.5x10-3

3.0x10-3

x= 0.00 y=0.0

x=0.02 y= 0.2

x=0.04 y=0.4

x=0.06 y=0.6

x=0.08 y=0.8

x=0.10 y=1.0

ac

(-c

m)-1

lnf(Hz)

ac

(-c

m)-1

lnf(Hz)

Fig.4. 14: Variation in AC Conductivity Vs frequency of (Tb-Mn) substituted Co2Sr2Fe12O22

hexa ferrites at room temperature.

6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

x= 0.00 y=0.0

x=0.02 y= 0.2

x=0.04 y=0.4

x=0.06 y=0.6

x=0.08 y=0.8

x=0.10 y=1.0

Lo

g

ac

Log()

Fig.4. 15: Variation in logσAC with logω of (Tb-Mn) substituted Co2Sr2Fe12O22 hexa ferrites.


81

The dependence of the AC conductivity on frequency can be expressed by the following the

power law [41];

σtot (ω) =σDC+Aωn (4.2)

Where A is a pre-exponential factor has the units of electrical conductivity and n is the frequency

exponent a dimensionless quantity, which generally is less than or equal to one. When n = 0, the

electrical conduction is frequency independent or dc conduction and for n ≤1, the conduction is

frequency dependent or AC conduction [42]. This value of n is used to explain the conduction

mechanism operative in the studied samples. The hopping of electron between Fe2+/ Fe3+ ions is

responsible for conduction mechanism in ferrites. The value of exponent ‘n’ was extracted from

the slope of log(σ) versus log(ω). Fig.4.14. shows plot of log(σ) versus log(ω) and values are

listed Table 4.5 showing variation of exponent ‘n’ with composition. In the present study, the

value of exponent varies between 0.81-0.97, suggesting that the conduction phenomena in the

studied samples follow hopping conduction.

For ions vibrating in their cages and hopping to immediate sites through barriers of energy EAC

will follow the following equation.

τ0(T) = τ∞exp(EAC/kT) (4.3 )

Where τ∞ the reciprocal of the attempt frequency of ions and τ0 the relaxation time for

autonomous ion-hopping. Commonly the energy barrier (Ac activation energy) will be lesser

than the activation energy for the dc conductivity and given by the relation.

Edc = EAC/ (1− n) (4.4)

The higher values of “n” actually indicate the higher degree of cooperativity in the ion-hopping

process which is mainly due to the increase of interactions among the mobile ions [43, 44]. In

fact, by using the experimental values obtained for EDC and n, the activation energy EAC for the

barrier that ions must overcome to hop (independently) between neighboring vacant sites in the

Mn-Tb substituted Sr2Co2Fe12O22 ferrites, can thus be estimated according to Eq. (4.4). A value

EAC is found, dependent of Mn-Tb –contents and listed in the Table 4.5. Higher degree of

structural disorder is produced due to high rare earth-contents [40] which is accredited to the


82

difference in size of dopant and host ions at various hexagonal conduction sites. An enhanced

ion–ion interaction are expected and subsequently higher values of the exponent n. Higher the

value of n increase the energy penalty that these correlations impose on long-range or dc ionic

conductivity. This clarifies the increasing difference.

Table 4. 5: Real and imaginary parts of electric modulus and impedance at 1MHz and DC

activation energy, exponential n and AC activation energy of Tb-Mn substituted hexaferrites,


4.1.5.2 Impedance Analysis

Fig.4. 15 and inset show the variation of the impedance (Z) with frequency and follow the

equation;

|Z| = Z΄ + jZ΄΄ (4.5)

Z΄ and Z΄΄are real and imaginary parts of the impedance respectively. The values of Z΄ and Z΄΄ at

1MHz are listed in Table 4.5. It has been observed that values of impedance and its components

increase with (Tb-Mn) substitution which is inconsistent with compositional dependence of AC

conductivity, i.e increase in impedance results in decrease AC conductivity. It is found that, the

magnitude of Z decreases with the increase of frequency indicating increase in AC conductivity.

It also indicates the semiconducting type behavior in these systems.

Compositional Formula (EDC1+EDC2)/2 n (±0.01) EAC M΄ M΄΄ Z΄(Ω) Z΄΄(Ω)

Sr2Co2Fe12O22 0.431 0.81 0.078 0.061 0.008 32762 577

Sr2Co1.8Mn0.2Tb.02Fe11.98O22 0.438 0.76 0.102 0.075 0.012 34241 677

Sr2Co1.6Mn0.4Tb.04Fe11.96O22 0.464 0.58 0.193 0.089 0.016 40671 770

Sr2Co1.4Mn0.6Tb.06Fe11.94O22 0.471 0.96 0.019 0.102 0.019 52963 778

Sr2Co1.2Mn0.8Tb.08Fe11.92O22 0.477 0.90 0.046 0.117 0.020 60863 879

Sr2Co1Mn1Tb0.1Fe11.90O22 0.483 0.97 0.011 0.151 0.017 73632 941


83

0.0 5.0x108

1.0x109

1.5x109

2.0x109

2.5x109

3.0x109

-5.0x103

0.05.0x10

3

1.0x104

1.5x104

2.0x104

2.5x104

3.0x104

3.5x104

4.0x104

4.5x104

5.0x104

5.5x104

6.0x104

6.5x104

7.0x104

7.5x104

8.0x104

8.5x104

0.0 5.0x108

1.0x109

1.5x109

2.0x109

2.5x109

3.0x109

0

50

100

150

200

250

300

350

400

450

500

550

600

650Im

pid

en

ce

Z

Ferequency (Hz)

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Fig.4. 16: Variation of impedance with frequency of (Tb-Mn) substituted Co2Sr2Fe12O22 hexa

ferrites at room temperature.

4.1.5.2.1 Nyqiust plot (Cole - Cole plot)

The impedance spectroscopy is extensively used to describe the electrical properties of materials

and interfaces present in these materials. The impedance measurements data gives both resistive

(real) and reactive (imaginary) components of a material. It can be demonstrated in terms of any

of the four complex variables, permittivity (ε*), admittance (Y*), impedance (Z*), electric

modulus (M*) and dielectric loss (tan δ) in a complex plane plot (Nyqiust plot).Their relation to

one another is as follows [8, 45]:

tanδ = ε΄΄/ ε΄ = Y΄΄/Y΄ = Z΄΄/ Z΄= M΄΄/ M΄ (4.6)

In the present case Nyqiust plot of Complex electric modulus are plotted which is a powerful

technique to study relaxation phenomenon (i.e. contribution of bulk, grain boundary and material

electrode interface effect) in the material. Moreover, It helps in determining inter particle

interactions like grains, grain boundaries.In order to study the frequency dependence of the


84

interfacial polarization effect, electrical modulus (M) can be used which generates electric

charge accumulation around the ceramic particles by displacing relaxation peaks.

14 16 18 20 22

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

x= 0.0 y=0.0

x=0.2 y=0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

M

lnf(Hz)

Fig.4. 17: Variation in Real part of electric Modulus with frequency of (Tb-Mn) substituted

Co2Sr2Fe12O22 hexaferrites at room temperature.

M = 1/ε* = 1/(ε΄-jε΄΄) = M΄-jM΄΄ (4.7)

Figs. 4.16- 4.17 show the variation of both real and imaginary parts of electric modulus against

frequency. The Maxwell–Wagner model provides information for the behavior of complex

conductivity in heterogeneous systems with two or more phases [46]. In a heterogeneous system,

in the first case if the region of continuity of the grain boundary occupies a small volume, the

spectrum of impedance (Z΄΄ versus Z΄) provides better visualization of the semi circles in the

plane. There is a probable relationship between the behavior of grain boundary, and the

appearance of the peaks of Z΄΄ as functions of frequency, in second case if the region of grain

boundary occupies a large volume, the graph of the modulus (M*=1/ε*) M΄΄ versus M΄, provides

better information about the semicircles, suggesting that there is a probable relationship between

the behavior of grain boundary and the appearance of the peaks of M΄΄ as a function of frequency

[47] second case is in great agreement. The values of M΄ and M΄΄ are calculated for the Tb-Mn


85

doped samples and listed in the Table 4.5. These values of both real and imaginary part of the

electric modulus varies from 6.1×10-2 to 1.5×10-1 and 8×10-3 to 1.7×10-2 respectively. These

values are comparable with already reported values for Y-type hexaferrites [48].

14 16 18 20 22

0.000

0.005

0.010

0.015

0.020

0.025

0.030

x= 0.0 y=0.0

x=0.2 y=0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

M

ln(Hz)

Fig.4. 17: Variation in imaginary part of electric Modulus with frequency of (Tb-Mn) substituted


Fig.4.18. Shows the complex impedance (Cole-Cole) plots of the (Mn-Tb) substituted

Sr2Co2Fe12O22 ferrites. The left end (lower frequency) of the semicircle stands for the grain

resistance [49] while that at intermediate frequencies represents grain boundary contribution [50]

and the right one (higher frequency) stands for the whole resistance of the grains and grain

boundaries [49]. Substitution makes comparatively small difference on the grain resistance, but

leads to a remarkable rise of grain boundary resistance. Higher the Tb contents the higher the

grain boundary resistance. The dominant conduction mechanism in ferrites is the hopping

mechanism, which is an easy electron transfer between Fe2+ and Fe3+.. Increasing substitution

level of Tb at the expanse of Fe will restrain the electron transfer between Fe2+ and Fe3+ Thus,


86

0.080 0.088

0.010

0.012

0.014

x=0.0, y= 0.0M

M

0.095 0.100

0.005

0.006

0.007

x=0.2, y= 0.02

M

M

0.115 0.120 0.125

0.005

0.006

0.007

0.008 x=0.4, y= 0.04

M

M

0.13 0.14 0.15

0.017

0.018

0.019

0.020

0.021

0.022 x=0.6, y= 0.06

M

M

0.24 0.27

0.016

0.020

0.024

0.028 x=1.0, y= 0.10

M

M

Fig.4. 18: Cole–Cole plots of (Tb-Mn) substituted Co2Sr2Fe12O22 hexa ferrites at room

temperature


87

the resistivity of ferrite changes with the grain boundary content and composition. Obviously the

Tb substitution effects the grain boundary resistance. High resistance regions are formed at grain

boundaries to impede conductivity. The high resistance of the grain boundary will determine the

resistivity and dielectric properties.

4.1.5.3 Quality Factor

Fig. 4.19 shows the variation of Q values with frequency for Tb-Mn substituted

Co2Sr2Fe12O22ferrites. The maximum values of quality factor occurred above the 2GHz

frequency and the Q values were found quite high. This high Q values and a resonance frequency

above 2 GHz, clearly suggest that these materials can be used in high frequency multilayer chip

inductors [51].

0.0 5.0x108

1.0x109

1.5x109

2.0x109

2.5x109

3.0x109

0

500

1000

1500

2000

2500

3000

3500

4000

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Q fa

ctor

Frequency HZ

Fig. 4. 19: Variation of Q values with frequency of (Tb-Mn) substituted Co2Sr2Fe12O22 hexa

ferrites

4.1.6 Magnetic Properties

4.1.6.1 Hysteresis Loops

Figs. 4.20 - 4.21 represents the MH-loops for Sr2Co(2-x)MnxTbyFe(12-y)O22 ferrites for both in-

plane (H applied parallel to the sample surface) and out-of-plane (H applied perpendicular to the

sample surface) orientations. The values of saturation magnetization (Ms), coercivity (Hc) and


88

remanent magnetization (Mr) were taken from the M-H craves. The changes in magnetic

properties such as Ms, Hc, Mr and nB are due to the influence of the cationic stoichiometry and

their occupancy in the specific sites [52]. So the knowledge of distribution of metals ions in S

and T blocks among the distant sites is very essential to describe the magnetic properties of Y-

type hexaferrite.

In the present experimental findings the variation of the saturation magnetization (Ms)

has been explained on the basis of metal ions distribution in different sites laying in the both

block. There are six non-equivalent sites named by 6c1v, 3av1, 18hVI, 6cv1, 6cIV and 3bv1.

Crystallographic and magnetic properties of these six sites are listed in Table 4.6 (taken from

Ref.[53])

4.1.6.2 Saturation Magnetization (Ms)

The variation of the saturation magnetization (Ms) and remanence (Mr) are shown in the Figs.

4.22 - 4.23 for both cases i.e, in-plane and out-plane orientation. In general it is believed that

both Ms and Mr shows same trend which, is true in our present experimental case. The deep

observation of the spinel block of Y-type hexaferrite recalls the fact that super exchange

interaction is accountable for magnetic ordering between octahedral 3av1 and tetrahedral 6c1v

sites of the spinel block. In the present investigated samples Sr2Co(2-x)MnxTbyFe(12-y)O22, the

substitution of rare earth ion at iron site is mainly responsible for diluting magnetic interactions.

Replacement of Fe 5UB magnetic moment) by Tb (magnetic moment almost approaches

to Zero) [54] had strong preference to 3av1 octahedral site. Consequently, reducing the super

exchange interaction between 3av1and 6c1v sites. In this way we can conclude that increasing

concentration of Tb, the magnetization (M3av1) of 3av1-sites decreased while that of 6c1v site

M6c1v remained constant. As net magnetization is equal to M3av1- M6c1v so it was found to

decrease.

Moreover in the case of Sr2Co(2-x)MnxTbyFe(12-y)O22 the observed decrease of saturation

magnetization (Ms), can be explained by assuming a non-collinear magnetic order. T block

consist of two octahedral sub lattices, i.e 6cvI and 3bv1 along the vertical threefold axis [53], in

which three ions can reside per formula unit. It is more interesting to notice that the structural

configuration of octahedral site 3bv1 within the T block is such that, it shares two faces of its


89

coordination figure with the adjacent 6cvI, ions [53]. This arrangement of two distant octahedral

sites inside the T block is mainly responsible for constructing high potential energy structural

configuration. Furthermore the central octahedral 3bv1site connects the lower and upper part of

the unit cell through the strong interaction with ions in 6cvI site. It is worth noting to recognize

the established fact that Mn2+ has strong preference at the tetrahedral sites near about 80 ٪ of

the Mn2+ ions occupy the tetrahedral sites and remaining will reside at octahedral sites. Although

small amount of Mn2+ occupy the octahedral sites inside the T block still able to favor the

occurrence of drastic variations in the magnetic configuration with respect to the usual Gorter

scheme.

-10000 -5000 0 5000 10000

-80

-60

-40

-20

0

20

40

60

80

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Ms

(em

u/g)

Applied Field H(Oe)

Fig. 4. 20: In-plane MH-loop of Tb-Mn substituted Co2Sr2Fe12O22

It is believed that the partial occupancy of octahedral 3bv1site by Mn2+ ion is responsible for the

deviation from the collinear to non-collinear order. Breaking of inversion symmetry around 3bv1

sites results due to the partial replacement of iron ions by Tb3+ ions at 6cvI sites. As a result, there

exist a antisymmetric interactions between couples of ions on opposite sides of 3bv, sites like

6cv1 or 18hv, ions. In this view according to the Morija fifth rule [55] the antisymmetric

interactions are parallel to the C-axis thus favoring the occurrence of angles between the


90

different moments lying in the basal plane. The formation of angles between the magnetic

moments are responsible for spin canting or collinear to non- collinear transformation, which is

responsible for the reduction in the saturation magnetization.

-10000 -5000 0 5000 10000

-80

-60

-40

-20

0

20

40

60

80

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Ms

(em

u/g)

Applied Field H(Oe)

Fig.4. 21: Out-plane MH-loop of Tb-Mn substituted Co2Sr2Fe12O22.

Table 4. 6: Number of ions per unit formula, coordination and spin orientation for the various

metallic sublattices of Y-structure.

Sublattice Coordination Block Number of ions Spin

6c1v tetrahedral S 2 Down

3av1 octahedral S 1 UP

1 8hVI octahedral S-T 6 UP

6cvI octahedral T 2 Down

6cIV tetrahedral T 2 Down

3bv1 octahedral T 1 UP


91

0.00 0.02 0.04 0.06 0.08 0.10

10

20

30

40

50

60

700.0 0.2 0.4 0.6 0.8 1.0

Ms

(em

u/g)

Tb-Mn contents

In-plane

Out-plane

Mn contents

Fig.4. 22: In-plane and out-of-plane saturation magnetization versus (Tb-Mn) concentration for

Sr2Co(2-x)MnxTbyFe(12-y)O22 ferrites.

0.00 0.02 0.04 0.06 0.08 0.10

10

15

20

25

30

350.0 0.2 0.4 0.6 0.8 1.0

Mr (

emu/

g)

Tb Contents

In-plane

Out-plane

Mn Contents

Fig.4. 23: In-plane and out-of-plane Remanence versus (Tb-Mn) concentration for Sr2Co(2-

x)MnxTbyFe(12-y)O22 ferrites.


92

4.1.6.3 Coercivity Hc

Both inplane and out of plane coercivity was measured from the BH curves taken at in plane and

out- plane orientation respectively. Fig. 4.24 shows that coercivity increases with increasing

substitution level of Tb-Mn. It is observed that higher the porosity higher the coercivity [56],

which is also consistent with our present case. The saturation magnetization and coercivity are

related to each other through Browns relation Hc = K1/ μoMs [57, 58] where K1 is

magnetocrystalline anisotropy, μo is vacuum susceptibility, Ms is saturation magnetization and

Hc is coercivity. Our present experimental results of Ms and Hc satisfy this relation, i. e. Ms

decreases and coercivity increases with increasing substitution level. This inverse behavior of

magnetic parameters are reported by many researchers [56, 58].

0.00 0.02 0.04 0.06 0.08 0.10

500

1000

1500

2000

2500

3000

35000.0 0.2 0.4 0.6 0.8 1.0

Hc

(Oe)

Tb contents

In-plane

out-plane

Mn contents

Fig.4. 24: In-plane and out-of-plane coercivity versus (Tb-Mn) concentration for Sr2Co(2-

x)MnxTbyFe(12-y)O22 ferrites.


93

Figs.4,25(a-f): Fitted curve of Ms for (Tb-Mn) substituted hexaferrites, calculated by law of


4000 5000 6000 7000 8000 9000 10000

56

58

60

62

64

66

Data: Data2_B

Model: LoA

Equation: y = Ms * (1 - a/x - b/x^2) + chi * x

Weighting:

y No weighting

Chi^2/DoF = 0.0017

R^2 = 0.9999

Ms 77.11252 ±0.33049

a 1779.85621 ±49.94247

b -2159703.94919 ±175899.06031

chi 0 ±--

Ms (

em

u/g

)

applied field H (Oe)

a

4000 5000 6000 7000 8000 9000 10000

36

38

40

42

44

46

48

50

52

54

Data: Data2_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00237

R^2 = 0.99995

Ms 60.82415 ±0.3939

a 724.89916 ±82.25009

b 5186456.03987 ±312561.84418

chi 0 ±--

Ms (

em

u/g

)


b

4000 5000 6000 7000 8000 9000 10000

28

30

32

34

36

38

40

Data: Data4_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00395

R^2 = 0.9998

Ms 45.54564 ±0.50473

a 1082.79795 ±136.91023

b 3095129.27224 ±512636.54566

chi 0 ±--

Ms (

em

u/g

)

applied Field (Oe)

c

4000 5000 6000 7000 8000 9000 10000

18

20

22

24

26

28

Data: Data6_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00209

R^2 = 0.99988

Ms 32.42257 ±0.36093

a 590.8682 ±141.70783

b 6452419.58356 ±543286.41991

chi 0 ±--

Ms (

em

u/g

)

Applied Field H(Oe)

d

4000 5000 6000 7000 8000 9000 10000

10

12

14

16

18

20

Data: Data8_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00566

R^2 = 0.99974

Ms 23.28582 ±0.60511

a -185.58129 ±354.23735

b 14159403.25719 ±1488556.70927

chi 0 ±--

Ms (

mu

/g)

Applied Field H (Oe)

e

4000 5000 6000 7000 8000 9000 10000

8

9

10

11

12

13

14

15

16

Data: Data10_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00179

R^2 = 0.99984

Ms 16.95719 ±0.33935

a -938.33713 ±287.3208

b 16413706.24078 ±1187833.05064

chi 0 ±--

Ms (

em

u/g

)


f


94

Moreover, the increasing behavior of coercivity Hc with increasing substitution level can be

explained on the basis the aspect ratio (c/a) listed in the Table 4.1. In that case the coercivity

could be written interm of following equation [59]:

Hc= 0.48(K1/Ms− NdMs) (4.8)

Where Ms is the saturation magnetization, K1 is the magneto-crystalline anisotropy constant and

Nd is the demagnetizing coefficient relating to the shape anisotropy. As the aspect ratio decreases

with increasing substitution level could reduce the demagnetizing factor and thus enhance Hc

[60]. The “magnetic bits” are directed parallel to the surface of disk in conventional longitudinal

magnetic recording (LMR). Whereas, in perpendicular recording media (PRM), the ‘‘magnetic

bits’’ are arranged point up or down perpendicular to the surface of disk.

The well-known clarification for the usage of PMR is that it can deliver 3 times extra

storage density as compared to LMR. Fundamentally, magnetic samples having high values of

coercivity are thermally more stable. Thermal stability of the magnetic samples is proportional to

the product of uniaxial anisotropy constant K1 times volume, higher coercive material will have

large the product. In this regard we can assume that PRM demands a high coercivity medium. If

the coercivity is high enough above 1200Oe, then hexaferrite materials can be beneficial for the

perpendicular recording media which is an emerging technology in the recording media [61]. In

the present experimental findings the investigated samples which are Y-type hexaferrite can be

used in PRM due to high value of coercivity 3200Oe which is comparable to the those of M-type

and W-type hard magnetic materials. Materials are consider to be hard magnets, if Hc>Mr/2 and

if Hc<Mr/2, then the materials are semi-hard magnets[57, 62]. The synthesized hexaferrite

materials in the present study have Hc>Mr/2. Furthermore, it is believed that if samples have

Hc>Mr/2, can be used for high frequency applications [61].

The saturation magnetization (Ms) curves were fitted using law of approach for

Sr2Co(2-x)MnxTbyFe(12-y)O22 ferrites shown in Figs.4.25(a-f). The large difference between

experimental and theoretical values of saturation magnetization has been observed which is due

to the deficient field applied in the experimental case while in the theoretical case infinite field is

applied in order to attain maximum values of saturation magnetization. The estimated values of

saturation magnetization (Ms) are listed in the Table 4.7.Deep observation of the Figs.4.25(a-f)

shows that insufficient field is applied in the experimental case explaining that additional


95

might be realized by increasing the external field, which will provide the close agreement among

theoretical and experimental values.

The values of magnetic moment (nB) are listed in Table 4.7 for in-plane and out-plane. In general

both the magnetic moment (nB) and the saturation magnetization (Ms) show similar behavior. In

present findings the behavior of magnetic moment is consistent with the saturation magnetization

as both decrease with increasing (Tb-Mn) contents, the decrease of magnetic moment may be

due to the weakening of super exchange interactions, as Fe –o – Fe super exchange decreases

with Rare earth substitution at the expanse of Fe. Similar behavior has already been reported by

other researchers [63, 64].

Table 4. 7: Estimated saturation magnetization Ms, Anisotropy constant( K), Magnetic moments

(nB), Squareness Ratio and Grain size of Tb-Mn substituted Co2Sr2Fe12O22.

4.1.6.4 Squareness Ratio

Both in-plane and out of plane Squreness ratios (Mr/Ms) of (Tb-Mn) substituted Co2Sr2Fe12O22

hexaferrites were calculated from VSM data and tabulated in the Table 4.7. In-plane Squreness

ratios (Mr/Ms) ranging from 0.41 to 0.65 whereas for out of plane measurement it varies from

0.30 to 0.62. Even though squreness ratio is well below of typical value ~1 for single domain

isolated ferromagnetic particle. Still comparatively higher value of squareness ratio obtained

especially at higher substitution level suggests that some particles may reside as single domain

Co

mp

osi

tio

nal

Form

ula

Esti

mat

ed

Ms(

em

u/g

)

K(e

rg/c

m3 )

In-p

lan

e

K(e

rg/c

m3 )

Ou

t-p

lan

e

nB(e

mu

/g)

In-p

lan

e

nB(e

mu

/g)

Ou

t-p

lan

e

Ms/

Mr

in-

pla

ne

Ms/

Mr

Ou

t-p

lan

e

Sr2Co2Fe12O22 77.11 2.34×104 2.10×104 15.27 14.64 0.41 0.30

Sr2Co1.8Mn0.2Tb.02Fe11.98O22 60.82 5.71×104 5.68×104 12.38 12.20 0.59 0.53

Sr2Co1.6Mn0.4Tb.04Fe11.96O22 45.54 3.94×104 4.09×104 9.19 9.39 0.55 0.51

Sr2Co1.4Mn0.6Tb.06Fe11.94O22 32.42 3.60×104 3.62×104 6.67 6.75 0.60 0.59

Sr2Co1.2Mn0.8Tb.08Fe11.92O22 23.28 3.19×104 3.28×104 4.77 4.87 0.59 0.49

Sr2Co1Mn1Tb0.1Fe11.90O22 16.95 2.47×104 2.59×104 3.68 3.84 0.65 0.62


96

magnetization.whereas, in case of pure CoY ferrite lower value of squareness ratio shows that

particles are completely randomly oriented and exist in multi domains. By assuming magnetic

particles to be isolated (exchange interacting spin) single domains [65], the anisotropy constant

(K=HcMs/2) was calculated using the given relation and given in the Table 4.7. The values of

magnetocrystalline anisotropy constant are less than that of reported ones for single domain

different ferrites. This shows that grains are not single domains and anisotropy contribution is

not uniaxial [66, 67].

4.2 Eu-Ni Substituted Y-type Hexaferrite

4.2.1 Structural Analysis Typical X-ray diffractions patterns of Eu-Ni substituted Sr2Co2Fe12O22 samples at room

temperature are shown in the Fig. 4,26 and X΄ pert highscore was used to index the XRD

patterns. The indexing of each pattern indicates that the well-defined Y-type single phase

crystalline structure is formed. Enhanced intensity of peaks which is measure of improved

crystalline phase suggests that Eu - Ni ions in the nominated substitution range are entirely

dissolved in the Sr–Co–Y crystal lattice. Minor deviations are observed in the lattice parameters

with replacement of Co and Fe ions by Ni and Eu ions, respectively.

The lattice constant “a” slightly varies from 5.88 to 5.99Å, where as lattic parameter “c”

increases from 43.37 to 43.78Å as shown in the Fig. 4.27. It is understood that all hexagonal

ferrites exhibit slow variation in lattice parameter ‘a’ and rapid variation in parameter ‘c’ [68]

which is in great agreement with our experimental findings. The increment in lattice parameters

with varying Eu –Ni contents is accredited to the difference in ionic radii of the substituted and

the host ions. It has been anticipated that the influence of large ionic radius of the doped Eu3+

(0.947 Å) than that of the host Fe3+ (0.64Å) suppresses the negative effect of Ni2+ (0.69 Å)

substitution for Co2+ (0.745Å). This elucidation is in agreement with the finding of many

researchers [2, 69].

The calculated values of bulk density decreased with substitutions (Eu-Ni) in Sr2Co2 Fe12O22 as

listed in Table 4.8. The observed deterioration in bulk density may be due to the lower room

temperature density of Eu3+ (5.264 g cm-3) as compared to that of Fe3+ (7.874 g cm-3). The bulk

densities of Co2+ (8.90 g cm-3) and Ni2+ (8.908 g cm-3) are almost equal which have no effect on

the density of synthesized materials. Slight decreases in X-ray density (dx) from 5.05 to 4.85



97

g·cm-3 with the (Eu-Ni) substitution as shown in Table 4.8 is largely due to minute increase in

the cell volume of the respective samples, as the cell volume is inversely related to the X-ray

density [70]. Fig. 4.28 shows the variation of crystallite size VsEu-Ni contents. A suitable

explanation for increase in crystallite size may be attributed to smaller solid solubility of Ni2+

ions compared to Co2+ ions with increasing Eu-Ni contents [71].

The porosity of ceramic samples usually results from two sources, i.e. intragranular or

intergranular depending on whether pores remain within the grains or pores lie in the grain

boundaries.When the grain growth rate is very high, pores are left behind the rapidly moving

grain boundaries and are trapped within the grains. This intragranular porosity is almost

impossible to exclude, leading to poor mechanical properties [72]. It is evident that Eu-Ni

substitution promotes less densification, which enhances the porosity to a considerable extent as

shown in Table 4,8.

20 25 30 35 40 45 50 55 60 65 70

2 0

20

1 1

0 1 1

60

1 1

411

9

0 0

18

1 0

19

1 0

22

10

21

0 0

27

2 1

19

0 1

8

x= 0.0, y= 0.0

x= 0.2, y= 0.02

x= 0.4, y= 0.04

x= 0.6, y= 0.06

x= 0.8, y= 0.08

x= 1.0, y= 0.1

2Ɵ (degree)

Inti

nsi

ty (

a. u

.)

Fig.4. 26: XRD analysis of (Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x =

0.00–1.00; y = 0.00–0.10).


98

0.0 0.2 0.4 0.6 0.8 1.0

0.00 0.02 0.04 0.06 0.08 0.10

5.88

5.90

5.92

5.94

5.96

5.98

6.00

Eu. content

a (Å

)

Ni. content

43.3

43.4

43.5

43.6

43.7

43.8

C(Å)

Fig.4. 27: Variation of lattice parameters for (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

0.00 0.02 0.04 0.06 0.08 0.10

30

40

50

60

70

80

900.0 0.2 0.4 0.6 0.8 1.0

crys

tallit

e si

ze (n

m)

Eu. Content

Ni. content

Fig.4. 28: Variation of crystallite size for (Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-

y)O22, (x = 0.00–1.00; y = 0.00–0.10).


99

Table 4. 8: c/a , cell volume (Vcell), bulk density (db) X ray density (dx -ray),P(%) percentage

porosity and Room temperature DC resistivity of (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

Compositional Formula c/a V(Aᵒ)3 db(g/cm3) dx(g/cm3) P(%) ρ(Ω-cm)

Sr2Co2Fe12O22 7.37 1298.56 4.92 5.05 2.57 1.23×106

Sr2Co1.8Ni0.2Eu0.02Fe11.98O22 7.34 1313.35 4.86 5.0 2.8 1.02×107

Sr2Co1.6Ni0.4Eu0.04Fe11.96O22 7.33 1324.69 4.81 4.96 3.02 3.95×107

Sr2Co1.4Ni0.6Eu0.06Fe11.94O22 7.34 1333.14 4.79 4.94 3.04 1.44×108

Sr2Co1.2Ni0.8Eu0.08Fe11.92O22 7.31 1348.49 4.73 4.89 3.27 4.76×108

Sr2Co1Ni1.0 Eu0.1Fe11.90O22 7.30 1360.33 4.69 4.85 3.3 3.07×109

4.2.2 EDX Analysis For the determination of the composition for present inspected samples, EDX technique was

employed. The stoichiometric analysis reveals the fact that Eu and Ni contents were increased

while Fe and Co content decreased. Fig.4.29 (a-f) elucidates EDX spectra of Y-type hexaferrites.

The numerous peaks observed in the EDXS spectra certify the presence of Sr, Co, Eu, Ni and Fe.

All the calculated metallic contents of the present synthesized samples as listed in Table 4.9. A

careful observation of the Table 4.9 suggests that experimental and theoretical calculation of all

stoichiometric contents is in close agreement with each other. The increment in dopant and

decrease in the substituted contents at systematic rate obviously suggest that present inspected

samples conserved the accurate contents stoichiometry. The Sr2+ ions have slightly lower

solubility in the in the pure Sr2Co2Fe12O22 sample. This can be illuminated by remembering the

fact that the maximum solubility of Sr2+ ions in the hexagonal Y-type ferrite is 1.8, owing to the

lesser size of Sr2+ (0.127 nm) as compared with Ba2+ (0.143 nm). Nevertheless, the occupation

of the Sr2+ in Y-type hexagonal lattice can be improved to 2.0 (theoretical) as exposed in Table

4.9. EDX studies is confirmatory study for the analysis which suggest that the presence of

dopants (Ni2+ and Eu3+) is actually responsible for the incorporation of Sr2+ ions in the hexagonal

lattice by creating more space inside the T block of Y-type hexaferrites. Substitution of Eu and

Ni in the hexagonal Y-type ferrite is mainly responsible for the creation of vacancies in the


100

hexagonal lattices which really ensure the maximum solubility of Sr ions in the hexagonal

lattice.

Fig.4. 29: (a-f) EDX spectra for (Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x

= 0.00–1.00; y = 0.00–0.10).


101

Table 4. 9: Comparison of the Observed and Theoretical Weight Percents and Content

Determined by EDX Analysis of the (Eu-Ni) substituted Co2Sr2Fe12O22.

The EDX data suggest that contents of and Eu and Ni in T block increases. It is important to

distinguish the well-known fact that both Ni and Eu have strong preference at the octahedral

sites. Most significantly Sr ions occupy positions in the basic oxygen layers of the T block. The

better approximation of the Sr in the Y- type ferrite has been achieved at higher substitution

level.

Sample Name Elements

Fe

Sr

Co Ni

Eu

Sr2Co2Fe12O22

Theoretical 69.57 18.19 12.23 0 0

Experimentl 69.39 18 11.99 0 0

Content 11.98 1.97 1.96 0 0

Sr2Co1.8Ni0.2Eu0.02Fe11.98O22

Theoretical 69.32 18.19 10.99 1.21 0.31

Experimentl 69.27 18.01 10.87 1.19 0.29

Content 11.97 1.98 1.78 0.19 0.018

Sr2Co1.6Ni0.4Eu0.04Fe11.96O22

Theoretical 69.07 18.19 9.75 2.42 0.62

Experimentl 69.01 18.08 9.69 2.37 0.59

Content 11.94 1.99 1.59 0.39 0.037

Sr2Co1.4Ni0.6Eu0.06Fe11.94O22

Theoretical 68.82 18.19 8.51 3.63 0.94

Experimentl 68.78 18.05 8.42 3.54 0.91

Content 11.93 1.99 1.38 0.58 0.058

Sr2Co1.2Ni0.8Eu0.08Fe11.92O22

Theoretical 68.57 18.19 7.28 4.83 1.25

Experimentl 68.48 18.13 7.15 4.76 1.23

Content 11.90 2.00 1.17 0.78 0.078

Sr2Co1Ni1.0 Eu0.1Fe11.90O22

Theoretical 68.32 18.19 6.05 6.03 1.56

Experimentl 68.20 18.13 5.00 5.99 1.51

Content 11.87 2.00 0.82 0.99 0.096


102

4.2.3 Scanning Electron Microscopy Fig. 4.30 (a-f) displays a series of SEM micrograph of the samples sintered at 1050Cº. The grain

morphology looks plate-like for almost every composition of the present investigated samples.

Fig.4. 30: (a-f) SEM images for (Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x =

0.00–1.00; y = 0.00–0.10).


103

This particular shape very advantageous for microwave absorbing purposes [5]. Keen

observation of these micrographs suggest that with increasing substitution level grain size

becomes lager and intercrystalline porosities increased and grain boundaries becomes more

sharp which consequently result in increasing grain size. Furthermore, very few agglomerates

were examined. The formation of these agglomerates is mainly governed due to the Chemical

reaction during sintering process. It is obvious to recall the fact that comparatively weak Van

der Waals bonds and magnetic forces are the major factors which really holds hold together these

agglomerates [6]. Under the influence of the insistent forces the uniqueness of the agglomerates

can be conserved. Average grain size calculated from the SEM images was found in the range of

92- 256nm and listed in the Table 4.11. Whereas ionic radii of Ni and Co are approximately

equal. It has been anticipated that the influence of large ionic radius of the doped Eu3+ (0.947 Å)

than that of the host Fe3+ (0.64A˚) may be the main source of the enhancement in the grain size.



DC resistivity is one of the most significant aspects of ferrite ceramics, usually high resistivity is

prerequisite for most electronic applications. Ferrites are considered to be consisted of highly

conductive grains and less-conductive grain boundaries. The values of room temperature

resistivity as a function of (Ni-Eu) contents are given in the Table 4.8. In this case, the resistivity

of grain boundaries is mainly responsible for DC resistivity of ferrite ceramics [73]. Moreover,

the DC resistivity of ferrite ceramics is strongly influenced by many factors like microstructural

homogeneity, stoichiometric composition, grain size, density (porosity), impurity levels, and

crystal structure perfection [74].Among all, the porosity is predominantly significant. DC

resistivity of ferrite increases due to the existence of porosity because air/vacuum acts as

insulator, if the pores are closely trapped and homogenously distributed. In the present

investigation, increase in the DC resistivity may be due to the slight increase in porosity with

(Eu-Ni) substitution.The observed rise in DC resistivity with (Eu-Ni) concentration might be due

to the fact that nickel (Ni) and europium (Eu) are slightly more resistive (6.93 x10-8 and 9.0x10-7

ohm m, respectively) when compared to cobalt and iron (6.2 x10-8and 9.71 x10-8 ohm m). The

changes in electrical resistivity can be explained due to the influence of the cationic

stoichiometry and their occupancy in the specific sites. So the knowledge of distribution of

metals ions in S and T blocks among the two distinct sites (tetrahedral and octahedral) is very


104

vital to describe the resistivity of Y- type hexaferrites. There are four non-equivalent octahedral

sites namely 3av1, 18hVI, 6cv1 and 3bv1 and two tetrahedral sites named by 6c1v and 6cIV.

In the present experimental findings the variation of the resistivity has been explained on the

basis of metal ions distribution at different sites. Room temperature resistivity increases with

increasing the Eu-Ni concentration, as Eu3+ ions prefer to occupy octahedral sites followed by

the migration of some Fe3+ ions to tetrahedral sites and altering them into Fe2+ ions. As a result

Fe3+ ions concentration is lowered at octahedral sites. This would limit the hopping probability

between Fe3+ and Fe2+ ions thereby enhancing the resistivity.


Fig. 4.31 elucidates temperature dependence DC resistivity for the investigated samples.

Temperature dependent resistivity which actually decreases with increasing temperature showing

the typical semiconducting behavior [56] as the temperature increases the charge carriers are

thermally activated and hop between the various hexagonal sites. As a result the resistivity of the

materials decreases with the increase in temperature. The conduction in ferrites at room

temperature is due to impurities, whereas at high temperature, it is attributed to polaron hopping

[75].The variation of electrical resistivity with temperature following the Arrhenius equation.

ρ = ρ0 exp ΔE/kBT (4.9)

where “ρ” is resistivity, “kB”represents Boltzmann’s constant and “∆E” is the activation energy,

which is needed for electron hopping from one metallic ion to the next [76]. The activation

energies in the ferri and para regions are calculated from the slopes of the plot log ρ versus 103/T

and their values are presented in Table 4.10. The variation of activation energy as a function of

Eu- Ni concentration is in agreement with the variation of room temperature resistivity.

Moreover, the explanation offered for electrical resistivity stands same for activation energy. It

can be viewed from Table 4.10 that the activation energy in the ferrimagnetic region is lesser

than that in the paramagnetic region. This result is in agreement with the theory developed by

Irkin and Turov [13].The conduction at a lower temperature i.e. below Curie temperature

(ferrimagnetic region ) is due to hopping of electrons [77] between Fe2+ and Fe3+ ions, whereas at

a higher temperature i.e. above Curie temperature (paramagnetic region ) is due to hopping of


105

polarons [77]. It is obvious to recall the fact that poloran hopping required comparatively more

energy than that of electrons hopping as in electron hopping the both types of charges freely

move in the crystal lattice. This is the main factor for lowering of the activation energy in

ferrimagnetic than paramagnetic region. It is also observed that the transition temperature is in

good agreement with the Curie temperature and shows kink at about Curie point. Variation in

slops (kinks) in the resistivity curves is also examined by many researchers [9, 78]. The

magnitude of the kink is measure of difference in activation energies between ferri-magnetic and

paramagnetic regions. It is observed that ∆E increases as resistivity increases (Table 4.10).

Structural peculiarities, and value of electrical resistance is also determined, whether the size of

kink is smaller or larger in various ferrites. The earlier experiments have shown that the smaller

kinks are characteristics of ferrite which have large resistance and are in good agreement with

our present experimental results. Generally the change of slope is attributed to change in

conductivity mechanism as ferrimagnetic material transforms to paramagnetic at the Curie

temperature. The hopping of electrons between Fe2+ and Fe3+ ions and jumping of holes between

Co3+ and Co2+ ions are responsible for conduction at lower temperature i.e. below Curie

temperature. Whereas at (higher temperature) above Curie temperature is due to polaron hopping

[25, 26, 79]. The measured values of activation energies in the paramagnetic region (E2) are

greater than 0.40 eV, which obviously propose that the conduction is due to polaron hopping [78,

80].

Table 4. 10: M1 (slopeof ferrimagnetic region), M2 (slopeof paramegnetic region), E1

(Activation energy of ferrimagnetic region) and E2 (Activation energy of paramegnetic region)

of (Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y


Slop Activation energy

FM PM

Region Region

M1 M2

FM PM

Region Region

E1 (ev) E2 (ev) ∆E=E2-E1(ev)

Sr2Co2Fe12O22 1.78 2.61 0.352 0.51 0.164

Sr2Co1.8Ni0.2Eu.02Fe11.98O22 1.82 2.66 0.360 0.526 0.166

Sr2Co1.6Ni0.4Eu.04Fe11.96O22 1.86 2.85 0.368 0.564 0.196

Sr2Co1.4Ni0.6Eu.06Fe11.94O22 1.87 2.87 0.370 0.568 0.198

Sr2Co1.2Ni0.8Eu.08Fe11.92O22 1.88 2.91 0.372 0.576 0.203

Sr2Co1Ni1Eu0.1Fe11.90O22 1.9 3.0 0.376 0.594 0.217


106

1.5 2.0 2.5 3.0 3.5

10

12

14

16

18

20

22

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Log

(o

hm-c

m)

1000/T (K-1

)

Fig.4. 31: Temperature dependent resistivity of (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

0.0 0.2 0.4 0.6 0.8 1.0

0.00 0.02 0.04 0.06 0.08 0.10

440

460

480

500

520

540

560

Eu. contants

Tc (K

)

Ni. content

Fig.4. 32: Variation of Curie temperature (Tc) for (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).


107

Two different regions are observed in temperature dependent resistivity plots, it can be

concluded that thermal energy in first region (ferro region) is not sufficient to disturb the aligned

spins of electrons. Whereas, in the second region (para region) the thermal energy is too

sufficient to disturb all the aligned spins of electrons. Fig. 4.32 elucidates the concentration

dependence of Curie temperature (Tc) for the investigated samples. It is clear from the figure that

the values of Curie temperature (Tc) decreases successively with increasing (Eu-Ni) contents. It

is understood fact that Fe3+–O–Fe3+ superexchange interactions and the Fe3+-Fe3+ direct

exchange interactions are the fundamental interactions in ferrimagnetic material [15]. The

decrease in curie temperature (Tc) with increasing Eu- Ni contents may be caused by changes in

Fe3+–O–Fe3+ and Fe3+–Fe3+ angles, which leads to a decrease in the magnetic moment

interaction. It has been reported that RE ions containing samples exhibit lower Curie

temperatures than those without RE contents [78]. This is in tremendous resemblance with our

present experimental observation. It is also recognized that magnetic moments of Fe3+ ions are

settled collinearly due to the persistence of super exchange interaction.

250 300 350 400 450 500 550 600 650 700

0.0

2.0x10-11

4.0x10-11

6.0x10-11

8.0x10-11

1.0x10-10

1.2x10-10

1.4x10-10

1.6x10-10

1.8x10-10

x= 0.00 y=0.0

x=0.02 y= 0.2

x=0.04 y=0.4

x=0.06 y=0.6

x=0.08 y=0.8

x=0.10 y=1.0

Mob

ility

(cm

2 v-1s-1

)

T(K)

Fig.4. 33: Change in Drift mobility with temperature for (Eu-Ni) substituted hexaferrites,


The substitution of RE for Fe3+ ions causes partial disorder and weakens Fe3+ –O–Fe3+ super

exchange interactions, where the valence of the iron ion changes from Fe3+ with a high spin state


108

(3d5with 5µB) to Fe2+ with a low spin state (3d6with 4µB) [2, 81], such valence change results in

deviation from collinear to non-collinear arrangement, this supervenes to a decrease in the Curie

temperature TC [2, 82]. Moreover, the decrease in TC may be due to the fact that Eu–Fe

interactions on the B sites are smaller than Fe–Fe interactions [17, 18].


Using the experimental data of electrical resistivity, the mobility for the charge carriers was

calculated for Sr2NixCo(2-x)EuyFe(12-y)O22 ferrites. The drift mobility is related to the temperature

by following relation;

µd = µ0 exp(-Eµ/kBT) (4.10)

Where µ0 is pre-exponential constant, kB is Boltzmann constant and Eµ is the activation energy

for mobility of ions. The values of the charge carrier mobility for the different compositions are

also included in Table 4.11. The data of electrical conductivity and drift mobility are in good

agreement with each other. It can be seen from the table that the mobility is maximum for un

substituted ferrite. The values of mobility are found to be very low when compared with those of

typical semiconductors. However, such low values are not new as far as ferrite semiconductors

are concerned; such low mobility values have already been reported by several researchers[83-

86]. The variation of mobility with temperature is shown in Fig.4.33. It can be seen from the

figure that the charge carrier mobility values increase continuously with the increase of

temperature. The increase in mobility with increasing temperature suggests that the conduction

in these ferrites is due to the hopping mechanism of electrons from Fe2+ to Fe3+ and holes

transfer from Co3+ to Co2+ and Ni3+ to Ni2+. Similar behavior, has also been reported by many

researchers [83-86].

4.2.5 Dielectric Properties The effect of frequency on the dielectric constant έ is shown in Fig. 4.34. It can be seen that with

increasing frequency the value of dielectric constant decreases continuously. The decrease of

dielectric constant with frequency is a normal dielectric behavior of the ferrite ceramics and

extensively examined by many researchers [23, 84, 87]. The dielectric constant decreases

massively in the low frequency region and it becomes almost independent of frequency in the

intermediate frequency region. Resonance peaks are observed above the frequency of 2GHz.

These peaks appeared when the jumping frequency of electrons between Fe2+ and Fe3+ is equal to


109

the frequency of applied ac field [88].The lowering of the dielectric constant with increasing

frequency is attributed to the fact that under the influence of external electric field the dielectric

material exhibits induced electric moment. But as the frequency increases the polarization of

induced moments or electron exchange between Fe2+ and Fe3+ ions could not synchronize with

the frequency of applied electric field [89].

14 16 18 20 22

4

5

6

7

8

9

10

11

12

13

14

15

16 x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Die

lect

ric

Con

stan

t (

lnf (Hz)

Fig.4. 34: Dielectric constant of Eu-Ni substituted, Sr2Co(2-x)NixEuyFe(12-y)O22,(x = 0.00–1.00; y

= 0.00–0.10) hexaferrites.

Fig. 4.35 shows the plot of ε″ Vs frequency and values are listed in the Table 4.11. The behavior

of dielectric Loss ε″ with frequency is qualitatively analogous with the deviation of έ with

frequency. Maxwell–Wagner’s bi-layered model elucidated that dielectric behavior of ferrites

ceramic exists in non-homogeneous layered structure [90, 91]. Deep inspection of this modal

reveals that, ferrite consists of ideally conducting grains separated by insulating grain

boundaries. Movements of charge carriers take place under the action of an applied field. Large

resistance of grain boundaries supports the charge carriers to align themselves at grain

boundaries. Thus availability of free charges on grain boundaries pile up the space charge

polarization at grain boundary. This leads to large dielectric constant. Grain boundaries are


110

effective in the low frequency region and grains are effective in the high frequency region [29].

Thus at higher frequency in the material low value of polarization builds up in the material which

tends to deteriorates the dielectric constant. The dispersion in the dielectric constant favor

occurrence of peaks in the tanδ(f) [24].

14 16 18 20 22

0.0

0.5

1.0

1.5

2.0

2.5

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Die

lect

ric

Los

s

lnf (Hz)

Fig.4.35: Dielectric loss of Eu-Ni substituted, Sr2Co(2-x)NixEuyFe(12-y)O22,(x = 0.00–1.00; y =

0.00–0.10) hexaferrites.

.The dielectric loss (tan δ) can be expressed in terms of the real and imaginary parts of the

dielectric constant [74]. Concentration dependence of tanδ values are list in Table 4.11.

έ = 𝜀∞ +𝜀𝑠−𝜀∞

1+𝜔2𝜏2 (4.11)

ε″ = (𝜀𝑠−𝜀∞)𝜔𝜏

1+𝜔2𝜏2 (4.12)

where 𝜏, 𝜀𝑠,and𝜀∞ are the relaxation time, dielectric constant at very low and very high

frequencies, respectively and “ω” is the angular frequency. The intense observation of Eq. (4.11)


111

propose that dielectric constant (έ ) decreases more rapidly with increasing frequency, since (έ )

is proportional to the 1/ ω2. While Eq. (4.12) implies that decrease of ε″ is comparatively slow,

because (ε″) is proportional 1/ ω [92]. Therefore, Comparatively fast decline of the (έ) than that

of (έ΄) in the given frequency range may favor the existence of peaks in the tanδ(f) plot as shown

in Fig.4.36 inset. Similar behavior has already been reported earlier[27]. Moreover suitable

clarification for existence of the peaks in plot of tanδ against the frequency can be given on the

basis of the previous hypothesis [31, 32] that in ferrite a strong correspondence exist between the

dielectric polarization and the conduction mechanism. In this case the peaks in tanδ(f) curves are

detected when the external electric field becomes equal to the hopping frequency of charge

carriers [27].

14 16 18 20 22

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

21.2 21.3 21.4 21.5 21.6 21.7

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

diel

ectr

ic lo

ss fa

ctor

(ta

n)

ln f (Hz)

diel

ectr

ic lo

ss f

acto

r (t

an)

ln f (Hz)

Fig.4. 36: Dielectric loss Factor ofEu-Ni substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x

= 0.00–1.00; y = 0.00–0.10).

Fig. 4.37 shows the comparative variation of dielectric constant and Dc resistivity with Eu-Ni

contents. The decrease in dielectric constant with increase in Eu-Ni contents is attributed to

increase in resistivity of the samples. The role of microstructure (grain boundary, porosity etc.) is


112

very decisive for the discussion of resistivity and dielectric behavior. The decrease of dielectric

constant may be accredited to the lowering of density which is in great agreement with our

present experimental observation [93].

0.0 0.2 0.4 0.6 0.8 1.0

0.00 0.02 0.04 0.06 0.08 0.10

0.0

5.0x108

1.0x109

1.5x109

2.0x109

2.5x109

3.0x109

3.5x109

Eu. content

(o

hm-c

m)

Ni. content

10

12

14

16

Dielectric C

onstant (

Fig.4.37: Comparison of dielectric constant and resistivity of Eu-Ni substituted

hexaferrites,Sr2Co(2-x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).


The dependence of AC conductivity on frequency can be expressed by the following power law

[41];

σtot(ω) =σDC+Aωn (4.13)

Where A is a pre-exponential factor with electrical conductivity units where as n is the frequency

exponent which is dimensionless quantity. It is observed that AC conductivity increases with

increasing frequency of the applied field as shown in the Fig. 4.38. Since the increase in

frequency enhances the hopping frequency of the charge carriers between Fe2+ and Fe3+, which

subsequently increases AC conductivity. This behavior of AC conductivity can be explained on

the basis of Maxwell–Wagner model and Koop’s phenomenological theory. According to which

the ferrites are imagined to act as a multilayer capacitor in which the ferrite samples are


113

characterized by a microstructure consisting of conducting grains separated by highly resistive

thin layers (grain boundaries). According to this model our results of AC conductivity at low

frequencies describe the grain boundary behavior, while the dispersion at high frequency may be

attributed to the conductivity of grains [39]. The AC conductivity decreases with increasing (Eu-

Ni) contents and is listed in the table 4.11.

0.0 5.0x108

1.0x109

1.5x109

2.0x109

2.5x109

3.0x109

0.00

0.05

0.10

0.15

0.20

0.25

0.30

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.10

ac(

-cm

)-1

Frequency (Hz)

Fig.4. 38: Variation in AC Conductivity with frequency of (Eu-Ni) substituted hexaferrites,

Sr2Co(2-x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10)

Fig 4.39. Illustrates a typical log–log demonstration of the frequency dependence of electrical

conductivity of Eu–Ni substituted in Sr2Co2Fe12O22 ferrites. It is clear from the figure that Ac

conductivity shows very minute variations at low frequencies however at relatively high

frequencies AC conductivity follows the power law by following the empirical expression

σ′(ω)∝ωn, where n is a fractional exponent (0 ≤ n ≤ 1), associated with the dynamic of hopping

ions [94]; thus, the value of n progressively increases with increasing interactions among mobile

ions and vice versa whereas at very low value i.e. n=0, Debye-like behavior, completely

independent of frequency has been observed.


114

6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Log ac

(-c

m)-1

Log()

Fig.4. 39: Variation of logσ with logω of (Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-

y)O22, (x = 0.00–1.00; y = 0.00–0.10).

In the present experimental findings, the values of exponent vary between 0.81–0.97 and are

listed in Table 4.12. High values of n (0 ≤n ≤ 1) suggest that the conduction phenomena in the

studied samples follow hopping mechanism [40].For vibration of ions, in their hexagonal sites

coupled with hopping to immediate sites through barriers of energy, following equation has been

applied [40];

τ0 (T) = τ∞ exp (EAC / kT) (4.14)

Where τ∞ is the reciprocal of the attempted frequency of ions and τ0 is the relaxation time for

independent ion-hopping and EAC is the AC activation energy. Usually the energy barrier (AC

activation energy) will be lesser than that of activation energy for the dc conductivity and is

given by the relation [40].

E dc = EAC / (1− n) (4.15)

Enhanced interactions between the mobile ions result in higher value of “n”. Furthermore higher

values of “n” are measure of higher degree of cooperatively in the ion-hopping process [43, 95].

In fact, by using the experimental values, obtained for EDC and n, the activation energy EAC for

the barrier that oxygen ions must overcome to hope (independently) between neighboring vacant


115

hexagonal sites in the Eu-Ni substituted Sr2Co2Fe12O22 ferrites, can thus be calculated by

Eqn(4.15). A value of EAC is found which is dependent on (Eu-Ni) concentration and is listed in

the Table 4.12. Higher degree of structural disorder is produced due to high rare earth-contents

[40] which is accredited to the difference in size of dopant and host ions at various hexagonal

conduction sites. Enhanced ion–ion interactions are expected and subsequently higher values of

the exponent n. Higher value of n increase the energy penalty that these correlations impose on

long-range or dc ionic conductivity. This elucidates the increasing difference found between Edc

and EAC (larger value of n).

Table 4. 11: Grain size, Drift mobility, Dielectric loss, Tangent Loss, AC conductivity (at 1MHz)

of (Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10)


Impedance can be written in terms of real and imaginary components of impedance and follow

the relation;

|Z| =Z΄+ j Z΄΄ (4.16)

The values of both resistive (Z΄) and reactive (Z΄΄) components of impedance are listed in Table

4.12. It has been found that the values of impedance and its components increase with (Eu-Ni)

substitution which is very much consistent with compositional dependence of AC conductivity,

i.e increase in impedance results in decrease in AC conductivity. . Fig. 4.40 and inset show the

variation of the impedance (Z) with frequency. It is found that the magnitude of Z decreases with

Compositional Formula Grain

size (nm)

µd (cm2v-1s-1) Dielectric

loss

Tangent

Loss

σAC(Ω-cm)-1

Sr2Co2Fe12O22 73 3.3×10-12 2.31 0.14 1.3×10-4

Sr2Co1.8Ni0.2Eu.02Fe11.98O22 92 4.1×10-13 2.00 0.13 1.1×10-4

Sr2Co1.6Ni0.4Eu.04Fe11.96O22 165 1.1×10-13 1.73 0.12 9.6×10-5

Sr2Co1.4Ni0.6Eu.06Fe11.94O22 184 2.9×10-14 1.44 0.11 8.0×10-5

Sr2Co1.2Ni0.8Eu.08Fe11.92O22 269 9.1×10-15 1.16 0.09 6.5×10-5

Sr2Co1Ni1Eu0.1Fe11.90O22 225 2.9×10-15 0.96 0.08 5.4×10-5


116

the increase of frequency, indicating increase in AC conductivity. It also indicates the

semiconducting type behavior in these systems.

Table 4. 12: DC activation energy, exponential factor n, AC activation energy, real and

imaginary parts of electric modulus and impedance at frequency of 1MHz of (Eu-Ni) substituted

hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

0.0 5.0x108

1.0x109

1.5x109

2.0x109

2.5x109

3.0x109

0

20000

40000

60000

80000

100000

120000

0.0 5.0x108

1.0x109

1.5x109

2.0x109

2.5x109

3.0x109

0

50

100

150

200

250

300

350

400

450

500

550

600

650

Impi

denc

e Z

Ferequency (Hz)

x= 0.0 y=0.0

x=0.2 y=0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Fig.4. 40: Variation in impedance with frequency of (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

Compositional Formula (EDC1+EDC2)/2 n EAC M΄ M΄΄ Z΄ Z΄΄

Sr2Co2Fe12O22 0.431 0.818 0.078 0.061 0.0089 32762 577

Sr2Co1.8Ni0.2Eu0.02Fe11.98O22 0.443 0.924 0.033 0.067 0.0092 36143 771

Sr2Co1.6Ni0.4Eu0.04Fe11.96O22 0.466 0.989 0.004 0.073 0.0095 44669 971

Sr2Co1.4Ni0.6Eu0.06Fe11.94O22 0.469 0.958 0.019 0.077 0.0087 62660 819

Sr2Co1.2Ni0.8Eu0.08Fe11.92O22 0.474 0.974 0.012 0.082 0.0080 89319 886

Sr2Co1Ni1.0 Eu0.1Fe11.90O22 0.484 0.976 0.011 0.087 0.0074 96183 1141


117

3.5.2 Cole–Cole plots

The appearance of broad Debye peak in these plots shows the existence of relaxation process in

all the studied samples. To separate the grain and grain boundary contributions, complex

impedance plane plots (Cole–Cole plots) have been investigated. The total electrical conductivity

is governed by the grain and grain boundary contributions for ferrites. The impedance

measurements data gives both resistive (real) and reactive (imaginary) components for a

material. It can be demonstrated in terms of any of the four complex variables i.e. permittivity

(ε*), admittance (Y*), impedance (Z*), electric modulus (M*) and dielectric loss (tan δ) in a

complex plane plot (Nyquistplot).Their relation to one another is as follows [8, 45];

tanδ = ε΄΄/ ε΄ = Y΄΄/Y = Z΄΄/ Z΄΄= M΄΄/ M΄ (4.17)

In the present studies only one semicircle was obtained , proposing a major contribution from the

grain boundary and one incomplete semicircle at x= 0.2, y= 0.02 is due to grain boundary

conduction, The incomplete circular arc in the given frequency region shows that grain boundary

resistance is out of measurement scale or presence of some additional relaxation phenomena

which occurred outside the measured frequency range [8, 96]. Hence, conductivity in the

investigated samples is mainly governed due to the grain boundary contribution. The

nanocrystalline samples are characterized by small grain size and large number of grain

boundaries. In order to study the frequency dependences of the interfacial polarization effect,

which generates electric charge accumulation around the ceramic particles, displacing relaxation

peaks, electrical modulus (M) was used and can be written in term of both resistive (real) and

reactive (imaginary) components as given below;

M = 1/ε* = 1/(ε΄-jε΄΄)= M΄-jM΄΄ (4.18)

Figs.4.41-4.42 show the variation of both real and imaginary parts of electric modulus against

frequency and concentration dependent values are listed in the Table 4.12. The appearance of

loss peaks in imaginary parts of electric modulus against frequency for the present investigated

samples show the relaxation process occurred with the change in frequency in the polycrystalline

(Eu–Ni) substituted Sr2Co2Fe12O22 hexaferrites. The peak formed when the jumping frequency of


118

charge carriers approximately becomes equal to external applied AC field [97]. The Maxwell–

Wagner model provides for the behavior of complex conductivity in heterogeneous systems with

two or more phases [46, 98]. In a heterogeneous system, if the region of grain boundary occupies

a large volume, the graph of the modulus (M*=1/ε*) M΄΄ versus M΄ provides better information

about the semicircles. It suggests that there is a probable relationship between the behavior of

grain boundary and the appearance of the peaks of M΄΄as a function of frequency. In second case,

if the region of continuity of the grain boundary occupies a small volume, the spectrum of

impedance (Z΄΄ versus Z΄) provides better visualization of the semi circles in the plane. Since

there is a probable relationship between the behavior of grain boundary and the appearance of

the peaks of Z΄΄ as a functions of frequency, first case is in great agreement with our present

experimental findings.

The complex impedance (Cole- Cole) plots are shown in Fig. 4.43. At lower frequency i.e. left

side of the semicircle is as a result of grain resistance [3]. While the intermediate frequencies

represent grain boundary contribution [50]. Taking into account higher frequency region,

extreme right side is plotted for the whole resistance of both grain and grain boundaries[3].

Substitution makes comparatively low influence on the grain resistance, but leads to a

remarkable rise of grain boundary resistance. Therefore, the conduction mechanism observed in

complex impedance measurement is in agreement with the AC conductivity, mentioned earlier.

Critically speaking, only grain boundary contribution is clearly observed from cole-cole plots as

compared to the grain contribution. Moreover the resistance of the grain boundary increases with

increasing Eu contents. The dominant conduction mechanism in ferrites is the hopping

mechanism, which is an easy electron transfer between Fe2+ and Fe3+. Increasing substitution

level of Eu at the expanse of Fe will restrain the electron transfer between Fe2+ and Fe3+ thereby

having effect on grain boundary resistance by subsequent substitution of Eu element. High

resistance regions are found at grain boundaries to impede conductivity. The high resistance of

the grain boundary will determine the resistivity and dielectric properties.


119

14 16 18 20 22

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

x= 0.0 y=0.0

x=0.2 y=0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

real

par

t of e

lect

ric m

odul

us(M

)

lnf (Hz)

Fig.4. 41: Variation in Real part of electric Modulus with frequency of (Eu-Ni) substituted

hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10) at room temperature.

14 16 18 20 22

0.00

0.01

0.02

0.03

0.04

0.05

0.06

imag

inar

y pa

rt of

ele

ctric

mod

ulus

(M)

lnf(Hz)

x= 0.0 y=0.0

x=0.2 y=0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Fig.4. 42: Variation in imaginary part of electric Modulus with frequency of (Eu-Ni) substituted

hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10) at room temperature.


120

0.080 0.088

0.010

0.012

0.014

x=0.0, y= 0.0M

M

0.11 0.12 0.13

0.009

0.010

0.011

0.012

x=0.4, y= 0.04

M

M

0.150 0.155 0.160 0.165

0.0075

0.0080

0.0085

0.0090 x=0.6, y= 0.06

M

M

0.190 0.195 0.200 0.205 0.210 0.215

0.0100

0.0105

0.0110

0.0115

0.0120

x=0.8, y= 0.08

M

M

0.22 0.24

0.030

0.035

0.040

0.045

0.050

0.055

0.060

x=1.0, y= 0.10

M

M

Fig.4. 43: Cole–Cole plots of (Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x =

0.00–1.00; y = 0.00–0.10).


121

0.0 5.0x108

1.0x109

1.5x109

2.0x109

2.5x109

3.0x109

0

1000

2000

3000

4000

5000

6000

7000

8000

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Q fa

ctor

Frequency (HZ)

Fig.4. 44: Variation of Q values with frequency of (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).


Fig. 4.44. Shows the variation in Quality Factor (Q values) with change in frequency for Eu-Ni

substituted Co2Sr2Fe12O22 ferrites. It is observed that the values of quality factor (Q) are

maximum at higher frequency region. Thus high Q values and a resonance frequency above 2

GHz clearly suggest that these materials can be used in high frequency multilayer chip inductors

[99].


4.2.6.1 Hysteresis Loops

Figs.4.45-4.46 represents the MH-loops for Sr2Co(2-x)NixEuyFe(12-y)O22 ferrites for both in-plane

(H applied parallel to the sample surface) and out-of-plane (H applied perpendicular to the

sample surface) orientations. The values of saturation magnetization (Ms), coercivity (Hc) and

remanent magnetization (Mr) were calculated from the MH-craves. The changes in magnetic

properties such as Ms, Hc, Mr and nB are due to the influence of the cationic stoichiometry and

their occupancy in the specific sites [100]. So the knowledge of distribution of metals ions in S

and T blocks among the distant sites is very essential to describe the magnetic properties of Y-

type hexaferrite.


122


The variation of the saturation magnetization (Ms) and remanence (Mr) are shown in the

Figs.4.47-4.48 for both cases i.e, in-plane and out-plane orientation. In the present experimental

findings the variation of the saturation magnetization (Ms) has been explained on the basis of

metal ions distribution in different sites lying in the both block.There are six non-equivalent sites

named as 6c1v, 3av1, 18hVI, 6cv1, 6cIV and 3bv1. Crystallographic and magnetic properties of these

six sites are listed in Table 4.6 (taken from Ref.[53])

The super exchange interaction play a crusal role in the magnetic ordering of S-block

magnetization between octahedral 3av1 and tetrahedral 6c1v sites of metal ions. The replacement

of Co2+ (having magnetic moment (3uB) by Ni2+ ions (having magnetic moment 2.3uB) which

had preferred octahedral 3av1 -site occupancy consequently reduced the super exchange

interaction between 6c1v and 3av1 sites. In the second case as Eu (Zero magnetic moment )

replaced Fe (5uB magnetic moment), it is interesting to recall the fact that Eu3+ had also

octahedral 3av1-site occupancy, it can concluded that replacement of Co2+ and Fe3+ by Ni2+ and

Eu3+ at octahedral site dilute the magnetization of this site. Whereas magnetization of tetrahedral

6c1v site remain constant. As net magnetization is equal to M (3av1)tet-M (6c1v)oct so it was found to

decrease.

T block consist of three octahedral ions per unit formula belonging to the two different sub

lattices, i.e 6cvI and 3bv1. Both these octahedral sites lay on a vertical threefold axis [53]. It is

worth noting that the configuration of the ion at octahedral site 3bv1 issuch that, it shears two

faces of its coordination figure with the adjacent 6cvI, ions [53]. Such a structural configuration is

accountable for a higher potential energy. These sites are likely to be filled by low charge ions

because the existences of a stronger electrostatic repulsion between the cations [53]. As a results

a less magnetic divalent ions with a marked preference for the octahedral coordination.


123

-10000 -5000 0 5000 10000

-80

-60

-40

-20

0

20

40

60

80

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Ms

(em

u/g)

Applied Feild H(Oe)

Fig.4. 45 : In-plane MH-loop of (Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x

= 0.00–1.00; y = 0.00–0.10).

-10000 -5000 0 5000 10000

-80

-60

-40

-20

0

20

40

60

80

x= 0.0 y=0.0

x=0.2 y= 0.02

x=0.4 y=0.04

x=0.6 y=0.06

x=0.8 y=0.08

x=1.0 y=0.1

Ms

(em

u/g)

Applied Feild H (Oe)

Fig.4. 46: Out-plane MH-loop of (Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x

= 0.00–1.00; y = 0.00–0.10).


124

Fig.4. 47: In-plane and out-of-plane saturation magnetization versus (Eu-Ni) substituted

hexaferrites, Sr2Co(2-x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).

In the present case Sr2Co(2-x)NixEuyFe(12-y) O22, metallic ions such as Fe3+, Co2+ and Ni2+ are

located, in non-equivalent octahedral and tetrahedral sites. The Co2+ cation having strongly

magnetocrystalline anisotropic element and its partial replacement by Ni2+ may lead to complex

magnetic properties. The Ni2+ and Co2+ ions have stronger octahedral site preference than

Fe3+[101] and, thus, Ni2+, and Co2+ occupy octahedral sites. It is understood fact that Ni2+ has a

smaller magnetic moment than Co2+ [101], therefore the substitution of nickel for cobalt leads to

the reduction of saturation magnetization. The decrease of saturation magnetization (Ms) and

retentivity (Mr) may also be explained on the basis of the fact that occupation of either 6cv, or

3bv by Ni ion may results in drastic variations in the magnetic configuration with respect to the

usual Gorter scheme. Moreover it is believed that occupation of octahedral sites in T block by

Ni2+ ion leads to the cancellation of the antiferromagnetic bv1-clv*, interaction which is the

strongest one in the Y-structure.

0.00 0.02 0.04 0.06 0.08 0.10

20

30

40

50

60

700.0 0.2 0.4 0.6 0.8 1.0

In-plane

Out-plane

Ms

(em

u/g)

Eu-Ni contents

Ni contents


125

0.00 0.02 0.04 0.06 0.08 0.10

10

12

14

16

18

20

22

24

26

0.0 0.2 0.4 0.6 0.8 1.0

Mr (

emu/

g)

Eu contents

In-plane

out-plane

Ni contents

Fig.4. 48: In-plane and out-of-plane remanence versus (Eu-Ni) concentration for Sr2Co(2-

x)NixEuyFe(12-y)O22 ferrites.

0.00 0.02 0.04 0.06 0.08 0.10

600

800

1000

1200

1400

1600

1800

2000

2200

2400

0.0 0.2 0.4 0.6 0.8 1.0

out-plane

Out-plane

Coe

rciv

ity (O

e)

Eu contents

Ni contents

Fig.4. 49: In-plane and out-of-plane coercivity of (Eu-Ni) substituted hexaferrites, Sr2Co(2-

x)NixEuyFe(12-y)O22, (x = 0.00–1.00; y = 0.00–0.10).


126

4.2.6.3 Coercivity Hc

Coercivity measured both in-plane and out-of-plane orientations of ferrite system increases as a

function of Eu-Ni content as shown in Fig. 4.49. The increase in coercivity may be attributed to

to the fact that higher the porosity higher will be the coercivity [102]. Domain wall pining may

take place at the grain boundaries, which results in increase in coericivity The saturation

magnetization is related to Hc through the Brown’s relation [58, 103] Hc = K1/ μoMs where K1 is

magnetocrystalline anisotropy, μo is vacuum susceptibility, Ms is saturation magnetization and

Hc is coercivity. Here Hc is inversely proportional to Ms, this is consistent with our experimental

results and with the results reported by other researchers. The coercivity (Hc) increases rapidly

with Eu-Ni contents. Furthermore, the increasing behavior of coercivity Hc with increasing

substitution level can be clarified on the basis the aspect ratio (a/c) and values of aspect ratio are

listed in the Table 4.8. In that case the coercivity could be written interm of following equation

[59]:

Hc= 0.48(K1/Ms− NdMs) (4.19)

Where Ms is the saturation magnetization, K1 is the magneto-crystalline anisotropy constant and

Nd is the demagnetizing coefficient relating to the shape anisotropy. As the aspect ratio decreases

with increasing substitution level could reduce the demagnetizing factor and thus enhance Hc

[59].In conventional longitudinal magnetic recording (LMR), the magnetization in the bits is

directed parallel to the surface of disk. While in perpendicular recording media (PRM), the

‘‘magnetic bits’’ are arranged point up or down perpendicular to the surface of disk. The well-

liked elucidation for the usage of PMR is that it can provide 3 times additional storage density of

LMR. It is understood that inherently thermally more stable magnetic samples have high values

of the coercivity.

Thermal stability of the magnetic samples is proportional to the product of uniaxial anisotropy

constant K1 times volume, higher coercive material will have large the product. So in this way,

we can conclude that PRM needs a high coercivity medium. If the coercivity is high enough

above 1200 Oe, then hexaferrite materials can be useful for the perpendicular recording media

which is a new developing technology in the recording media [61]. In the present case

investigated samples which are Y-type hexaferrite can be used in PRM due to high value of

coercivity 2300Oe which is analogous to the those of M-type and W-type hard magnetic


127

materials. If Hc>Mr/2, the materials are hard magnets and if Hc<Mr/2, then the materials are

semi-hard magnets [62, 104].

Fig.4. 50 (a-f): Fitted curve of Ms for (Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-

y)O22, (x = 0.00–1.00; y = 0.00–0.10) calculated by law of approach to saturation.

4000 5000 6000 7000 8000 9000 10000

56

58

60

62

64

66

Data: Data2_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.0017

R^2 = 0.9999

Ms 77.11252 ±0.33049

a 1779.85621 ±49.94247

b -2159703.94919 ±175899.06031

chi 0 ±--

Ms

(em

u/g

)


a

4000 5000 6000 7000 8000 9000 10000

38

39

40

41

42

43

44

45

46

Data: Data4_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00006

R^2 = 0.99999

Ms 54.21506 ±0.06072

a 1628.74133 ±13.29685

b -1124673.07141 ±47625.17467

chi 0 ±--

Ms (

em

u/g

)


b

4000 5000 6000 7000 8000 9000 10000

35

36

37

38

39

40

41

42

Data: Data6_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00005

R^2 = 0.99999

Ms 49.58694 ±0.05553

a 1628.74133 ±13.29685

b -1124673.07141 ±47625.17467

chi 0 ±--

Ms (

em

u/g

)


c

4000 5000 6000 7000 8000 9000 10000

25

26

27

28

29

30

31

32

Data: Data8_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00112

R^2 = 0.99984

Ms 36.61002 ±0.26944

a 1404.33686 ±88.70483

b 291374.82713 ±321059.2783

chi 0 ±--

Ms

(em

u/g

)


d

4000 5000 6000 7000 8000 9000 10000

22

23

24

25

26

27

28

Data: Data10_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00087

R^2 = 0.99984

Ms 32.30295 ±0.23774

a 1404.33686 ±88.70483

b 291374.82295 ±321059.27825

chi 0 ±--

Ms

(em

u/g

)


e

4000 5000 6000 7000 8000 9000 10000

19

20

21

22

23

24

25

26

Data: Data12_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00097

R^2 = 0.99988

Ms 30.60276 ±0.25169

a 1493.62556 ±98.74433

b 1568063.2184 ±371423.46513

chi 0 ±--

Ms

(em

u/g

)


f


128

The synthesized hexaferrite materials in the present study have Hc>Mr/2. It is believed

that if samples have Hc>Mr/2, can be used for high frequency applications.

Fitted curves for saturation magnetization of Eu-Ni substituted Sr2Co(2-x)MnxTbyFe(12-y)O22

hexaferrites system are shown in the Figs.50(a-f). The observed difference in the estimated and

calculated values of saturation magnetization is ascribed to the insufficient field applied in the

experimental case which is deficient to align the all randomly oriented magnetic moments in one

direction (in the direction of external field). Whereas in theoretical case infinite field is applied

to orient all the magnetic moment in the direction of the external field to get the maximum

saturation magnetization. Intensive inspection of the Figs. 50(a-f).indicates that insufficient field

is applied in experimental case. Which clearly suggests that further magnetization might be

accomplished by increasing the external field, which will offer the close agreement among

experimental and theoretical values of saturation magnetization. The estimated values of

saturation magnetization are listed in the Table 4.13.

Table 4. 13: Estimated saturation magnetization (Ms), Anisotropy constant( K), Magnetic

moments (nB) and Squareness Ratio of (Eu-Ni) substituted hexaferrites, Sr2Co(2-x)NixEuyFe(12-

y)O22, (x = 0.00–1.00; y = 0.00–0.10).

Co

mp

osi

tio

nal

Form

ula

Esti

mat

ed

Ms

(em

u/g

)

K (

erg

/cm

3)

(in

-pal

ne

)

K (

erg

/cm

3)

(ou

t-p

lan

e)

nB (

em

u/g

)

(in

-pla

ne

)

nB(e

mu

/g)

(Ou

t-p

lan

e)

Ms/

Mr

in-

pla

ne

Ms/

Mr

0u

t-p

lan

e

Sr2Co2Fe12O22 77.11 2.34×104 2.10×104 15.27 14.64 0.41 0.30

Sr2Co1.8Ni0.2Eu.02Fe11.98O22 54.21 2.16×104 2.61×104 10.86 10.43 0.38 0.43

Sr2Co1.6Ni0.4Eu.04Fe11.96O22 59.58 2.14×104 2.54×104 9.94 9.63 0.39 0.42

Sr2Co1.4Ni0.6Eu.06Fe11.94O22 36.61 2.38×104 2.81×104 7.38 7.15 0.37 0.53

Sr2Co1.2Ni0.8Eu.08Fe11.92O22 32.30 2.16×104 2.78×104 6.51 6.30 0.41 0.42

Sr2Co1Ni1Eu0.1Fe11.90O22 30.69 2.45×104 2.84×104 6.07 5.86 0.44 0.56


129

4.2.6.3 1 Magnetic Moment (nB)

The values of magnetic moment (nB) for Both in-plane and out of planemeasurement are listed in

Table 4.13. Generally speaking both the the saturation magnetization (Ms) and magnetic moment

(nB) show alike behavior. In our present experimental work, behavior of magnetic moment is

consistent with the saturation magnetization as both decrease with increasing (Eu-Ni) contents,

Decreasing behavior of magnetic moment is attributed to the weakening of super exchange

interactions, because Fe–O–Fe super exchange weaken with Re substitution at the expanse Fe

Similar behavior has already been reported by many researchers [65-67].


Squreness ratios (Mr/Ms) of (Eu-Ni) substituted Co2Sr2Fe12O22 hexaferrites were calculated from

VSM data for Both in-plane and out of plane magnetic measurement and are presented in the

Table 4.13. The values of inplane squreness ratios (Mr/Ms) ranging from 0.41 to 0.65 whereas in

case of out of plane measurement it varies from 0.30 to 0.62. However, squreness ratio is well

below of common value ~1 for single domain isolated ferromagnetic particle. The relative higher

values of squareness ratio is obtained particularly at higher substitution level proposes that some

particles may belong to as single domain. While, in case of pure CoY ferrite lower value of

squareness ratio indicates that the particles are entirely randomly oriented and exist in multi

domains. By assuming magnetic particles to be isolated (exchange interacting spin) single

domains [65], using the equation (K=HcMs/2) anisotropy constant was calculated. The values of

anisotropy constant for both in-plane and out of plane measurementare listed in the Table 4.13.

The values of magnetocrysatlline anisotropy constant are less than already reported for single

doman different ferrites. This shows that grains are not single domains and anisotropy

contribution is not uniaxial [66, 67].

4.3 Sm-Ni Substituted Y-type Hexaferrites.

4.3.1 Structural Analysis Fig.4.51. shows the typical X-ray diffraction patterns of Y-type hexaferrite samples with

chemical composition Sr(2-x)Sm(x)Co2NiyFe(12-y)O22 (x= 0.00-0.10; y= 0.00-1.25). X΄ pert

highscore software was used to index the diffraction patterns. The indexing of patterns reveals


130

that there are no traces of impurities present in the samples, confirming single phase Y-type

hexagonal ferrite (JCPDS card number 00-019-019-0100). Increased intensity of the peaks

indicate batter crystallinaty single phase substituted Sr-Co-Y type hexaferrites indicates that Sm-

Ni are completely soluble in the lattice..

Substitution of Sm and Ni at the cost of Sr and Fe respectively results in very small variation in

the lattice parameters. Slight decrease in “a” and “c” that varies from 5.88 to 5.802Å and 43.37 to

43.17Å respectively as shown in the Table 4.14. The variation in lattice parameter “c” as

compared to lattice parameter “a”[68], is in tremendous agreement with the already reported

results

The distortion in lattice parameters with increasing Sm –Ni concentration is mainly attributed

to the difference in ionic radii of the host ions and the substituted ones. The decrease in lattice

parameters is mainly due to the smaller ionic radius of the doped Sm3+ (0.964 Å) than that of the

host Sr2+ (1.12Å)suppresses the negative effect of Ni2+ (0.69 Å) substitution for Fe3+ (0.645Å).

Similar elucidation has already been reported by many researchers [2, 69]. The values of bulk

density increased with substitutions (Sm-Ni) in Sr2Co2 Fe12O22 as listed in Table 4.14.

Table 4. 14: Compresses the compositional formula, Lattice parameters a and c, c/a, volume of

cell, Bulk density, X-ray density and percentage porosityof Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x =

0.00–0.10; y = 0.00–1.25).

Compositional Formula a(Aᵒ) c(Aᵒ) c/a V(Aᵒ)3 db(g/cm3) dx(g/cm3) P٪ Crystallite size( D)

Sr2Co2Fe12O22 5.883 43.37 7.37 1298 4.92 5.05 2.57 31.8

Sr1.98 Sm.02Co2Ni0.25Fe11.75O22 5.835 43.31 7.42 1276 4.93 5.14 4.08 35.5

Sr1.96 Sm.04Co2Ni0.50Fe11.50O22 5.831 43.25 7.41 1273 4.95 5.16 4.06 45.2

Sr1.94 Sm.06Co2Ni0.75Fe11.25O22 5.826 43.22 7.41 1270 4.96 5.18 4.24 57.7

Sr1.92 Sm.08Co2Ni1.00Fe11.00O22 5.812 43.19 7.43 1263 4.97 5.20 4.42 63.3

Sr1.90 Sm0.1Co2Ni1.25Fe10.75O22 5.802 43.17 7.44 1258 4.99 5.24 4.77 70.4


131

The higher value of density of the substituted ions Sm3+ and Ni2+ 7.52 and 8.908 g·cm−3

respectively as compared to the host density i.e. of Sr2+ and Fe3+ 2.64 and 7.874 g cm-3

respectively is mainly responsible for the increased bulk densities. The X-ray density (dx) varies

from 5.05 to 5.24g·cm-3 with the (Sm-Ni) substitution as shown in Table 4.14. This increasing

behavior of X-ray density (dx) is principally due to slight decrease in the cell volume of the

corresponding samples, as the X-ray density is inversely related to cell volume the [105]. It is

evident that Sm-Ni substitutions increase the porosity to a considerable extent as shown in Table

4.14. This slight increase in porosity may be consequences from two distant sources, i.e.

intragranular or intergranular depending on whether pores exist within the grains or pores occurs

in the grain boundaries. When the grain growth rate is rapid, pores are remained behind the fastly

moving grain boundaries and are trapped within the grains. Exclusion of this intragranular

porosity is practically impossible, giving poor mechanical properties [72]. Table 4.14, shows

that crystallite size (D) increases with (Sm-Ni) substitution due to grain growth at high sintering

temperature.

Fig.4. 51: XRD patterns of (Sm-Ni) substituted hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x =

0.00–0.10; y = 0.00–1.25), hexaferrites.




132

4.3.2 EDX Analysis The chemical composition of the investigated samples was determined by EDX analysis. The

elemental analyses of the EDX profiles of the samples are given in Table 4.15. It is clear from

the stoichiometric analysis that Sm and Ni contents increased whereas Sr2+ and Fe3+contents

decreased. EDX spectrums of the all the studied Y-type hexaferrites samples are depicted in the

Figs.4.52 (a-f). The presence of the contents i.e. Sr, Co, Sm, Ni and Fe are confirmed by the

characteristic peaks in the EDXS spectra.

Table 4. 15: Elemental analysis of Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–1.25),

hexaferrites. Obtained from EDX.

Sample Name Elements

Fe Sr Co Ni Sm

Sr2Co2Fe12O22

Theoretical 69.57 18.19 12.23 0 0

Experimental 69.51 18.04 12.18 0 0

Content 11.98 1.98 1.96 0 0

Sr1.98 Sm.02Co2Ni0.25Fe11.75O22

Theoretical 67.98 17.97 12.21 1.52 0.31

Experimental 66.99 17.85 12.19 1.65 0.28

Content 11.71 1.97 1.97 0.23 0,019

Sr1.96 Sm.04Co2Ni0.50Fe11.50O22

Theoretical 66.40 17.75 12.18 3.03 0.62

Experimental 65.87 17.78 12.01 2.99 0,57

Content 11.45 1.95 1.96 0.48 0.037

Sr1.94 Sm.06Co2Ni0.75Fe11.25O22

Theoretical 64.82 17.53 12.16 4.54 0.93

Experimental 64.32 17.45 12.09 4.31 0.85

Content 11.19 1.94 1.98 0.74 0.057

Sr1.92 Sm.08Co2Ni1.00Fe11.00O22

Theoretical 63.25 17.32 12.13 6.04 1.23

Experimental 62,87 17.17 12.06 5.95 1.18

Content 10.98 1.91 1.98 0.98 0.077

Sr1.90 Sm0.1Co2Ni1.25Fe10.75O22

Theoretical 61.69 17.10 12.11 7.54 1.54

Experimental 61.41 17.03 12.12 7.11 1.48

Content 10.74 1.89 1.99 1.19 0.095


133

Experimental calculation and theoretical calculation of all stoichiometric contents are in close

agreement with each other. The increment in substituents (Sm-Ni) and decrease in the (Fe-Sr)

contents at systematic rate undoubtedly propose that the prepared samples conserved the precise

contents stoichiometry.

Fig.4. 52 (a-f): EDX spectra for Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–1.25),

hexaferrites.


134

4.3.2 Scanning Electron Microscopy

Fig.4.53 shows SEM micrographs of Sm-Ni substituted hexferrite samples. The calculated grain

size is found to be in the range of 73.81–246.41nm and listed in the table 4.16. However some

particles exist in agglomerates, this may be attributed to the fact that chemical reaction during the

sintering course play very critical part for the creation of these agglomerates.

Fig. 4. 53 (a-f): SEM images for Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x= 0.00–0.10; y = 0.00–1.25).


135

It is very substantial to understand that relatively weak Van der Waals bonds and magnetic

forces play vibrant role to grip together these agglomerates.Under the influnce of the persistent

forces the individuality of the agglomerates can be preserved. All the investigated hexagonal

ferrite samples reflect a well-defined platelet-like shape of the grains,which is in agreement with

already reported work by many researchers [3, 106]. This typical shape of grains is a suitable for

microwave absorbing purposes [107].



The room temperature resistivity values as a function of Sm-Ni contents are given in the

Table 4.16 for Sr(2-x)Sm(x)Co2NiyFe(12-y)O22.. Structurally ferrite ceramics consist of less-

conductive grain boundaries and highly conductive grains. In general, the resistivity of grain

boundaries is chiefly accountable for DC resistivity of ferrites [73]. Numerous aspect like crystal

structure perfection, microstructural homogeneity, grain size, stoichiometric composition,

impurity levels, density (porosity) are responsible for the DC resistivity of ferrite ceramics [74].

Table 4. 16: Values of Grain size, Resistivity, Mobility, Dielectric Loss, Tangent loss and AC

conductivity at 1MHZ of Sm-Ni substituted hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x= 0.00–

0.10; y = 0.00–1.25).

Out of these, the porosity is primarily substantial.The increases in DC resistivity of ferrite owing

to the presence of porosity because vacuum/air behaves like insulator, if the pores are closely

stuck and their distribution is homogenous. In the present experimental findings, the slight

increase in porosity with varying Sm-Ni contents may be responsible for increase in the DC

Compositional Formula Grain size

(nm)

Resistivity

(ohm-cm)

Mobility

(cm2v-1s-1)

Dielectric

Loss (ε΄΄)

Tangent loss

( ε΄΄/ ε΄)

σAC (Ω-cm)-1

Sr2Co2Fe12O22 73.81 1.23×106 3.36×10-12 2.31 0.166 1.29×10-4

Sr1.98 Sm.02 Co2Ni0.25Fe11.75O22 103.84 1.54×107 1.53×10-13 1.90 0.160 1.06×10-4

Sr1.96 Sm.04 Co2Ni0.50Fe11.50O22 163.40 2.42×107 9.94×10-14 1.50 0.148 8.35×10-5

Sr1.94 Sm.06 Co2Ni0.75Fe11.25O22 213.97 1.91×108 1.28×10-14 1.07 0.130 5.99×10-5

Sr1.92 Sm.08 Co2Ni1.00Fe11.00O22 215.76 3.80×108 6.61×10-15 0.86 0.119 4.79×10-5

Sr1.90 Sm0.1 Co2Ni1.25Fe10.75O22 246.41 3.72×109 6.89×10-16 0.59 0.100 3.29×10-5


136

resistivity. The increase in DC resistivity with Sm-Ni contents might be attributed to the fact that

samarium (Sm) and nickel (Ni) are more resistive (9.4x10-7and 6.93 x10-8ohm-mat 293K,

respectively) when matched to Strontium (Sr) and Iron(3.64 x10-8and 9.71 x10-8ohm-mat 293K,

respectively). Room temperature resistivity improve with varying (Sm-Ni) contents, as Sm3+

contents prefer to reside in octahedral sites followed by the migration of some Fe3+ ions to

hexagonal tetrahedral sites and converting them into Fe2+ ions. As a consequence concentration

of Fe3+ ions is lowered at octahedral sites. Hence the resistivity increased by lowering the

hopping probability between Fe3+ and Fe2+ ions at octahedral sites.

1.5 2.0 2.5 3.0 3.5

10

12

14

16

18

20

22

x= 0.00 y=0.0

x=0.02 y= 0.25

x=0.04 y=0.04

x=0.06 y=0.06

x=0.08 y=0.08

x=0.10 y=0.1

Log

(o

hm-c

m)

1000/T (K-1

)

Fig. 4.54: Temperature dependent resistivity of Sm-Ni substituted hexaferrites, Sr(2-



Temperature dependent DC resistivity for the present investigated samples is shown in the

Fig.4.54 With increasing temperature the resistivity decreases showing the typical

semiconducting behavior [102]. The hoping probability of thermally activated charge carriers

increase between the various hexagonal sites with rise of temperature. Consequently the


137

resistivity of the ferrite materials decreases with the rise of temperature. Impurities play a crucial

role in conduction of ferrites at room temperature, whereas at high temperature, it is ascribed to

polaron hopping [75]. The temperature dependent electrical resistivity follows the Arrhenius

equation.

ρ = ρ0 exp ΔE/kBT (4.20)

where “ρ” is resistivity, “KB” represents Boltzmann’s constant and “∆E” is the activation

energy, which is required for electron hopping [76]. The activation energies are calculated in the

both ferri and para regions from the slopes of the Arrhenius plot and their values are listed in

Table 4.17. The variation in activation energy as a function of Sm-Ni contents exibits a similar

behavior as that of room temperature resistivity. Furthermore, the elucidation offered for

electrical resistivity stands same for activation energy. It can be observed from Table 4.17, that

the activation energy in the paramagnetic region is greater than that in the ferrimagnetic region.

Table 4. 17: Slopes and activation energies of farrimagnetic and paramagnetic regions of Sm-Ni

substituted hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–1.25).

This result is consistent with the experimental findings purposed by Irkin and Turov

[13].The hopping of electrons between Fe2+ and Fe3+ ions play a cruel role in the conduction of

ferrite at a lower temperature i.e. below Curie temperature (ferrimagnetic region ) [13], whereas

above Curie temperature (paramagnetic region ) i.e. at a higher temperature is due to hopping of

polarons [102]. It is clear that poloran hopping required relatively more energy than that of


Slope Activation energy FM PM

Region Region

M1 M2

FM PM

Region Region

E1 (eV) E2 (eV) ∆E=E2-E1(eV)

Sr2Co2Fe12O22 1.78 2.61 0.352 0.51 0.164

Sr1.98Sm.02Co2Ni0.25Fe11.75O22 1.79 2.69 0.354 0.53 0.158

Sr1.96Sm.04Co2Ni0.50Fe11.50O22 1.80 2.87 0.356 0.56 0.204

Sr1.94Sm.06Co2Ni0.75Fe11.25O22 1.84 2.92 0.364 0.57 0.206

Sr1.92Sm.08Co2Ni1.00Fe11.00O22 1.86 2.99 0.368 0.59 0.222

Sr1.90Sm0.1Co2Ni1.25Fe10.75O22 1.88 3.10 0.372 0.61 0.238


138

electrons hopping because in electron hopping the both types of charges freely move in the

crystal lattice. This is the key reason for lowering of the activation energy in ferrimagnetic than

paramagnetic region.

It can be seen that the transition temperature is in good agreement with the Curie temperature

and shows kink at about Curie point. Changes in slops of these resistivity curves are also

reported by several investigators [9, 108]. The size of the kink is degree of difference in

activation energies between ferri-magnetic and para-magnetic regions. It is observed from Table

4.17, that ∆E increases as resistivity increases. It has been reported in the various ferrites system

that value of electrical resistance and Structural peculiarities play a cruel role in determining the

size of kinks.

The previous investigations have shown that smaller kinks are characteristics of ferrite

which have large resistance and are in good agreement with our experimental findings. Normally

the variation in the slope is ascribed to change in conductivity mechanism as ferri-magnetic

material changes to para-magnetic at the Curie temperature. The jumping of holes between Ni3+

and Ni2+ ions and hopping of electrons between Fe2+ and Fe3+ ions are accountable for

conduction at lower temperature i.e. below Curie temperature. However at (higher temperature)

beyond Curie temperature is owing to polaron hopping [25, 26, 109]. The calculated values of

activation energies in the paramagnetic region (E2) are greater than 0.40 eV, which clearly

suggest that the conduction is due to polaron hopping [80].

Arrhenius plots show two distinct regions, it can be observed that thermal energy in first

region (ferro-region) is not enough to disturb the aligned spins of electrons. While, in the second

region (para-region) the thermal energy is appropriate to disturb all the aligned spins of

electrons. Fig. 4.55 shows the concentration dependence of Curie temperature (Tc) for the

present samples. It can be seen from the figure that Curie temperature (Tc) decreases

continuously with increasing (Sm-Ni) concentration. It is obvious to recall the fact that super

exchange interactions Fe3+–O–Fe3+ and the direct exchange interactions Fe3+-Fe3+ are the major

interactions in ferrimagnetic material[15, 108]. The decrease in curie temperature (Tc) with

increasing (Sm- Ni) concentrations may be caused by fluctuations in Fe3+–O–Fe3+ and Fe3+–Fe3+

angles, which tends to a decrease the magnetic interaction. Moreover, it has been examined that

samples containing RE ions show lower Curie temperatures than those without RE contents

[107].This is in fabulous similarity with our experimental findings.


139

0.00 0.02 0.04 0.06 0.08 0.10

420

440

460

480

500

520

540

560

0.00 0.25 0.50 0.75 1.00 1.25

Tc

(K)

Sm. Contents

Ni. contents

Fig.4.55: Variation of curie Temperature (Tc) for Sm-Ni substituted hexaferrites, Sr(2-


It is also known that magnetic moments of Fe3+ ions are collinearly settled because of the

persistence of super exchange interaction. The substitution of rare earth ions (Sm3+) at the

expanse of Fe3+ ions create partial disorder and fades Fe3+ –O–Fe3+ super exchange interactions,

where the valence conversation of the iron ions from Fe3+ with a high spin state (3d5 with 5µB) to

Fe2+ with a low spin state (3d6 with 4µB) [2, 110], this valence conversion results in deviation

from collinear to non-collinear arrangement, this appears to a reduce in the Curie temperature TC

[82]. Furthermore, the lessening of TC may be owing to the fact that Eu–Fe interactions on the B

sites are lesser than Fe–Fe interactions [17, 18].


The temperature dependent drift mobility for (Sm-Ni) substituted Co2Sr2Fe12O22 ferrite samples

is shown in the Fig. 4.56 The present investigated samples show a transition as already

mentioned in resistivity data at a specific temperature i.e. the drift mobility increases with the


140

rise in temperature and beyond the specific temperature, the drift mobility increases brusquely

with increase in the temperature. The (Sm-Ni) concentration dependent drift mobility of prepared

samples decreases and values are tabulated in the Table 4.16. The decline of drift mobility is may

be attributed to increase in resistivity by doping (Sm-Ni) ions.

250 300 350 400 450 500 550 600 650 700

0.00E+000

2.00E-011

4.00E-011

6.00E-011

8.00E-011

1.00E-010

1.20E-010

1.40E-010

1.60E-010

1.80E-010

x= 0.00, y= 0.00

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x=0.06, y= 0.75

x= 0.08, y= 1.00

x=0.10, y= 1.25

Mob

ility

(cm

2 v-1s-1

)

Temperature T(K)

Fig.4. 56: Change in Drift mobility Vs temperature for Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–

0.10; y = 0.00–1.25), hexa ferrites.

The measured values of drift mobility for the (Sm-Ni) substituted samples are in the range 10-12 –

10-15 cm2v-1s-1 K-1, which are slightly lower than the reported values of 10-11–10-14 cm2v-1s-1 K-1

[105]. These results can be elucidated on the basis of the electrical resistivity data of these

investigated samples. The initial increase in the drift mobility with increase in the temperature is

due to the decrease in the electrical resistivity in the temperature range which enhances the

mobility of the charge carriers. The increase in drift mobility above transition temperature is

attributed to the fact that the electrical resistivity further increases above this temperature and

consequently the mobility of charge carrier increases speedily.

4.3.5 Dielectric Properties The frequency dependent dielectric constant έ is presented in Fig. 4.57. It can be viewed that

dielectric constant decreases successively with increase of frequency, which is quite common

dielectric behavior of the ferrite ceramics and widely inspected by many investigators [18, 111,


141

112]. Tremendous decrease in the dielectric constant value in the low frequency region has been

observed.

14 16 18 20 22

-1

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

x= 0.0 y=0.00

x=0.2 y= 0.25

x=0.4 y=0.50

x=0.6 y=0.75

x=0.8 y=1.00

x=1.0 y= 1.25

Die

lect

ric C

onsta

nt (

lnf (Hz)

Fig.4. 57: Dielectric constant of (Sm-Ni) substituted Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–

0.10; y = 0.00–1.25), hexaferrites.

It is very remarkable to mention the occurrence of resonance peaks above the 2GH frequency.

Existence of these peaks at higher frequency may be justified as frequency of applied ac field

becomes equal to the jumping frequency of electrons between Fe2+ and Fe3+ [88]. The decrease

of dielectric constant with rise of frequency is due to the fact that under the action of applied

electric field the dielectric material shows induced electric moment. However the further

increases in frequency disturb synchronization of electron exchange between Fe2+ and Fe3+ ions

or the polarization of induced moments which consequently decrease the dielectric constant [89].

Fig. 4.58 shows the variation imaginary part of the dielectric constant (ε″) Vs frequency and its

concentration dependent values are listed in the Table 4.16. The variation of imaginary part of

the dielectric constant (ε″) with frequency is quite similar to the variation frequency dependent έ.

Maxwell–Wagner’s bi-layered model is quite beneficial to clarify the dielectric conduct of

ferrites ceramic [90, 91]. Deep review of this modal exposes that, ferrite contains obviously

insulating grain boundaries separated by conducting grains. Movements of charge carriers take


142

place under the action of an applied field. Massive resistance offered by grain boundaries is

advantageous to align the charge carriers at grain boundaries.

14 16 18 20 22

0.0

0.5

1.0

1.5

2.0

2.5

Die

lect

ric

Los

s

lnf (hz)

x= 0.00, y= 0.00

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x=0.06, y= 0.75

x= 0.08, y= 1.00

x=0.10, y= 1.25

Fig.4. 58: Dielectric loss of Sm-Ni substituted hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x =

0.00–0.10; y = 0.00–1.25).

Thus space charge polarization on grain boundaries is mainly due to the availability of free

charges at the grain boundary consequently, large dielectric constant is obtained. It is proven fact

that grain boundaries are dynamic in the low frequency region and grains are effective in the

high frequency region [29]. Thus at higher frequency low values of polarization are obtained

which tends to deteriorates the dielectric constant.

The dispersion in the dielectric constant favor existence of peaks in the tanδ(f) [24]. The (tan δ)

can be written in terms of the real and imaginary parts of the dielectric constant [74].

έ = (4.21)

ε″ = (4.22)


143

“ω” is the angular frequency where and are the relaxation time, dielectric constant at

very low and very high frequencies, respectively.

14 16 18 20 22

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

14 16 18 20 22

0

1

2

3

4

x= 0.00, y= 0.00

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x=0.06, y= 0.75

x= 0.08, y= 1.00

diel

ectri

c lo

ss fa

ctor

(tan

)

lnf (hz)

diel

ectri

c lo

ss fa

ctor

(tan

)

lnf (hz)

x=0.10, y= 1.25

Fig.4. 59: Dielectric loss Factor of (Sm-Ni) substituted hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22,

(x = 0.00–0.10; y = 0.00–1.25).

The deep observation of equation (4.21) suggest that the decrease of dielectric constant (έ ) is

more fast with increasing frequency, subsequently (έ ) is proportional to the 1/ ω2, where as

equation (4.22) shows that decay of ε″ is fairly slow, since (ε″) is proportional 1/ ω [92]. Hence,

the relatively quick decline of the (έ) than that of (έ΄) in the particular frequency range may be

due to the presence of peaks in the tanδ(f) ploted as in Fig.4.59. It shows variation of the tangent

loss factor Vs frequency, inset also shows the variation of the tangent loss factor Vs frequency

for the (x= 0.10, y= 1.25 substitution). Analogous variation of tanδ vs frequency has already

been reported previously [27]. Additionally appropriate illumination for presence of the peaks in

plot of tanδ vs the frequency can be assumed on the basis of the earlier theory [31, 32] that in

ferrite a strong correspondence happen among the conduction mechanism and the dielectric

polarization. In this situation the resonance peaks in tanδ(f) plats are identified once the external

applied electric field becomes approximately equal to the hopping frequency of charge carriers

[113].The Concentration dependent values of tanδ are listed in the Table 4.16.


144

Fig. 4.60 illustrates the relative variation of Dc resistivity and dielectric constant with (Sm-Ni)

concentrations. The decrease of dielectric constant with the increase in (Sm-Ni) contents is

ascribed to increase in DC resistivity of the ferrite samples.

0.00 0.25 0.50 0.75 1.00 1.25

0.00 0.02 0.04 0.06 0.08 0.10

-5.0x108

0.0

5.0x108

1.0x109

1.5x109

2.0x109

2.5x109

3.0x109

3.5x109

4.0x109

Sm. Contents

(o

hm

-cm

)

6

8

10

12

14

16

Dielectric C

onstant (

Fig.4.60: Comparison of dielectric constant and DC resistivity of (Sm-Ni) substituted , Sr(2-

x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–1.25), hexaferrites at room temperature.


The variation of AC conductivity with increasing concentration of (Sm-Ni) at 1MHZ is given in

the Table 4.16. The increase of impedance with increase of substitution level is mainly

responsible for decrease of AC conductivity. It is viewed that AC conductivity of the present

samples increases with increasing frequency of the applied field as depicted in the Fig. 4.61.

Meanwhile the increase in frequency improves the hopping of the charge carriers between Fe2+

and Fe3+in ferrite ceramic, which consequently increases the conductivity. Such a variation of ac

conductivity can be described on the basis of Maxwell–Wagner model and Koop’s

phenomenological theory. According to which the ferrites ceramic behave like a multilayer

capacitor in which the ferrite samples are assumed to consist of heterogeneous structure i. e

highly resistive thin layers (grain boundaries) separated by conducting grains. Keeping in the


145

view this model our present results of ac conductivity at low frequencies define the grain

boundary behavior, whereas the dispersion at high frequency may be accredited to the

conductivity of grains [39]. At low frequencies, the low conductivity is obviously noticeable

which is ascribed to the blocking effects at grain boundaries in the present experimental findings

[40] and relatively high values of the AC conductivity perceived at higher frequencies is owing

to the bulk contribution [40].

The frequency dependent AC conductivity can be written by the following equation [41];

σtot (ω) = σDC+Aωn (4.23)

Where A is a pre-exponential factor carry the units of electrical conductivity and n is the

frequency exponent a dimensionless quantity, which normally equal to one or less than one. For

n ≤1, the conduction is frequency dependent or AC conduction and when n = 0, the conduction is

frequency independent or dc conduction [42]. The value of n is quite supportive to elucidate the

conduction mechanism operative in the investigated samples. The hopping of electron between

Fe2+/ Fe3+ ions is accountable for conduction mechanism in ferrites. From the slope of log(σ) vs

log(ω), the value of exponent ‘n’ was extracted as shown in the Fig.4.62. and values are given

Table 4.18. The variation has been observed in the values of ‘n’ with increasing composition of

Sm-Ni. In the present findings, the value of exponent varies between 0.81-0.98, proposing that

the conduction phenomena in the present investigated samples follow hopping conduction.

The numerous ions hopping to immediate sites through barriers of energy EAC will obey the

subsequent equation (4.24).

τ0(T) = τ∞exp(EAC/kT) (4.24 )

Where τ0 the relaxation time for independent ion-hopping and τ∞the reciprocal of the attempt

frequency of ions. Frequently the energy barrier (Ac activation energy) will be smaller than the

activation energy for the dc conductivity and given by the relation.

Edc = EAC/ (1− n) (4.25)

The greater values of “n” essentially specify the greater degree of cooperativity in the ion-

hopping process which is primarily owing to the increase of interactions between the mobile

oxygen ions [45, 114]. By using the calculated values of EDC and n, the activation energy EAC for

the barrier that oxygen ions must overcome to hop (independently) amongadjacentempty sites in


146

the Sm-Ni substituted ferrite samples, can thus be estimated according to Eq. (4.25). The values

EAC is found, dependent of Sm-Ni contents and given in the Table 4.18.

0.00E+000 1.00E+009 2.00E+009 3.00E+009

0.0

5.0x10-2

1.0x10-1

1.5x10-1

2.0x10-1

2.5x10-1

3.0x10-1

x= 0.00, y= 0.00

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x=0.06, y= 0.75

x= 0.08, y= 1.00

x=0.10, y= 1.25

ac

(-c

m)-1

f(Hz)

Fig.4. 61: Variation in AC Conductivity with frequency of (Sm-Ni) substituted Sr(2-

x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–1.25), hexaferrites at room temperature.

Table 4. 18: Compresses the DC activation energy, exponent n , AC activation energy, real and

imaginary parts of electric modulus and impedance, at 1MHz of Sm-Ni substituted hexaferrites,

Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10, y = 0.00–1.25; )

Compositional Formula (EDC1+EDC2)/2 n (±0.01) EAC M΄ M΄΄ Z΄(Ω) Z΄΄(Ω)

Sr2Co2Fe12O22 0.431 0.81 0.078 0.061 0.008 32762 577

Sr1.98Sm.02Co2Ni0.25Fe11.75O22 0.443 0.92 0.031 0.081 0.013 34671 610

Sr1.96Sm.04Co2Ni0.50Fe11.50O22 0.458 0.95 0.019 0.096 0.014 36241 701

Sr1.94Sm.06Co2Ni0.75Fe11.25O22 0.467 0.91 0.041 0.119 0.015 39671 740

Sr1.92Sm.08Co2Ni1.00Fe11.00O22 0.479 0.98 0.007 0.136 0.016 59754 842

Sr1.90Sm0.1Co2Ni1.25Fe10.75O22 0.491 0.94 0.025 0.169 0.017 60663 859


147

The rare earth-contents produced higher degree of structural disorder [40], which is accredited to

the difference in size of host ion and dopant at numerous hexagonal conduction sites. An

improved ion–ion interaction are predictable and then greater values of the exponent n. Greater

the value of n enhance the energy penalty that these correlations impose on long-range or dc

ionic conductivity. This explains the increasing difference found between Edc and EAC..

6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

x= 0.00, y= 0.00

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x=0.06, y= 0.75

x= 0.08, y= 1.00

x=0.10, y= 1.25

log

ac(

-cm

)-1

log()

Fig.4. 62: Variation in logσ with logω of (Sm-Ni) substituted Co2Sr2Fe12O22 hexa ferrites.

4.3.5.2 Impedance analysis

Frequency dependent variation of the impedance (Z) is depicted in Fig.4.63 and inset shows the

individual behavior of impedance following the relation;

|Z| = Z΄ + jZ΄΄ (4.26)

whereas Z΄ and Z΄΄are real and imaginary parts of the impedance respectively. The values at

1MHz of Z΄ and Z΄΄are listed in Table 4.18. It has been perceived that values of impedance and

its components increase with (Sm-Ni) substitution are quite consistent with compositional

dependence of AC conductivity, i.e AC conductivity decrease within crease in impedance. The


148

decrease of impedance and increase of AC conductivity with frequency obviously exposing the

semiconducting type behavior of present investigated sample

0.00E+000 1.00E+009 2.00E+009 3.00E+009

-5.0x103

0.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

3.0x104

3.5x104

4.0x104

4.5x104

5.0x104

5.5x104

6.0x104

6.5x104

7.0x104

7.5x104

8.0x104

0.0 5.0x108

1.0x109

1.5x109

2.0x109

2.5x109

3.0x109

0

50

100

150

200

250

300

350

400

450

500

550

600

650

Imp

ide

nce

Z

Ferequency (Hz)

x= 0.00, y= 0.00

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x=0.06, y= 0.75

x= 0.08, y= 1.00

x=0.10, y= 1.25

Fig.4. 63: Variation of impedance with frequency of (Sm-Ni) substituted Co2Sr2Fe12O22 hexa

ferrites at room temperature.

Nyqiust plot (Cole - Cole plot)

The impedance spectroscopy is widely used tool to explain the electrical properties of ferrite

ceramic materials and interfaces existent in these materials. Both real and imaginary parts of

material are given by the impedance measurements data. It can be written in terms of any of the

four complex variables, admittance (Y*), permittivity (ε*), electric modulus (M*),impedance

(Z*) and dielectric loss (tan δ) in a complex plane plot (Nyqiust plot).Their relation to one

another is as follows [8, 45]:

tanδ = ε΄΄/ ε΄ = Y΄΄/Y΄ = Z΄΄/ Z΄= M΄΄/ M΄ (4.27)

Nyqiust plot of Complex electric modulus are plotted because it is a powerful technique to study

material electrode interface effect, contribution of bulk and grain boundary in the dielectric


149

behavior of the material Additionally, It is very beneficial in determining inter particle

interactions like grain boundaries and grains. The electrical modulus (M) plays a very decisive

role in the accumulating the electric charge around the grains by condensing the relaxation

peaks, in order to calculate the precise relaxation time. In short we can conclude that electrical

modulus is quite versatile approach related to the solution of relaxation phenomenon.

M = 1/ε* = 1/(ε΄-jε΄΄)= M΄-jM΄΄ (4.28)

The variations of both real and imaginary parts of electric modulus against frequency are

reflected in the Figs. 4.64-4.65.

14 16 18 20 22

0.0

0.5

1.0

1.5

2.0

2.5

3.0

14 16 18 20 22

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

0.30

0.32

0.34

x= 0.00, y= 0.00

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x=0.06, y= 0.75

x= 0.08, y= 1.00

x=0.10, y= 1.25

M

lnf(Hz)

x= 0.00, y= 0.00

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x=0.06, y= 0.75

x= 0.08, y= 1.00

x=0.10, y= 1.25

Y A

xis

Title

X Axis Title

Fig.4. 64: Variation in Real part of electric Modulus with frequency of (Sm-Ni) substituted

hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10, y = 0.00–1.25).


150

14 16 18 20 22

-1

0

1

2

3

4

14 16 18 20 22

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

x= 0.00, y= 0.00

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x=0.06, y= 0.75

x= 0.08, y= 1.00

x=0.10, y= 1.25

M

lnf(Hz)

x= 0.00, y= 0.00

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x=0.06, y= 0.75

x= 0.08, y= 1.00

Y A

xis

Title

X Axis Title

Fig.4. 65; Variation in Imaginary part of electric Modulus with frequency of (Sm-Ni) substituted

hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10, y = 0.00–1.25; ).:

The Maxwell–Wagner model offers complete detail about the conduct of complex conductivity

in heterogeneous ferrite systems with two or more phases [46]. In a heterogeneous system, in the

first case if the region of continuity of the grain boundary occupies a small volume, the plot of

impedance (Z΄΄ versus Z΄) offers more detailed information of the semi circles in the plane.

There is a suitable relationship between the behavior of grain boundary, and the existence of the

peaks in the plat of Z΄΄ as functions of frequency, in second case if the region of grain boundary

occupies large volume, the plot of the electric modulus

(M*=1/ε*) M΄΄ vs M΄, offers more information about the semicircles, proposing that there is a

probable relationship between the behavior of grain boundary and the presence of the peaks of

M΄΄ as a function of frequency [47] latter case is consistent with our present samples.

The calculated values of M΄ and M΄΄ for the (Sm-Ni) doped ferrite samples are listed in the Table

4.18 at 1MHZ. The calculated values of both real and imaginary part of the electric modulus

changes from 6.1×10-2 to 1.5×10-1 and 8×10-3 to 1.7×10-2 respectively. These values are

analogous with earlier reported values for Y-type hexaferrites [48].


151

The complex impedance (Cole-Cole) plots of the (Sm-Ni) substituted Sr2Co2Fe12O22 ferrites are

shown the Fig. 4.66. The left end (lower frequency) of the semicircle stands for the grain

resistance [3] however the intermediate frequency represents grain boundary resistance [50]

whereas the whole resistance of the grains and grain boundaries is determined by right one

(higher frequency) [3]. Substitution makes fairly minor influence on the grain resistance, but

leads to a extraordinary rise of grain boundary resistance. Greater the Sm contents greater will be

the grain boundary resistance. The hopping mechanism is leading conduction mechanism in

ferrites cermics, which is governed by the electron transfer between Fe2+ and Fe3+.The restriction

of electron transfer between Fe2+ and Fe3+is found to increase with increasing concentration of

Sm at the expanse of Fe. Thus, the variation has been observed in resistivity of the present

samples with the grain boundary content and composition. High resistance regions are found at

grain boundaries which hamper the conductivity. The high resistance of the grain boundary has

remarkable importance for determination the resistivity and dielectric properties.

0.05 0.10 0.15 0.20 0.25 0.30 0.35

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.080 0.088

0.010

0.012

0.014

x=0.0 , y= 0 .0

M

M

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x= 0.06, y= 0.75

x=0.08, y= 1.00

M

M

Fig.4. 66: Cole–Cole plots of (Sm-Ni) substituted hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x =

0.00–0.10, y = 0.00–1.25; ).


152

0.00E+000 1.00E+009 2.00E+009 3.00E+009

0

200

400

600

800

x= 0.00, y= 0.00

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x=0.06, y= 0.75

x= 0.08, y= 1.00

x=0.10, y= 1.25

Q fa

ctor

Frequency (HZ)

Fig.4. 67: Variation of Q values with frequency of (Sm-Ni) substituted hexaferrites, Sr(2-

x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10, y = 0.00–1.25 ).


Fig.4.67 shows the variation of Q values with increasing frequency for Sm-Ni substituted

Co2Sr2Fe12O22hexaferrites. Improved values of quality factor occures beyond the 2GHz

frequency and quite extraordinary Q values were observed. This high Q values and a resonance

frequency above 2 GHz, obviously propose that these ferrite materials can be used at high

frequency multilayer chip inductors [51].


4.3.6.1 Hysterious Loop

Figs. 4.68-4.69 shows the MH-loops for Sr(2-x)Sm(x)Co2NiyFe(12-y)O22 ferrites for both in-plane (H

applied parallel to the sample surface) and out-of-plane (H applied perpendicular to the sample

surface) orientations. The values of saturation magnetization (Ms), coercivity (Hc) and remanent

magnetization (Mr) were taken from the MH-loop..


153

-10000 -5000 0 5000 10000

-80

-60

-40

-20

0

20

40

60

80

x= 0.00, y= 0.00

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x=0.06, y= 0.75

x= 0.08, y= 1.00

x=0.10, y= 1.25

Ms

(em

u/g

)

Applied Feild H(Oe)

Fig.4. 68: In-plane MH-loop of (Sm-Ni) substituted hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x

= 0.00–0.10, y = 0.00–1.25 ).

The changes in magnetic properties such as Ms, Hc, Mr and nB are due to the cationic

stoichiometry and their occupancy in the specific sites [100]. So the knowledge of distribution of

metals ions in S and T blocks among the distant sites is very essential to describe the magnetic

properties of Y- type hexaferrite. In the present experimental findings the variation of the

saturation magnetization (Ms) has been explained on the basis of metal ions distribution in

different sites laying in the both block. There are six non-equivalent sites namely 6c1v, 3av1,

18hVI, 6cv1, 6cIV and 3bv1. Crystallographic and magnetic properties of these six sites are listed

in Table 4.6 (taken from Ref.[53].


The variant of the saturation magnetization (Ms) and remanence (Mr) are shown in the Figs.

4.70-4.71 for both cases i.e, in-plane and out-plane orientation. The increase of saturation

magnetization at lower concentration of (Sm-Ni) contents may be justified as. The presence of

the large number of Sr2+ in the T block indicates that substitution of Sr2+ions with smaller ions


154

could result in a less preferential occupation of tetrahedral sites, within the blocks the

equilibrium of super exchange interactions does not change and the spins remain collinearly

oriented. Hence in an increase of saturation magnetization (Ms), such kind of justification has

already been given [115]. It has been observed that such magnetic collinear ordering exist up to

some extend as the dopant level increase, some change in magnetic ordering and field induced

transition occur similar phenomenology has already been explained by many researchers [116]

-10000 -5000 0 5000 10000

-80

-60

-40

-20

0

20

40

60

80

x= 0.00, y= 0.00

x= 0.02, y= 0.25

x= 0.04, y= 0.50

x=0.06, y= 0.75

x= 0.08, y= 1.00

x=0.10, y= 1.25

Ms

(em

u/g)

applied Field H(Oe)

Fig.4.69: Out-plane MH-loop of (Sm-Ni) substituted hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22,

(x = 0.00–0.10, y = 0.00–1.25; ).

However it seems that up to now some uncertainties remain concerning the localization of the

layers representing the boundary between the magnetic blocks in the Y-type structures. The

decrease of saturation magnetization (Ms) at higher concentration of Sm-Ni explained flowingly.

The super exchange interactions play a cruel role in the magnetic ordering of S-block between A

and B sites of metal ions. In the present experimental case Sr(2-x)Sm(x)Co2NiyFe(12-y)O22. The

replacement of Sr and Fe ions by Sm and Ni ions respectively (having less magnetic moment)

both of them had strong octahedral 3av1-site occupancy resulted in the reduction of super

exchange interaction between tetrahedral 6c1vand octahedral 3av1 sites. Inother sense with

increasing concentrations of Sm-Ni the magnetization of octahedral 3av1-sites M3av1 decreased


155

while that of tetrahedral 6c1vsite M6c1v remained constant. As net magnetization is equal to M3av1-

M6c1v so it was found to decrease.

0.00 0.02 0.04 0.06 0.08 0.10

30

35

40

45

50

55

60

65

70

0.00 0.25 0.50 0.75 1.00 1.25

Ms (e

mu/g)

Sm contents

In-plane

Out-plane

Ni Contents

Fig.4. 70: In-plane and out-of-plane saturation magnetization versus (Sm-Ni) concentration for

Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–1.25), ferrites.

0.00 0.02 0.04 0.06 0.08 0.10

14

16

18

20

22

24

26

280.00 0.25 0.50 0.75 1.00 1.25

Mr (

emu/

g)

Sm contents

In-plane

Out-Plane

Ni contents

Fig.4. 71: In-plane and out-of-plane Remanence versus (Sm-Ni) concentration for Sr(2-

x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10; y = 0.00–1.25), ferrites.


156

4000 5000 6000 7000 8000 9000 10000

56

58

60

62

64

66

Data: Data2_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.0017

R^2 = 0.9999

Ms 77.11252 ±0.33049

a 1779.85621 ±49.94247

b -2159703.94919 ±175899.06031

chi 0 ±--

Ms

(em

u/g

)


a

4000 5000 6000 7000 8000 9000 10000

60

61

62

63

64

65

66

67

68

Data: Data2_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00007

R^2 = 0.99999

Ms 74.55773 ±0.06783

a 920.58183 ±11.37704

b -155665.20372 ±39118.8661

chi 0 ±--

Ms (

em

u/g

)


b

4000 5000 6000 7000 8000 9000

61

62

63

64

65

66

67

68

69

Data: Data4_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00001

R^2 = 1

Ms 77.39236 ±0.03891

a 1062.77052 ±5.97739

b -325756.25084 ±20172.62008

chi 0 ±--

Ms (

em

u/g

)

applied Feild H (Oe)

c

4000 5000 6000 7000 8000 9000

46

47

48

49

50

51

52

53

54

55

Data: Data6_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00026

R^2 = 0.99998

Ms 66.38414 ±0.16858

a 1846.11899 ±28.2592

b -2144578.72773 ±97660.19715

chi 0 ±--

Ms

(em

u/g

)


d

4000 5000 6000 7000 8000 9000

36

37

38

39

40

41

42

43

Data: Data8_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00073

R^2 = 0.99993

Ms 53.42645 ±0.2835

a 2084.78897 ±58.01481

b -2715902.00445 ±202540.33247

chi 0 ±--

Ms (

em

u/g

)

applied Field H (Oe)

e

4000 5000 6000 7000 8000 9000 10000

28

29

30

31

32

33

34

35

36

Data: Data10_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00209

R^2 = 0.99978

Ms 43.36702 ±0.47684

a 2101.04519 ±119.43967

b -2072264.12525 ±422770.75168

chi 0 ±--

Ms

(em

u/g

)

Applied Feild H(Oe)

f

Figs.4.72 (a-f). Fitted curve of Ms for (Sm-Ni) substituted hexaferrites, calculated by law of



157

Equation (3) was used to elaborate estimated values of saturation magnetization Ms from the

loops at infinite field, using a least squares procedure the data were fitted by the following law of

approach to saturation [70, 117].

M = Ms(1- A/H - B/H2) + χH (4.30)

Where Ms is saturation magnetization, H is the applied external field, A is in homogeneity

parameter, χ is the susceptibility, B is the factor which is proportional to K2(K is the anisotropy

constant) and supposition has been made to take the Brillion function is equal to unity. In order

to apply this law in polycrystalline samples The magnetization M(H) in the above is replaced by

the specific name polarization. Fitted curves of Sm-Ni substituted Sr(2-x)Sm(x)Co2NiyFe(12-y)O22

hexaferrites for saturation magnetization deliberated by above mentioned law at room

temperature are shown in the Figs.4.72 (a-f). The estimated values of saturation magnetization

are listed in the Table 4.19.

Table 4. 19: Estimated saturation magnetization (Ms), Anisotropy constant( K), Magnetic

moments (nB), Squareness Ratio for in-plane and out-plane orientation of (Sm-Ni) substituted

hexaferrites, Sr(2-x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10, y = 0.00–1.25; ).

The observed variance in the estimated and calculated values of saturation magnetization is

endorsed to the lacking field applied in the experimental case which is not enough to in line the

all randomly spread magnetic moment in one direction (in the direction of external field). but in

theoretical case infinite field is applied to orient the all magnetic moment in the direction of the


Estimate

d

Ms (e

mu

/g)

K(e

rg/cm3)(I

n p

ane

)

K (e

rg/cm3)

(Ou

t-plan

e)

nB

(em

u/g)

(In p

ane

)

nB

(em

u/g)

(Ou

t-plan

e)

Ms/M

r

in- p

lane

Ms/M

r

Ou

t-plan

e

Sr2Co2Fe12O22 77.11 2.34×104 2.10×104 15.27 14.64 0.41 0.30

Sr1.98 Sm.02Co2Ni0.25Fe11.75O22 74.55 2.30×104 1.77×104 16.00 15.67 0.36 0.21

Sr1.96 Sm.04Co2Ni0.50Fe11.50O22 77.39 1.96×104 1.94×104 16.32 16.27 0.29 0.27

Sr1.94 Sm.06Co2Ni0.75Fe11.25O22 66.38 2.12×104 1.97×104 13.05 12.47 0.38 0.33

Sr1.92 Sm.08Co2Ni1.00Fe11.00O22 53.42 2.06×104 2.35×104 10.31 10.04 0.48 0.40

Sr1.90 Sm0.1Co2Ni1.25Fe10.75O22 43.36 2.37×104 2.34×104 8.31 8.08 0.49 0.42


158

external field to obtain the maximum saturation magnetization. The careful observation of the

Figs.4.72 (a-f) hints that insufficient field is applied in experimental case. Which obviously

purpose that remarkable magnetization might be achieved by increasing the external field,

consequently the values of experimental and theoretical saturation magnetization will become

more close to each other.

4.3.6.3 Coericivity

Both in-plane and out of plane coericivity was measured from the BH curves of Sm-Ni

substituted Sr(2-x)Sm(x)Co2NiyFe(12-y)O22 and are shown in the Fig. 4.73. Thecoericivity first

decreases with increasing substitution level of Sm-Ni up to the x = 0.04, y = 0.50. and then

increases. This typical behavior of the Hc can be explained on the the basis of Browns relation

Hc=K1/μoMs[58, 103] where K1 is magnetocrystalline anisotropy, μo is vacuum susceptibility,

Ms is saturation magnetization and Hc is coercivity. As saturation magnetization Ms is inversely

related to the coericivity Hc. This is true in our present experimental case. Alike behavior has

already been reported by many researchers [57, 62]. It is understood fact that higher the porosity

higher will be the Coercivity [102], which is quite justified with our present experimental case

both coericivity and porosity has increasing trend.

In conventional longitudinal magnetic recording (LMR), the “magnetic bits” are directed parallel

to the surface of disk. Whereas, in perpendicular recording media (PRM), the ‘‘magnetic bits’’

are settled perpendicular to the surface of disk. The distinguished elucidation for the usage of

PMR is that it can supply 3 times extra storage density of LMR. Basically, magnetic samples

having extraordinary values of coercivity are thermally stable. Thermal durability of the

magnetic samples is proportional to the product of uniaxial anisotropy constant K1times volume,

greater coercive material will have more the product. In this regard we can assume that PRM

required more coercive medium.

The hexaferrite materials can be beneficial for the perpendicular recording media which is an

emerging technology in the recording media [61]. In the present experimental outcomes the

studied ferrite samples which are Y-type hexaferrite can be used in PRM due to high value of

coercivity (1400Oe). Materials are consider to be hard magnets, if Hc>Mr/2 and if Hc<Mr/2,

then the materials are semi-hard magnets [11, 62]. The prepared materials in the present study


159

have Hc>Mr/2. Additionally, it is supposed that if samples have Hc>Mr/2, can be used for high

frequency applications [61].

0.00 0.02 0.04 0.06 0.08 0.10

400

600

800

1000

1200

1400

0.00 0.25 0.50 0.75 1.00 1.25

Hc

(Oe

)

Sm contents

In-plane

Out-plane

Ni Contents

Fig.4. 73: In-plane and out-of-plane coercivity versus (Sm-Ni) substituted hexaferrites, Sr(2-

x)Sm(x)Co2NiyFe(12-y)O22, (x = 0.00–0.10, y = 0.00–1.25; )

Both in-plane and out-plane values of magnetic moment (nB) are summarized in Table 4.19. By

and large both the saturation magnetization (Ms) and the magnetic moment (nB) show parallel

behavior. In the present hexaferrite samples behavior of magnetic moment is consistent with the

saturation magnetization as both first increases and then decreases at higher concentration of

(Sm-Ni) contents, the deterioration of magnetic moment may be owing to the weakening of

super exchange interactions, as Fe –O – Fe super exchange decreases with rare earth substitution

at the expanse of Fe. Alike behavior has already reported by various researchers [63, 64]


From the VSM data, both in-plane and out of plane Squreness ratios (Mr/Ms) of (Sm-Ni)

substituted Co2Sr2Fe12O22ferritesamples was calculated and presented in the Table 4.19. In-plane

Squreness ratios (Mr/Ms) ranging from 0.41 to 0.49 however for out of plane measurement it


160

varies from 0.30 to 0.42. The squreness ratio is well below of typical value ~1 for single domain

isolated ferromagnetic ferrite particle. The lower values of squareness ratio show that particles

are totally randomly oriented and be in multi domains. However by assuming magnetic particles

to be isolated (exchange interacting spin) single domains [65], the anisotropy constant was

calculated using the given relation, K=HcMs/2. The values of magnetocrystalline anisotropy

constant for both cases are less than that of already reported results for single domain ferrite

systems. Also the values are given in the Table 4.19. This shows that grains are not single

domains and anisotropy contribution is not uniaxial [67, 118].

4.4 Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22 /PST Composite Samples

4.4.1 Structural Analysis

Fig. 4.74 shows the XRD patterns of Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22 hexaferrite powder,

polystyrene and their composites.

10 20 30 40 50 60

p

FP1

FP2

FP3

FP4

F

0 0

13

2 1

19

0 0

27

2 0

19

2 0

20

0 2

161 0

19

0 0

18

1 1 9

01

14

1 0

13

1 1

0

Inte

nsi

ty (

a. u

.)

2 degree

Fig.4. 74: X-ray Diffraction Patterns of PST, FP1, FP2,FP3, FP4 and Y-type



161

The broad peak in the XRD pattern of the PST sample, is the characteristic of amorphous

structure over the 2Ɵ range 7–28º. Similar XRD patterns were already observed by many

researchers [119-121]. All the reflections of Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22 were compared with

JCPDS card.No. 00-019-019-0100. It depict that the Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22 formed well

defined Y-type structure. In case of composites FP1, FP2, FP3, FP4 both the amorphous and

crystalline phases co-exist. The broad peak at low angles is due to the PST and the crystalline

peaks of Y-type ferrite are also observed. With increasing concentration of ferrite the peaks

become more intense and less broadening which suggests that crystallinty is improved with the

addition of ferrite. Peaks in the XRD patterns of composite samples are broad. The broadening

of the peaks is due to the nanometer size of the crystallites.

4.4.2 Scanning Electron Microscopy SEM is useful tool to study the surface morphology. Figs. 75(a-f) show the SEM micrographs of

polystyrene, FP1, FP2, FP3, FP4 and Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22. The pure ferrite sample

Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22 grains are plate-like and most of them have hexagonal shape. A

ferrite particle comprises of separate grains with clear-cut boundaries. The grain morphology in

the composites can be perceived using SEM images, as ferrite and PST grains are uniquely

distinguishable in the shape. Whereas a continuous over layer of polymers is produced on the

ferrite particle surface. This agreed with the XRD results.. A keen observation of these SEM

images clearly suggests that the grain morphology changes noticeably with increasing ferrite

filler contents. In the composite samples the planar grains of Y-type hexagonal ferrite ceramics

become equiaxed crystal. The equiaxed crystal is more auspicious for a compact microstructure

than planar crystal. Furthermore, equiaxed grains of polymer will restrain the grain growth of Y-

type hexagonal ferrite and assimilate their grain shape in to equiaxed crystal. It is understood

fact that the inside stress cannot be eluded in the composite ferrite ceramic samples, owing to

diverse densification rate of two phases.


162

Figs.4. 75 (a-f): SEM Image of PST, FP1, FP2,FP3, FP4 and Y-type hexaferrite(Sr1.8Sm0.2Co2

Ni1.50 Fe10.50O22).


163



Normally all ferrites behave like semiconductors with the temperature. Though their resistivity

performance is elucidated by localized electron model rather than collective band model. The

exchange of valancies between Fe3+ and Fe2+ play a very decisive role in order to explain the

resistivity behavior of ferrite ceramics [122] and occasionally also the existence of Co2+ and Ni2+

ions in Ni and Co comprising ferrites. It is supposed that the deterioration of resistivity is

predominantly accountable due to the mobility of the surplus electron, which comes from Fe2+

(or sometimes extra holes in positive charge containing ferrites), through the crystallattice. The

hopping mechanism between divalent and trivalent ions is principally accountable for conduction

mechanism in ferrite ceramics.

DC resistivity of the composites samples is higher than that of the pure ferrites and decreases by

increasing ferrite filler into the polymer. This decrease of resistivity is mainly due to the addition

of comparatively less resistive ferrite into the highly insulating polymer matrix of PST. Room

temperature resistivity of PST is 3.57×1014 (Ω-cm). This high value of resistivity obtained in the

present composite samples is not new but it is within the desire range of already reported data

[123, 124]. While resistivity of pure ferrite (filler) is 4.81×109 (Ω-cm). Room temperature

resistivity of the present investigated samples are presented in the Table 4.20.

Table 4. 20: Crystallite size, Grain size (nm), resistivity and Activation energy ofPST, FP1,

FP2,FP3, FP4 and Y-type hexaferrite(Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22).

Parameters PST FP1 FP2 FP3 FP4 Ferrite

Resistivity(Ω-cm) 3.5 x1014 2.1 x1014 1.2 x1013 1.3 x1012 7.2 x1010 4.8 x109

Activation energy 0.69 0.66 0.64 0.61 0.59 0.58

Fig.4.76 shows the temperature dependence of DC resistivity of the the investigated samples in

the temperature range (292 to 342K). The Arhenius plots reveal semiconducting nature of the

samples. The values of activation energy for the different samples are presented in Table 4.20.

The values of activation energies shows similar trend as that of room temperature resistivity. The


164

variation of lnρ vs. 1000/T specifies that the electrical resistivity is due to thermally activated

charge carriers. A decrease in resistivity with temperature is linked with the improvement in the

drift mobility [125].

2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3

20

22

24

26

28

30

32

34

ln

(ohm

-cm

)

1000/T (K-1

)

p

FP1

FP2

FP3

FP4

F

Fig.4.76: Arrhenius plot of DC resistivityof PST, FP1, FP2,FP3, FP4 and Y-type


4.4.4 Dielectric Properties It is well established fact that PST shows an amorphous structure. Moreover it is believed

that real non-crystalline materials (amorphous structure) are supposed to have imperfection, most

prominent imperfection are point defects and micro voids. Point defect are thought to contain

impurities or dangling bonds, whereas exemplary rule of micro voids is to create the levels

inside the band gap, just as in crystals [126].

Energy gain is mainly due to the lattice distortion which is direct measure of the coulomb

repulsion energy amongst electrons. Furthermore the Van der Waals bonds in the dielectric

materials of organic origin like PST between the neighboring molecules play a crucial rule to

govern the very narrow allowed bonds with corresponding high effective masses of the resulting

charge carriers and this together with the existence of disorder, gives extra to the process of


165

localization. It is more obvious to enlighten the established fact that these localized centers are

responsible for enhancement in the electrical hopping transport phenomena. Two distinct kinds

of relaxations are examined in PST [127] mainly initiating from chain motion, wagging motions

and rotational motions of pendent phenyl group.

In ferrite the different dipoles have different relaxation time in different frequency range, giving

rise to different relaxation frequencies. The electron hopping between Fe+3 and Fe+2 ions also add

to the dielectric loss due to enhanced conduction mechanisms giving rise to another relaxation

time [128, 129]. Whereas, in case of a ferrite–polymer composite, the contribution to dielectric

constant and dielectric loss also occur due to interfacial polarization and its relaxation as the

semiconducting ferrite particles separated by insulating matrix molecules giving rise to

heterogeneity.

4.4.4 1 Concentration Dependent Dielectric Constant

The values of dielectric constant are listed in the Table 4.21. The observed increase in the

dielectric constant (permittivity) with increasing concentration ratio of ferrites is mainly due to

the electron exchange between Fe2+ ↔ Fe3++ē which consequently results in enhancement of

electric polarization as well as dielectric constant. In this way we conclude that more the iron

ions more will be the polarization.

The cations Ni2+, Co2+, Sm3+ and Fe3+ at their own respective conduction sites (B and A sites)

in different S and T blocks of Y type hexaferrites are responsible for the formation of dipole

moments with adjacent O2- ions contributing to the dielectric behavior through dipole

polarization, interfacial polarization and dipole relaxation.

It is quite realistic approach to imagine that the well-known Koops model [29] be obeyed in

these present investigated samples, where the Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22 ferrite particles

behave like high conducting grains sandwiched between comparatively lower conducting PST

molecules.

The plot of real part of permittivity with respect to frequency is shown in Fig. 4.77. The real part

of permittivity spectra of present investigated samples show insignificant variation in the whole

frequency range. However, observed small variation in the values of real part of permittivity

largely owing to lagging of dipole moment of polaron/bipolaron with respect to external

frequency and magnetocrystalline anisotropy of nano-ferrite [130].


166

Table 4.21: Dielectric constant, Dielectric Loss, Tangent Loss and AC conductivity of PST, FP1,

FP2,FP3, FP4 and Y-type hexaferrite(Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22).


Dielectric constant 12.17 13.10 14.24 15.03 15.89 16.17

Dielectric Loss 2.99 3.77 4.52 4.90 5.39 5.68

Tangent Loss 0.24 0.28 0.31 0.32 0.33 0.35

AC conductivity 1.3 x10-1 1.7 x10-1 2.1 x10-1 2.7 x10-1 3.2 x10-1 3.7 x10-1

14 16 18 20 22

10

12

14

16

PST

FP1

FP2

FP3

FP4

F

Applied frequency lnf (Hz)

Fig.4.77: The variation of dielectric constant versus applied field frequency of PST, FP1,

FP2,FP3, FP4 and Y-type hexaferrite (Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22).

The plot of imaginary part of permittivity with respect to frequency is reflected in Fig. 4.78.

Various relaxation frequencies of several dipoles occur in the typical hexagonal ferrite structure

are mainly accredited to the relaxation due to interfacial polarization the and hopping of

electrons. All these factors are accountable for oscillatory behavior of absorption in the present

investigated samples. Nevertheless, as the ferrite ratio is enhanced in the composite dominance

of relaxation due to interfacial polarization favor the smoothening of loss curves especially at

higher ferrite ratio. Finally, we can conclude that higher the ferrite content more will be the


167

overlapping of precise motion of several crystallites which consequently smoothened absorption

curve. The observed zig–zag manner, giving rise to oscillatory behavior of absorption is not new.

Such behavior is also observed by many researchers [105]. The values of dielectric loss with

increasing ferrite ratio are listed in the Table 4.21.

14 16 18 20 22

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

PST

FP1

FP2

FP3

FP4

F

Die

lect

ric L

oss

Applied Frequency lnf(Hz)

Fig.4.78: The variation of dielectric loss versus applied field frequencyof PST, FP1, FP2, FP3,

FP4 and Y-type hexaferrite (Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22).

Fig. 4.79 demonstrates the resonance peaks in the plot of dielectric tangent loss. The existence of

resonances peaks in the dielectric loss tangent spectra are due to the fact, when the external

applied frequency becomes equal to the jumping frequency of electrons between Fe2+ and Fe3+

[88]. The presence of resonance peaks in the spectra was elucidated by an analogy: if an ion has

two distant equilibrium conduction states A and B with identical potential energies, then in these

circumstances it is more likely that the jumping probability of an ion should be identical in both

cases: from A to B and from B to A. The jumping frequency of various ions between the

equilibrium states A and B is called the natural frequency of jump between these two distant

states. When the applied external frequency becomes equal to the natural frequency of ions, then

the probability of transformation of the electrical energy to the oscillating ions is maximum and

the power losses becomes maximum, which consequently favor the occurrences of the

phenomenon of resonance. Hence, the resonance peaks appear [131]. Moreover, according to the


168

Rezlescu’s theory the existences of the relaxation peaks are mainly due to the both n-type and p-

type charge carriers [132]. The values of dielectric loss tangent with increasing ferrite contents

are listed in the Table 4. 21.

14 16 18 20 22

0.10

0.15

0.20

0.25

0.30

0.35 PST

FP1

FP2

FP3

FP4

F

tan

Applied Field lnf(Hz)

Fig.4. 79: Variation of dielectric tangent loss versus applied field frequency of PST, FP1,

FP2,FP3, FP4 and Y-type hexaferrite (Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22).


Fig.4.80 depicts the variation of the AC conductivity for the present examined samples. It is clear

from the figure that AC conductivity increases with increasing frequency for all present

examined samples. The frequency dependence of AC conductivity can be expressed by the

following power law,

σtot(ω) = σDC+Aωn (4.31)

Whereas n is the frequency exponent and is a dimensionless quantity. Where A is a pre-

exponential factor with electrical conductivity units. This behavior of AC conductivity can be

explained on the basis of Maxwell–Wagner model and Koop’s phenomenological theory.

According to this model at low frequencies the grain boundary contribution is dominant, while

the dispersion at high frequency may be accredited to the conducting grains [39].


169

Table 4.21 shows that σAC increases as a function of ferrite concentration, this increasing

behavior of AC conductivity may be attributed to the increasing behavior of the dielectric as the

dielectric constant (έ) is proportional to the conductivity (σAC). The change of dielectric constant

of these hexaferrite materials runs analogous to the variation of the hopping of electrons at the

different conduction sites. It is therefore concluded that the process of the dielectric polarization

in the present studied samples is analogous to that of the electrical conduction which is

consistent with the Iwauchi’s hypothesis [133].

14 16 18 20 22

0.0

1.0x10-1

2.0x10-1

3.0x10-1

4.0x10-1

5.0x10-1

14 16 18

0.0

2.0x10-2

4.0x10-2

6.0x10-2

PST

FP1

FP2

FP3

FP4

F

ac(

-cm

)-1

lnf (Hz)

Fig.4. 80: The variation of AC conductivity versus applied field frequencyof PST, FP1, FP2,FP3,

FP4 and Y-type hexaferrite(Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22).

Fig 4.81 shows a log–log behavior of the frequency dependence of electrical conductivity for the

present samples following the eq (4.31). The fractional exponent n lies (0 ≤ n ≤ 1), linked with

the dynamics of various hopping ions [94] and values of n is listed in the Table 4.22; low value

i.e. n=0, entirely independent of frequency. In the current studies the values of exponent are high

which reflect conduction phenomena in the studied samples follow hopping mechanism [40].

The increasing behavior of the dielectric constant, dielectric loss and ac conductivity with

increasing ferrite ratio in PST matrix. One could propose their versite use in different

technological applications.


170

6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

PST

FP1

FP2

FP3

FP4

F

log

ac

log()

Fig.4.81: Log-Log variation of AC conductivity versus applied field frequencyof PST, FP1, FP2,

FP3, FP4 and Y-type hexaferrite(Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22).

Table 4. 22: Exponentail factor, AC activation energy, Real part of electric modulus, Imaginary

part of electric modulus and Impedance of PST, FP1, FP2,FP3, FP4 and Y-type



N 0.92 0.89 0.891 0.91 0.933 0.928

EAC 0.055 0.072 0.070 0.055 0.040 0.041

M΄ 0.07745 0.07046 0.06376 0.0601 0.05643 0.05502

M΄΄ 0.01903 0.02027 0.02025 0.01962 0.01914 0.0193

4.4.4 3 Frequency-Dependent Complex Electric Modulus.

Complex electric modulus formalism is powerful tool to determine, interpret, and analyze the

various electrical parameters like ions hopping and relaxation time, etc in materials. Hence the

complex electric modulus plots actually provide an alternative approach based on polarization

analysis. Fig. 4.82 reflects the variation of real part of modulus (M΄) versus applied field

frequency at a room temperature. It can be seen that the value of real modulus ((M΄) is smaller in

lower-frequency region and enhanced with the increase of applied field frequency and at higher


171

frequency modulus become constant, which might be due to a lack of restoring force responsible

for the mobility of charge carriers under the influence of an induced electrical field. Furthermore,

The observed behavior favored the conduction phenomena and might be due to long range

mobility of charge carriers [134].

14 16 18 20 22

0.054

0.060

0.066

0.072

0.078

0.084

0.090

PST

FP1

FP2

FP3

FP4

F

Rea

l par

t of e

lect

ric M

odul

us

Applied Frequency lnf (HZ)

Fig.4. 82: Variation of real part of eletric modulus (M΄) versus applied field frequencyof PST,

FP1, FP2,FP3, FP4 and Y-type hexaferrite(Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22).

Variation of imaginary part of complex modulus (M΄΄) versus applied field frequency is shown

in the Fig. 4.83. It can be seen that the values of M΄΄ decreases with increasing frequency. The

M΄΄ has higher values in the lower-frequency region and low for higher frequencies. For the

reason mentioned in the dielectric loss factor few resonance peaks appeared in the plot of M΄΄ vs

frequency in the composites samples actually indicates the relaxation phenomena in the

conductivity. The values of imaginary part of electric modulus (M΄΄) with increasing ferrite

content are listed in the Table 4.22.


172

14 16 18 20 22

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

0.022

PST

FP1

FP2

FP3

FP4

F

imag

inar

y pa

rt o

f ele

ctric

mol

ulus

Applied frequency lnf (Hz)

Fig.4. 83: Variation of imaginary part of electric modulus (M΄) versus applied field frequencyof

PST, FP1, FP2, FP3, FP4 and Y-type hexaferrite (Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22)


Fig. 4.84 shows the change of Q values with increasing frequency for investigated samples. High

values of quality factor(Q)occurred at higher frequency. The occurrence of resonance at high

frequency, noticeably suggest that the present investigated composite samples are best candidate

for multilayer chip inductors [99].


173

0.00E+000 1.00E+009 2.00E+009 3.00E+009

0

10000

20000

30000

PST

FP1

FP2

FP3

FP4

F

Q v

alue

s

Applied Frequency (HZ)

Fig.4. 84: Variations of Q values versus applied field frequency of PST, FP1, FP2,FP3, FP4 and

Y-type hexaferrite(Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22).


4.4.5.1 Hysteresis Loop

Figs. 4.85-4.86 show the MH-loops of the present investigated samples measured by applying

magnetic fields parallel to the sample surface (inplane) and (out of plane) by applying magnetic

field perpendicular to the sample surface. The magnetization reveals clear hysteresis loops

similar to ferromagnetic behavior. The MH-loops were obtained by applying magnetic field in

the different directions both in-plane and out of plane of the investigated composite samples

proves that magnetic particles are uniformly distributed within the polymer matrix. The

experimental values of saturation magnetization (Ms), coercivity (Hc) and retentivity (Mr) are

taken from these hysteresis loops for the both cases and are listed in the Table 4.23.

An increase in Ms, Mr and Hc values is obtained with increasing weight ratio of the magnetic

filler i.e Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22 from .25 to 1 molar ratio. It can be observed that the Ms

of SSCNF/PST nanocomposite samples increase evenly with filler Sr1.8Sm0.2Co2 Ni1.50 Fe12O22

contents. Bestowing to the equation Ms = φms, Ms is completely associated to the volume fraction

of the particles (φ) and single particle the saturation moment (mS) [135]. It is assumed that the

Ms of SSCNF/PST nanocomposites predominantly depends on the volume fraction of


174

Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22 magnetic ferrite particles, so increasing volume fraction

percentage of Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22 enhances the saturation magnetization (Ms) and Mr

of the composite samples. It may be concluded the net magnetic moment achieved by the

composites samples turns out to be directly the vector sum of the every individual ferrite grain

contributions inside the polymer matrix.

-10000 -5000 0 5000 10000

-40

-30

-20

-10

0

10

20

30

40

Ms

(em

u/g)

Applied Feild H(Oe)

PF1

PF2

PF3

PF4

F

Fig.4. 85 : In-plane MH-loop of FP1, FP2,FP3, FP4 and Y-type hexaferrite(Sr1.8Sm0.2Co2

Ni1.50Fe10.50O22).


175

It is believed that irreversible motion of domain walls is mainly responsible to originates

coercivity (Hc). The values of coercivity (Hc) must be at least a few hundred Oersteds to be of

some significance for beneficial applications [136]. Table 4.23, shows that the coercivity of

polymer ferrite nanocomposites increases gradually with Sr1.8Sm0.2Co2 Ni1.50Fe10.50O22 contents.

It is anticipated that improvement in the coercivity is owing to the magnetocrystalline anisotropy

[136] that exists in ferrite filler in the polymer matrix. However in the magnetic nanocomposites

at the lower percentage of Sr1.8Sm0.2Co2 Ni1.50Fe10.50O22, the coercivity is among the lower side

and the hysteresis show more or less zero loss. But at higher percentage i. e. 100 percntage of

Sr1.8Sm0.2Co2 Ni1.50Fe10.50O22magnetic powder, quite improved values of Hc is achieved which is

essentially advantageous for practical use of this composite sample for memory devices.

During the polymerization process, PST is placed on the surface of ferrite and crystallite

boundaries and defects of ferrite surface are minimized, such as cracks and pores, which

essentially reduce the internal stress of the composite samples. Besides this, there might be

surface spin pinning at ferrite nanoparticle of magnetic moments support interface[137], which

consequently reduce the surface anisotropy of ferrite particles. Subsequently, the Hc of

nanocomposites has lesser values compared with that of Sr1.8Sm0.2Co2 Ni1.50 Fe12O22 ferrite.

It is summarized that from hysteresis loops shape and linearity of MH-Loop in the Ms,

Mr, Hc values vs. ferrite content that the ferrite nanoparticles are evenly dispersed within the

composite and moreover the single ferrite grains perform like a individual centers of

magnetization in the polymer matrix.


176

-10000 -5000 0 5000 10000

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

35

40

45

PF1

PF2

PF3

PF4

F

Ms

(em

u/g)

Applied Feild H(Oe)

Fig.4. 186: Out-plane MH-loop of FP1, FP2,FP3, FP4 and Y-type hexaferrite(Sr1.8Sm0.2Co2

Ni1.50Fe10.50O22).


The saturation magnetization (Ms) fitted curve for studied samples are presented in Figs. 4.87(a-

e). The large difference between experimental and theoretical values of saturation magnetization

is essentially owing to the lacking field applied in the experimental case whereas in the

theoretical case infinite field is applied to achieve extreme values of saturation magnetization. A

care full inspection of the Figs.4.87 (a-e) reveals that applied field in experimental case is

insufficient. Clarifying that extra magnetization might be realized with increase of the external

field, which will actually offer the close agreement between theoretical and experimental values.

The estimated values of saturation magnetization are listed in the Table 4.23


177

Fig.4. 87: (a-e) Fitted curve of FP1, FP2,FP3, FP4 and Y-type hexaferrite(Sr1.8Sm0.2Co2

Ni1.50Fe10.50O22.calculated by law of approach to saturation.

4000 5000 6000 7000 8000 9000 10000

0

1

2

3

4

5

6

7

8

FP1

Data: Data10_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00179

R^2 = 0.99984

Ms 7.95719 ±0.33935

a -938.33713 ±287.3208

b 16413706.24078 ±1187833.05064

chi 0 ±--

Ms (

em

u/g

)


4000 5000 6000 7000 8000 9000 10000

8

9

10

11

12

13

14

15

16

Data: Data10_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00179

R^2 = 0.99984

Ms 16.95719 ±0.33935

a -938.33713 ±287.3208

b 16413706.24078 ±1187833.05064

chi 0 ±--

Ms (

em

u/g

)


FP2

4000 5000 6000 7000 8000 9000 10000

10

12

14

16

18

20

Data: Data8_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00566

R^2 = 0.99974

Ms 23.28582 ±0.60511

a -185.58129 ±354.23735

b 14159403.25719 ±1488556.70927

chi 0 ±--

Ms (

mu

/g)


FP3

4000 5000 6000 7000 8000 9000 10000

22

23

24

25

26

27

28

Data: Data10_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00087

R^2 = 0.99984

Ms 32.30295 ±0.23774

a 1404.33686 ±88.70483

b 291374.82295 ±321059.27825

chi 0 ±--

Ms (

em

u/g

)


FP4

4000 5000 6000 7000 8000 9000

36

37

38

39

40

41

42

43

Data: Data8_B

Model: LoA


Weighting:

y No weighting

Chi^2/DoF = 0.00073

R^2 = 0.99993

Ms 42.42645 ±0.2835

a 2084.78897 ±58.01481

b -2715902.00445 ±202540.33247

chi 0 ±--

Ms

(em

u/g

)

applied Field H (Oe)

F


178

Table 4. 23: Saturation Magnetization (Ms), Remenances(Mr), coercivity) (Hc), Squreness ratios

(Mr/Ms), magnetocrystalline anisotropy constant (K) and Estimated Saturation Magnetization

(Ms) of PST, FP1, FP2, FP3, FP4 and (Sr1.8Sm0.2Co2 Ni1.50Fe10.50O22) Y-type hexaferrites

Parameters FP1 FP2 FP3 FP4 Ferrite

Ms (emu/g) in-plane

Ms (emu/g) out-plane

5

4

10

9

14

12

25

20

39

38

Estimate Ms(emu/g) 7 16 20 27 42

Mr (emu/g) in-plane

Mr (emu/g) out-plane

2

1

4

3

6

4

12

9

20

19

Hc (Oe) in-plane

Hc (Oe) out-plane

339

273

387

335

447

395

1006

1066

1493

1492

Mr/Ms in-plane

Mr/Ms out-plane

0.40

0.25

0.40

0.33

0.42

0.33

0.48

0.60

0.51

0.50

K (erg/cm3) In pane 8.4×102 1.8×103 3.1×103 1.2×104 2.9×104

K (erg/cm3) Out-plane 5.4×102 1.5×103 2.3×103 1.0×104 2.8×104

4.5 Composite of Co2Sr2Fe12O22 with Ppy-DBSA

4.5.1 Structural Anaylsis. X-ray diffraction was carried out at room temperature to determine the structure

of the investigated samples. The X-ray diffraction patterns were recorded using a computer

controlled JDX-3532 JEOL Japan model operated at 40KV and at 30mA. The radiations used

were Cu-Kα (λ = 1.5406Å) with Ni filter and sample was scanned in 2θ range of 10 to 75o with

step size 0.02o and time per step was 0.5Sec. All peaks in XRD patterns of Sr2Co2Fe12O22, were

compared with JCPDS card.No. 00-019-019-0100. It reveals from XRD analysis that the

Sr2Co2Fe12O22 formed well defined Y-type structure. Whereas the PPy-DBSA shows amorphous

behavior. There was a co- existence of PPY-DBSA and Y-type Sr2Co2Fe12O22 phases in the

composite sample as indicated in the XRD patterns shown in Fig.4.88. The decrease in intensity

of the peaks in the composite sample may be attributed to the amorphous nature of PPY-DBSA

and few peaks disappeared with addition of PPY_DBSA in the pure ferrite. Pure-phase formation


179

of the Co2Y ferrite was achieved at the temperature of 1000ºC, which is lower than the 1100ºC

reported in the literature [138, 139]. The composite sample exhibit broad XRD paks. The

broadening of the peaks is due to the nanometer size of the crystallites [140]. The peaks of the

composite samples show on change in the peaks position. It means neither exfoliation takes place

nor interaction takes place. It may be concluded that the composite sample comprises of irlands

of ferrite and PPy-DBSA

The calculated crystallite size is presented in Table 4.24. Estimated crystallite size of ferrite,

composite and polymer is 34.66, 4.04 and 8.47 nm respectively. It is an established fact that the

grain growth depends upon grain boundary mobility. The mixing of polymer with pure ferrite

reduces the grain growth which decreases the grain size. The decreasing of the crystalline size

with the addition of polymer may be attributed to the fact that higher the porosity smaller the

crystallite size [102].

Fig.4.88: . XRD patterns of (a) Y-type hexaferrite, Sr2Co2Fe12O22, (b) composite (Sr2Co2Fe12O22

+PPy-DBSA) and (c) polymer PPy-DBSA.

Table 4. 24: Parameters measured from XRD patterns for ferrite (Sr2Co2Fe12O22), (b) composite

(Sr2Co2Fe12O22 +PPy-DBSA) and polymer (PPy-DBSA).

Parameters Ferrite Composite Polymer

Crystallite size (nm) 34.66 4.04 8.47

Resistivity(Ω-cm) 1.23×106 7.5×103 6.4×101

Activation energy(eV) 0.37 0.24 0.12


180

4.5.2 Scanning Electron Microscopy (SEM) The SEM micrograph of Co2Y ferrite sample, (Co2Y+ PPy-DBSA) and polymer PPy-DBSA are

shown in Fig. 4.89(a-c). The ferrite shows fine structure with few agglomeration. The

micrograph of composite presents a heterogeneous distribution of grain size. Some grains

agglomerate in different masses. Mixing polymer (PPy-DBSA) in the Co2Y ferrite, the grain size

as well as the density decreased than that in pure ferrite, a porous microstructure with little

densification was observed for composite ferrite. For the ferrite sample more dense and

heterogeneous distribution of grain sizes was observed, moreover the grain morphology appears

plate-like for ferrite sample. The particle size of composite ferrite is small enough to obtain the

suitable signal-to-noise ratio in the high density recording media. Therefore, the synthesized

samples can be used as potential material for application in recording media [141].

Fig.4. 89: SEM graphs for (a) Y-type hexaferrite Sr2Co2Fe12O22, (b) composite (Sr2Co2Fe12O22

+PPy-DBSA) and (c) polymer PPy-DBSA


181



Generally speaking all ferrites behave like semiconductors with temperature. However their

resistivity behavior is explained by localized electron model rather than collective band model.

The valence change of Fe ions play a very decisive rule in order to summarized the resistivity

behavior of ferrite materials [122] and sometimes also the presence of Co2+ and Ni2+ ions in Ni

and Co containing ferrites. It is believed that the decrease of resistivity is mainly responsible due

to the mobility of the additional electron, which originates from Fe2+ (or occasionally additional

holes in positive charge containing ferrites), through the crystallattice. The hopping mechanism

between divalent and trivalent ions of the same element present in the hexagonal crystallographic

sites (B-sites) is chiefly responsible for conduction mechanism in ferrite. Furthermore,

Ppy_DBSA composite is electro-active and its behavior is like ananion exchanger. During the

preparation of PPy to balance the positive charge established during the oxidized polymerchain,

numerous dopant anions are incorporated in to the structure of PPy [142]. Typically, in

polypyrrole the DC conductivity exhibits an exponential dependence on temperature [143] and it

is accredited to the presence of anionic dopant in the conducting polymer, which consequently

accountable for the delocalizationofthedoublebond electrons [144].

Also, it has been observed by comparing with the already published data[145] that, the resistivity

of the ferrite nanoparticles is higher than that of the corresponding polycrystalline bulk material

as expected because the ultrafine grains would provide lot of restriction in the flow of electron

because of large number of grain boundaries and consequently resistivity values enhanced [146].

DC resistivity of the composites is mostly lower than that of the pure ferrites and moreover

increases with temperature. This decline of resistivity is due to the insertion of ferrite into PPy-

DBSA in to the ferrite sample, as FeCl3 was used during the synthesis of polymer which

consequently formed a donor–acceptor complex in the conjugated system. Formation of this

donor-accepter conjugated system might be responsible for creation of quasi-particles (polarons

or bipolarons), which behave as charge carriers. The mobility of these charge carriers is

enhanced as temperature increases, so resistivity decreases. This is quite understandable as the

polarons or bipolarons move with higher diffusion velocity when the temperature is enhanced

and thus reducing the resistivity [147].The values of activation energy for the different samples


182

are listed in Table 4.24. Table 4.24 suggests that the values of activation energies of the samples

shows similar trend as that of room temperature resistivity.

Fig. 4.90 exhibits the temperatures dependent DC resistivity has been taken from 292 to 342K

for all samples. The temperature dependence of the DC resistivity executes the semiconducting

behavior of the samples. The behavior of lnρ vs. 1000/T indicates that the electrical resistivity is

thermally activated. A decrease in resistivity with temperature is linked with the mobility of

thermally generated carriers [125].

2.2 2.4 2.6 2.8 3.0 3.2 3.4

2

4

6

8

10

12

14

ln

(ohm

-cm

)

1000/T (K-1

)

ferrite

composite

polymer

Fig.4. 90: Temperature dependent resistivity for (a) y-type hexaferrite Sr2Co2Fe12O22, (b)

composite (Sr2Co2Fe12O22 +PPy-DBSA) and (c) polymer PPy-DBSA.

4.5.4 Dielectric Properties It is believed that Ppy-DBSA exhibit an amorphous structure. Furthermore it is thought that

amorphous structure (non-crystalline materials) consist of imperfections, most noticeable

imperfections are point defects and micro voids. Point defect are supposed to contain impurities

or dangling bonds, whereas typical rule of micro voids is to create the levels inside the band

gap, just as in crystals [126]. The coulomb repulsive energy between electrons is mainly due to

the lattice distortion. Additionally the Van der Waals bonds in the materials like Ppy-DBSA play

a crucial rule to the process of localization. It is more evident to educate the well-known fact that


183

these localized centers are accountable for improvement in the electrical hopping transport

process.

Fig.4.91 shows the variation of the dielectric constant of the present investigated samples with

frequency. A close inspection of the figure suggests that values of dielectric constant are high at

low frequency and vice versa. With increasing frequency values of dielectric constant decreases

which is common behavior of the ceramics like ferrifes. Identical behavior is also studied by

other researchers[1, 27, 28].The lowering of dielectric constant with increase of frequency is

accredited to the well-established fact that under the influence of external electric field the

dielectric material shows induced electric moment. But as the frequency increases the

polarization of induced moments could not synchronize with the frequency of applied electric

field [89]. The decrease of permittivity and dielectric loss in ferrite sample with increasing

frequency is mainly due to the lagging of electron exchange between Fe2+ and Fe3+ ions with

respect to the applied field. The permittivity spectra and dielectric loss of the polymer sample

PPy-DBSA shows significant variation in the whole frequency range. The observed variation in

the values of real part of permittivity and dielectric loss is basically owing to the lagging of

dipole moment of polaron/bipolaron with external applied frequency. Whereas, in case of a

ferrite–polymer composite, the contribution to dielectric constant and dielectric loss occur due to

interfacial polarization and its relaxation as the semiconducting ferrite particles separated by

insulating matrix molecules giving rise to heterogeneity. The values of dielectric constant 1MHz

for all the investigated samples are listed in the Table 4.25. The increase in dielectric constant

(permittivity) of composite sample compared to pure ferrite is attributed to the decrease in DC

resistivity as listed in table 2.

The plot of imaginary part of permittivity ( dielectric loss) with increasing frequency is shown in

the Fig.4.92. Numerous relaxation frequencies of number of dipoles in the hexagonal ferrite

structure are principally endorsed the relaxation because of interfacial polarization due to

hopping of electrons. The values of dielectric loss at 1MHz for all the investigated samples are

listed in the Table 4.25. The increasing behavior of the dielectric constant, dielectric loss is quite

encouraging to suggest their diversified technological applications. Particularly the improved


184

values of dielectric loss of the composite clearly suggest its use in electromagnetic shielding

effect.

14 16 18 20 22

0

10

20

30

40

50

60

70

80

90

100

110

120

130

Die

lect

ric C

onsta

nt (

lnf (Hz)

ferrite

composte

polymer

Fig.4. 91: Dielectric constant of (a) Y-type hexaferrite Sr2Co2Fe12O22, (b) composite


14 16 18 20 22

0

10

20

30

40

50

ferrite

composte

polymer

Diele

ctric

Loss

lnf (Hz)


185

Fig.4. 92: Dielectric loss Factor of (a) Y-type hexaferrite Sr2Co2Fe12O22, (b) composite


Fig.4.93 shows the plot of dielectric tangent loss .which exhibits the resonance peaks in the

spectra,. The presence of are due to the fact, It is believed that when the jumping frequency of

electrons becomes equal to the external applied frequency than resonances peaks occurred in the

dielectric tangent loss spectra [88]. The occurrence of resonance peaks may be understood by an

analogy: if an ion has two different equilibrium conduction states A and B with same potential

energies, then in these conditions it is more probable that the hopping probability of an ion

would be indistinguishable in both cases: from A to B or B to A. The hopping frequency of

several ions among the equilibrium states A and B is called the natural frequency of hop between

these two states.

14 16 18 20 22

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

ferrite

composte

polymer

diel

ectr

ic lo

ss fa

ctor

(ta

n)

lnf (Hz)

Fig.4. 93: Dielectric loss Factor of (a) Y-type hexaferrite Sr2Co2Fe12O22, (b) composite



186

As the external frequency becomes identical to the natural frequency of ions, then the possibility

of transformation of the electrical energy to the oscillating ions is maximum and the power

losses occurs, which subsequently favor the incidences of the phenomenon of resonance.

Therefore, the resonance peaks appear [131]. Moreover according to Rezlescu’s theory the

occurance of the relaxation peaks are predominantly owing to the both n-type and p-type charge

carriers in case of ferrite [132]. The values of dielectric tangent loss at IMHZ are listed in the

Table 4.25.

Table 4. 25: Real and imaginary parts of electric modulus and impedance,at 1MHz and DC

activation energy, exponential factor n and AC activation energy of (a) Y-type hexaferrite

Sr2Co2Fe12O22, (b) composite (Sr2Co2Fe12O22 +PPy-DBSA) and (c) polymer PPy:

4.5.4.1 AC conductivity

Fig. 4.94 reveals the change of the AC conductivity for the present studied samples. It is evident

from the figure that AC conductivity raises with increasing frequency for all the samples. It can

be seen that έ (permittivity) decreases with increasing frequency. This is because as the

frequency increases the electric polarization decreases as dipoles cannot follow up the applied

external AC field. Therefore one can conclude that weakening of induced dipole moment with

Parameters Ferrite composite Polymer

Dielectric constant(ε΄) 15.91 71.02 120.99

Dielectric loss(ε΄΄) 2.31 34.63 41.91

Tangent loss(ε΄΄/ ε΄) 0.16 0.48 0.34

AC conductivity(σAC) 1.29×10-4 1.92×10-3 2.33×10-3

Impudence |Z| (Ω) 38218 41247 42933

Real part of electric modulus (M΄) 0.061 0.011 0.006

Imaginary part of electric modulus(M΄΄) 0.008 0.005 0.002

Exponential Factor n (±0.01) 0.81 0.82 0.9

Estimated activation energy (EAC) 0.067 0.043 0.012


187

increasing applied external frequency. The frequency dependence of AC conductivity can be

stated by the subsequent equation [32];

σtot(ω) = σDC+ Aωn (4.32)

Whereas n is the frequency exponent and A is a pre-exponential factor. This behavior of AC

conductivity may be elucidated on the basis of Maxwell–Wagner model and Koop’s

phenomenological theory. Keeping in view of this theory the experimental results of AC

conductivity at low frequencies designate the grain boundary behavior, while the dispersion at

high frequency might be ascribed to the conductivity of grains [39]. The values of AC

conductivity at IMHZ are listed in the Table 4.25.

A typical log–log behavior of the frequency dependence of electrical conductivity of the

investigated samples is depicted in the Fig. 4.95. AC conductivity obeys the empirical law

σ′(ω)∝ωn, where n is a fractional exponent (0 ≤ n ≤ 1), linked with the dynamic of various

hopping ions [94]

0.00E+000 1.00E+009 2.00E+009 3.00E+009

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

ferrite

composte

polymer

ac

(-c

m)-1

f(Hz)

Fig.4. 94: Polt of AC Conductivity Vs frequency of (a) Y-type hexaferrite Sr2Co2Fe12O22, (b)



188

and values of n are given in the Table 4.25. The n=0 indicate the dc conduction. The high value

of “n” in the present samples reflects that conduction phenomena follow hopping mechanism

[40]. For vibrant ions moving in their lattice sites and hopping to the next immediate sites over

barriers of energy, EAC will obey the subsequent equation;

τ0(T) = τ∞exp(EAC/kT) (4.33)

Where τ∞ is the reciprocal of the frequency of different ions and τ0 the relaxation time for

independent ion-hopping. Usually the energy barrier (Ac activation energy) will be smaller than

the activation energy for the dc conductivity and given by the relation;

Edc = EAC/ (1− n) (4.34)

The estimated activation energy EAC can be calculated by using the values of EDC and nA value

EAC are listed in the Table 4.25.

6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

ferrite

composte

polymer

log

ac(

-cm

)-1

log()

Fig.4. 95: Variation in logσ with logω of (a) Y-type hexaferrite Sr2Co2Fe12O22, (b) composite



Fig. 4.96 and inset reflect the change of the impedance (Z) with frequency and follow the

equation.


189

|Z| = Z΄ + j Z΄΄ (4.34)

Z΄ and Z΄΄ are real and imaginary parts of the impedance respectively. The decrease in the

magnitude of Z with the increase of frequency demonstrating the increase in AC conductivity. It

also specifies the semiconducting nature of the present investigated samples.

4.5.4.2.1 Nyqiust plot (Cole - Cole plot)

The impedance spectroscopy is widely employed to describe the electrical properties of dielectric

materials and interfaces exist in these materials. The impedance data gives both resistive (real)

and reactive (imaginary) components of a material. It may be visualized in terms of any of the

four complex variables admittance (Y*), permittivity (ε*), impedance (Z*), electric modulus

0.00E+000 1.00E+009 2.00E+009 3.00E+009

-5000

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

5.00E+008 1.00E+009 1.50E+009

90

180

Impi

denc

e Z

applied feriquency (Hz)

Impi

denc

e

Z

Applied feriquency (Hz)

ferrite

composite

polymer

Fig.4. 96: Polt of impedance with frequency of (a) Y-type hexaferrite Sr2Co2Fe12O22, (b)


(M*) and dielectric loss (tan δ) in a complex plane plot (Nyqiust plot).Their relation to one

another is as follows:

tanδ = ε΄΄/ ε΄ = Y΄΄/Y΄ = Z΄΄/ Z΄= M΄΄/ M΄ (4.35)


190

Complex electric modulus is fairly strong technique to understand and investigate the several

electrical parameters such as relaxation time, conductivity and ions hopping etc in dielectric

materials. Moreover, it is very obvious to mention the versatile use of Complex electric modulus

plots for analyzing low capacitance dielectric material. So we can easily conclude that Complex

electric modulus plots essentially offer the substitute method to solve the different problem based

on polarization analysis.

The variation of real part of modulus (M΄) and imaginary part of electric modulus (M΄΄) with

increasing frequency at room temperature can be seen in the Figs.4.97-4.98. It can be observed

that the value of (M΄) is smaller in lower-frequency region and improved with the rise of applied

field.

The existence of resonance peaks in the plot of M΄΄ vs frequency in the present samples truly

signifies the relaxation phenomena. The Maxwell–Wagner model offers batter evidence for the

conduct of complex conductivity in heterogeneous systems with two or more phases [46].In the

first case if the grain boundary reside in a small volume, the spectrum of impedance (Z΄΄ versus

Z΄) gives better information about the semi circles in the plane.

14 16 18 20 22

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

ferrite

composte

polymer

M

ln(F)

Fig.4. 97: Variation in Real part of electric Modulus with frequency of (a) Y-type hexaferrite

Sr2Co2Fe12O22, (b) composite (Sr2Co2Fe12O22 +PPy-DBSA) and (c) polymer PPy-DBSA.


191

14 16 18 20 22

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

ferrite

composte

polymerM

ln(F)

Fig.4. 98: Variation in imaginary parts of electric Modulus with frequency of (a) f Y-type

hexaferrite Sr2Co2Fe12O22, (b) composite (Sr2Co2Fe12O22 +PPy-DBSA) and (c) polymer PPy-

DBSA.

There is a probable correlation among the behavior of grain boundary, and the exsistance of the

peaks of Z΄΄ as functions of frequency, in second case if grain boundary reside in a large volume,

the plot of the electric modulus (M*=1/ε*) M΄΄ versus M΄, gives more detail about the

semicircles, indicating that there is a probable relationship between the conduct of grain

boundary and the presence of the peaks in M΄΄ plot as a function of frequency [47] second case is

more consistent with our present experimental findings.

Fig.4.99. shows the complex impedance (Cole-Cole) plots of the under investigated samples.

The the grain resistance is determined by the left end (lower frequency) of the semicircle [3]

where as that at intermediate frequencies denotes grain boundary contribution [50] and the whole

resistance of the grains and grain boundaries is determined by right one (higher frequency[3].


The variation of Q values with frequency of the present samples ii depicted in the Fig. 4.100. The

large values of quality factor occurs above the 2GHz frequency and quite high Q values were

found. This high Q values and a resonance frequency above 2 GHz, evidently propose that these

samples can be used in high frequency multilayer chip inductors [51]


192

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.080 0.088

0.010

0.012

0.014

x=0.0, y= 0.0

M

M

m

m

composite

polymer

Fig.4. 99: Cole–Cole plots of electric Modulus with frequency of (a) Y-type hexaferrite


0.00E+000 1.00E+009 2.00E+009 3.00E+009

150

200

250

300

350

400

0.00E+000 1.00E+009 2.00E+009 3.00E+009

171

172

173

174

175

176

177

178

179

180

Q v

alue

s

Applied feriquency (Hz)

ferrite

composite

polymer

Q v

alue

s

Applied friquency (Hz)

composite

polymer

Fig.4.100: Variation of quality factor (Q) values with frequency of (a) Y-type hexaferrite



193


4.5.5.1 Hysteresis Loop

The hysteresis loops were recorded on VSM at temperature 300K. Hysteresis loops recorded for

pure ferrite and composite sample are shown in Fig.4.101. The remanence and coercive force

for polymer magnetic composite were determined from magnetic hysteresis loops accordingly.

The magnetic properties that were obtained for these measured quantities are listed in the table

(2).The increase in coerecivity (Hc) of composite ferrite may be due to the smaller particle size

as compared to the pure ferrite indicating that polymer inhibit the grain growth of the filler.

Fig.4. 101: M–H loops for (a) Y-type hexaferrite Sr2Co2Fe12O22 and (b) (Sr2Co2Fe12O22

+PPyDBSA) composite.

It is well established fact that particle size has a significant effect on the magnetic properties of

magnetic materials [72]. When particles are larger than the critical single domain size then they

in essence exist in multi- domain. On contrary, when particle size become smaller than the

critical value they are mainly exist in single domain form. Mixing of ferrite with polymer results

in decrease of particle size which consequently increases the Hc. Clearly the Ms and Mr values


194

decrease with the mixing of pure ferrite in polymer as shown in the Table 4.26. The decrease in

Ms and Mr is due to the fact that polymer is nonmagnetic, mixing of ferrite in polymer reduce the

total magnetic moment and as the result saturation magnetization and renanant decreases The

saturation magnetization is related to Hc through Browns relation [58], Hc =2x1/μoMswhere Hc

is inversely proportional to Ms, this is consistent with our experimental results and with the

results reported earlier [58]. Magnetic moment (nB) is calculated according to relation (5) and the

values are tabulated in Table 4.26. Decreasing of Magnetic moment (nB) may be attributed due to

the weakening of exchange interactions.

Magnetic moment (nB) = molecular weight ×saturation magnetization/5585 (4.36)

In conventional longitudinal magnetic recording (LMR), the magnetization in the bits is directed

circumferentially along the track direction. In perpendicular recording media (PRM) the

“magnetic bits” point up or down perpendicular to the disk surface. The well-liked explanation

for the advantage of perpendicular recording is that it can deliver more than three times

the storage density of traditional longitudinal recording. Magnetic samples with higher coercivity

are inherently thermally more stable which is proportional to the product of volume times

the uniaxial anisotropy constant Ku, the product is of course larger for higher coercive material.

PRM requires high coercivity medium because of the fact mentioned above. In the present study,

the composite ferrites can be used in PRM due to large value of coercivity ~1896(Oe)

From the loops, the values of saturation magnetization (MS) were deliberated by the law of

approach to saturation [58] and fitted curve for ferrite Co2Sr2Fe12O22 and composite

(Co2Sr2Fe12O22+PPy-DBSA) of Ms calculated by above mentioned law at the room temperature

is shown in Figs.4.102-4.103. For the polycrystalline sample, the magnetization M(H) in the

following is replaced by the precise name polarization in order to apply this law. The data were

fitted using a least squares procedure by the following law of approach to saturation [70, 117].

M = Ms(1- A/H - B/H2) + χH (4.37)

Where Ms is saturation magnetization assuming that the Brillion function is equal to unity, A is

in homogeneity parameter, B factor which is proportional to K2 (K is the anisotropy constant), H

is the applied field and χ is the susceptibility. Substianal difference in the experimental and

theoretical values of saturation magnetization of ferrite and composite ferrite may be due to the

insufficient field applied in the former case while in the latter case infinite field is applied in

http://en.wikipedia.org/wiki/Computer_storage_density


195

order to achieve aforementioned property. Insufficient field in experimental case can be viewed

from the Fig.4.101. Elaborating that further magnetization might be achieved by enhancing the

external field, which will give close agreement between theoretical and experimental values.

Fig.4. 102: Fitted curve for Ms of (Co2Sr2Fe12O22 +PPy-DBSA) calculated by law of approach to

saturation.

Fig.4. 103: Fitted curve for Ms of Co2Sr2Fe12O22 calculated by law of approach to saturation.


196


Squreness ratio (Mr/Ms) ranging from 0.4247 to 0.6045 for CoY- ferrite and composite ferrite

respectively and tabulated in Table.4.26. Although squreness ratio is well below of typical

value~1 for single domain isolated ferromagnetic particle. Still comparatively higher value of

squareness ratio of composite ferrites reveal that some particles may reside as single domain

whereas in CoY ferrite lower value of squareness ratio shows that particles are completely

randomly oriented and exist in multi domains.

Table 4. 26: Saturation magnetization (Ms), remanance (Mr), coercivity (Hc), Squareness ratio,

anisotropy constants (K) and magnetic moment For Co2Sr2Fe12O22 and composite ferrite.

The anisotropy constant (K=HcMs/2) was calculated using the given relation and assuming them

magnetic particles to be isolated (exchange interacting spin) single domains [65]. The anisotropy

constants for CoY ferrite K= 2.34× 104

(erg/cm3) and for composi te K=

2.82× 104(erg/cm

3). These values of K are less than that reported for different ferrites. This

shows that grains are not single domains and anisotropy contribution is not uniaxial [67, 118].

Ms(emu/g)

(experimental)

Ms(emu/g)

estimated

Mr

(emu/g)

Hc

(Oe)

Mr/Ms K

(erg/cm3

)

nB

(emu/g)

Ferrite 64.88 80.28 27.56 724 0.42 2.34× 10

4

15.279

composite 29.84 37.22 18.13 1896 0.60 2.82× 10

4

4.565


197

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Conclusions

Thesis Summary and Conclusions The present research work comprises of synthesis of Y-type hexaferrites doped with different

trivalent and divalent metal cations. Simple and economical wet chemical method has been

adopted for the synthesis of these ferrites. Three series of Strontium-Cobalt base Y-type

hexaferrites; Sr2Co2-xMnx TbyFe12-yO22 , Sr2Co2-xNix EuyFe12-yO22 (x = 0.0–1, y = 0.0–0.1) and

Sr(2-x)Sm(x)Co2NiyFe(12-y)O22 (x= 0.00-0.10; y= 0.00-1.25) have been prepared by normal

microemulsion method sintered at 1050°C. Two series of ferrite-polymer composites were also

prepared by mixing the ferrite fller in polymer matrix; (1) ferrite-polymer thick film composites

were prepared by mixing the Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22 hexaferrite with different ferrite

filler ratio 1:0, 1:0.25, 1:0.50, 1:0.75 and 1:1 in the Polystyrene (2) A composite prepared by

mixing Co2Sr2Fe12O22 with conducting polymer PPy-DBSA (1:1) and the results are compared

with the pure ferrite and polymer.

Structural analysis has been carried out by X-ray diffraction. XRD patterns recorded at room

temperature were indexed by comparing with JCPDS cards for pure hexagonal phase. The XRD

analysis reveals single phase Y-type hexagonal ferrites without any traces of impurities in the

XRD patterns. Lattice parameters a and c were observed to increase due to (Tb-Mn) and (Eu-Ni)

substitution with larger ionic radii as compared to pure sample (x = 0.0). Whereas the lattice

parameters were found to decrease for (Sm-Ni) substitution due to smaller ionic radii. The XRD

pattern of the polystyrene sample shows typical amorphous behavior. With increasing

concentration of ferrite in Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22 the peaks becoming more intensive and

less broadening which suggest that crystallinty is improved with the addition of ferrite filler. A

co-existence of both phases in PPY-DBSA+Sr2Co2Fe12O22 composite sample was also observed.

The intensity of X-ray reflections of ferrite filler in composite sample is diminished due to the

amorphous nature of the polymer, PPY-DBSA.

Average crystallite size was calculated using the Scherer’s formula for all the substituted ferrite

samples and the crystallite size was found in the range of 30-89 nm. An inhomogeneous grain

size distribution has been observed by the microstructural analysis (SEM) and grain size is found

to increase by substitution of Tb–Mn, Eu-Ni and Sm-Ni in Y-type hexaferrite, Sr2Co2Fe12O22.

CONCLUSION

204

The micrographs also exhibit a few agglomerates of platlet like particles of hexaferrites. The

EDX analysis indicates the elemental percentage of the elements present in each sample, that

reveals the stoichometry of the ferrites investigated.

In case of Polystyrene +ferrite composites, a keen observation of the SEM images clearly

suggests that the grain morphology changes noticeably with increasing ferrite ratio. The SEM

images of the composite samples shows the mixed contrast of both the phases due to continuous

overlayer of the polymer produced on the ferrite particles surface. Where as the planar grains of

the Y- type hexagonal ferrite ceramics become equiaxed crystals. Mixing polymer (PPy-DBSA)

in the Co2Sr2Fe12O22 ferrite, the SEM micrographs shows the mixed contrast of both the phase as

observed in the X-ray diffraction.

DC resistivity was found to increase from 106 -109 Ω-cm due to substitution of (Tb-Mn), (Eu-Ni)

and (Sm-Ni) cations in ferrite samples, since Tb3+, Eu3+ and Sm3+ ions prefer to occupy

octahedral sites followed by the migration of some Fe3+ ions to tetrahedral sites and converting

them into Fe2+ ions. As a result Fe3+ ions concentration is lowered at octahedral sites hence

limiting the hopping probability between Fe3+ and Fe2+ ions thereby increasing the resisitivity.

Two different regions were observed in the temperature dependent resistivity plots, ferro-region

and Para-region. The thermal energy in first region (ferro-region) is not sufficient to disturb the

aligned spins of electrons. Whereas, in the second region (para-region) the thermal energy is

sufficient to disturb all the aligned spins of electrons. The variation of activation energy as a

function of (Tb-Mn), (Eu-Ni) and (Sm-Ni)- concentration is in agreement with the room

temperature resistivity. The activation energy in ferromagnetic region is lower than the

paramagnetic region, due to the fact that poloran hopping required comparatively more energy

than that of electrons hopping. The measured values of activation energies in the paramagnetic

region are greater than 0.40 eV, which obviously propose that the conduction is due to polaron

hopping.

The decrease in Curie temperature (obtained from Arrhenius plots) with increasing (Tb-Mn),

(Eu-Ni) and (Sm-Ni) contents may be attributed to Fe3+–O– Fe3+ and Fe3+–Fe3+ angles, that leads

to a decrease in the magnetic moment interaction. Moreover the substitution of RE for Fe3+ ions

causes partial disorder and weakens Fe3+–O–Fe3+ superexchange interactions, where the valence

of the iron ion changes from Fe3+ with a high spin state (3d5 with 5 μB) to Fe2+ with a low spin

CONCLUSION

205

state (3d6 with 4μB), such valence change results in deviation from collinear to non-collinear

arrangement, this supervenes to a decrease in the Curie temperature.

The DC electrical resistivity of PST+ Sr1.8Sm0.2Co2 Ni1.50 Fe10.50O22 decreases from 1014-1010 Ω-

cm with increasing the ferrite filler concentration. This deterioration in resistivity is mainly due

to the insertion of comparatively less resistive ferrite into the highly insulating polymer matrix of

PST. The temperature dependent resistivity specifies that the charge species contributing in the

electrical resistivity are thermally activated. A decrease in resistivity with temperature indicating

semiconducting behavior and linked with the improvement in the drift mobility.

The resistivity of the ferrite-PPy/DBSA composite decreases to103 Ω-cm relative to the pure

ferrite having resistivity 106 Ω-cm due to conducting nature of the PPy/DBSA polymer. The

temperature dependent dc resistivity of ferrite, Polymer and composite samples decreases with

the increase of temperature that may be attributed to the semiconducting behavior of the samples.

It was observed that the samples having higher values of resistivity possess higher activation

energy and vice versa.

The dielectric constant of all the samples shows dispersion as a function of frequency. The

values of dielectric constant, complex dielectric constant and loss tangent are high at low

frequency and then decreases rapidly with the increase in frequency in accordance with Maxwel

Wagner model. The composition dependent dielectric properties of (Tb-Mn), (Eu-Ni) and (Sm-

Ni) substituted ferrites shows a systematic decrease up to the frequency of 2 GHz coupled with

few anomalous and resonance peaks at frequency greater than 2 GHz. The reduction in the

values of dielectric constant with increasing concentration is due to depleting concentration of

iron ions at octahedral sites that play a dominant role in the dielectric polarization. The electron

transfer between Fe2+ and Fe3+ ions (Fe2+ ↔ Fe3+ + e–) is hindered hence the polarization

decreases.

The dielectric constant decreases from 16-6 at 1 MHz for all the three series of substituted

samples. Dielectric losses have been found to decrease from 2.31 to 0.41 at 1 MHz. The decrease

in dielectric constant at fixed frequency is consistent with the increase of dc resistivity. The

resonance peaks in tanδ(f) are observed when the external electric field matches with the

hopping frequency of charge carriers.

The dielectric constant of the PST, composites (PST+ Sr1.8Sm0.2Co2 Ni1.50 Fe12O22) FP1, FP2,

FP3, FP4 and ferrite is 12.17, 13.10, 14.24, 15.03 15.89 and 16.17 at 1MHz respectively. The

CONCLUSION

206

observed enhancement in the dielectric constant (permittivity) with increasing ratio of ferrite

filler is mainly due to the electron exchange between Fe2+ ↔ Fe3++ē which consequently results

in enhancement of electric polarization as well as dielectric constant. It is observed that more the

number of iron ions more will be the polarization. The cations Ni2+, Co2+, Sm3+ and Fe3+ at their

own respective conduction sites (B and A sites) in different S and T blocks of Y type

hexaferrites are responsible for the formation of dipole moments with adjacent O2+ ions

contributing to the dielectric behavior through dipole polarization, interfacial polarization and

dipole relaxation. The variation of the dielectric loss with frequency shows oscillatory behavior

of the peaks. Higher the ferrite content more will be the overlapping of precise motion of several

crystallites which consequently smoothened absorption curve. The existence of resonances peaks

in the dielectric tangent loss spectra are due to the fact, that when the external applied frequency

becomes equal to the jumping frequency of electrons between Fe2+ and Fe3+ ions.

The dielectric constant of the polymer PPY/DBSA, composite sample (PPY/DBSA+

Sr2Co2Fe12O22) and pure ferrite Sr2Co2Fe12O22 is observed to be 16, 71 and 121 at 1MHz

respectively. The dielectric loss of the PPY/DBSA, composite (PPY/DBSA+ Sr2Co2Fe12O22) and

pure ferrite Sr2Co2Fe12O22 is 2, 35 and 42 at 1MHz respectively. The Composite sample (

PPY/DBSA+ Sr2Co2Fe12O22) exhibit larger dielectric constant and dielectric loss due to

conducting nature of the polymer PPY-DBSA as compared to ferrite filler.

The increasing behavior of the dielectric constant, dielectric loss is quite encouraging to suggest

their diversified technological applications. Particularly the improved values of dielectric loss of

the composite clearly suggest its use in electromagnetic shielding effect. The presence of

resonance peaks are observed because of jumping frequency of electrons becomes equal to the

external applied frequency.

The cole-cole plots between real and imaginary part of electrical modulus shows the semicircle

for most of the samples to elaborate the grain and grain boundary contribution towards the

dielectric relaxation phenomena. The frequency dependent AC conductivity follows power law

with large value of exponent, n that shows the polaron hopping is the likely conduction

mechanism. The estimated AC activation energy is found lower than the activation energy for

the dc conductivity.

Magnetic analysis of Tb–Mn and Eu-Ni substituted Y-type hexaferrites Sr2Co2Fe12O22 at room

temperature revealed that substitution causes a decrease in the Saturation magnetization from 64

CONCLUSION

207

to 16 (emu/g) and increase in coercivity from 724 to 3195 (Oe). While in case of Sm-Ni

substitution the Saturation magnetization decreases and coercivity increases up to the x= 0.04,

Y= 0.50 due to strengthening of superexchange interactions and collinear orientations of spins.

Whereas above x=0.04, Y= 0.50 substitution saturation magnetization decreases and coercivity

increases. The difference in ionic radii of the substituent and dopant (Sr-1.27 A. Sm = 0.964 Å)

play a crucial role in the interactions of various sites. The interaction 6c- 6c is reinforced while

the interaction 6cv,-3bv, is weakened. These results favor the occurrence the spin canting

between the successive spinel blocks that leads to the formation of a helicoidal spin order, which

consequently decrease the saturation magnetization. Overall the Saturation magnetization and

coercivity follows the Browns relation.

The values of magnetic moment (nB) of Tb–Mn, Eu-Ni and Sm-Ni substituted Y-type

hexaferrites Sr2Co2Fe12O22 varies from 15.27 to 3.68 (emu/g), 15.27 to 6.07(emu/g) and 15.27 to

8.31(emu/g) respectively . The decrease of magnetic moment follows saturation magnetization

that is governed by the weakening of super exchange interactions.

In case of Sr1.8Sm0.2Co2Ni1.50Fe12O22/PST composite samples the Saturation magnetization,

retentivity and coercivity increases with increasing concentration of ferrite. It is assumed that the

Ms of SSCNF/PST nanocomposites predominantly depends on the volume fraction of

Sr1.8Sm0.2Co2 Ni1.50 Fe12O22 magnetic ferrite particles, the increasing volume fraction percentage

of Sr1.8Sm0.2Co2 Ni1.50 Fe12O22 enhance the saturation magnetization (Ms) of the composite

samples. It may be conclude that the net magnetic moment achieved by the composite samples

turns out to be directly the vector sum of the every individual ferrite grain contributions inside

the polymer matrix. For composite, Sr1.8Sm0.2Co2 Ni1.50 Fe12O22/PST (1:1) quite improved values

of Hc is achieved which is essentially advantageous for practical use of this composite sample

for memory devices. The composite sample Co2Sr2Fe12O22/PPy-DBSA (1:1) executes low

magnetization and high coercivity with respect to ferrite filler, which is an interesting result with

an added advantage of flexibility of the composite material.

The synthesized hexaferrite materials and their composites in the present study have Hc>Mr/2

suitable for high frequency applications. The calculated values of magnetocrystalline anisotropy

constant are less than that reported for single domain ferrites. This shows that grains are not

single domains and anisotropy contribution is not uniaxial. Even though squareness ratio(Mr/Ms)

CONCLUSION

208

is well below of typical value ~1 for single domain isolated ferromagnetic particle. Still

comparatively higher value of squareness ratio is obtained especially at higher substitution level

suggesting that few particles may reside as single domain. The large difference between

experimental and theoretical values of saturation magnetization for all the samples observed by

fitting the law of approach is due to the deficient field applied in the experimental case while in

the theoretical case infinite field is applied in order to attain maximum value of saturation

magnetization.

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prr.hec.gov.pkprr.hec.gov.pk/jspui/bitstream/123456789/6699/1/Irshad_Ali_2015... · CERTIFICATE This is to certify that Mr. IRSHAD ALI has carried out experimental work in this dissertation