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"ALEXANDRU IOAN CUZA" UNIVERSITY
OF IAŞI
FACULTY OF PHYSICS
PhD Thesis Summary
Magnetoelectric systems based on ferroelectric perovskites
PhD student:
Alexandra Guzu (married Maftei)
Scientific coordinator:
Prof. Univ. Dr. Liliana Mitoşeriu
thesis presented
in partial fulfilment of the requirements
for the title of Doctor of Science in Physics
Iasi
September 2020
1
In the attention of
........................................................................................................
“ALEXANDRU IOAN CUZA” UNIVERSITY, IAȘI
FACULTY OF PHYSICS
we would like to inform you that on 03.09.2020, at 11:00, Mrs. GUZU
ALEXANDRA (married to MAFTEI) will defend, in a public online meeting,
the doctoral thesis entitled MAGNETOELECTRIC SYSTEMS BASED ON
FEROELECTRIC PEROVSKITS, in view of obtaining the scientific title of
doctor in the fundamental field: EXACT SCIENCES, field: PHYSICS.
The doctoral commission has the following composition:
President:
• Prof. univ. dr. Diana MARDARE, Director of the Doctoral School, Faculty
of Physics, “Alexandru Ioan Cuza” University of Iași
Scientific coordinator:
• Prof. univ. dr. Liliana MITOȘERIU, Faculty of Physics, “Alexandru Ioan
Cuza” University, Iași
Reviewers:
• Prof. univ. dr. Viorel POP, “Babeș Bolyai” University of Cluj Napoca
• Prof. univ. dr. Daniel VIZMAN, West University of Timișoara
• Prof. univ. dr. hab. Laurențiu STOLERIU, “Alexandru Ioan Cuza”
University of Iași
The thesis can be consulted at the Library of the Faculty of Physics.
2
The content of the thesis
Abstract 4
Thanks 5
I. Introduction 6
I.1 Introductory notions 8
I.1.1 Magnetoelectric systems 8
I.1.2 Ferroelectrics - general properties 10
I.1.3 Magnetic materials. Ferrite 17
I.1.3.1 Classification of ferrites 18
I.1.3.2 Spinel structure 19
I.2 Magnetoelectric multiferroics 20
I.2.1 Magnetoelectric composite materials 22
I.2.2 Applications of magnetoelectric composites 27
I.2.3 Percolation 33
Bibliography I 40
II. Description of the experimental methods used 45
II.1 Structural and phase analysis by X-ray diffraction 45
II.2 Microstructural analysis 46
II.3 Impedance spectroscopy 49
II.4 Determination of the ferroelectric hysteresis cycle P (E) and
nonlinear dielectric properties ("DC tunability") 51
II.5 Determination of magnetic and magnetoelectric properties
52
Bibliography II 55
III. Preparation of composite ceramics 56
Bibliography III 65
IV. Study of the role of the type of interconnectivity on the macroscopic
properties of composites 0.66BT-0.33CF 66
IV.1 Preparation, structural and microstructural characterization
of samples 66
IV.2 Estimation of effective permittivity 76
IV.3 Dielectric and ferroelectric properties 78
IV.3.1 Weak field electrical properties 78
IV.3.2 Ferroelectric properties 83
3
IV.4 Magnetic and magnetoelectric properties 85
IV.4.1 Magnetic and thermomagnetic properties 85
IV.4.2 Magnetoelectric coupling properties 86
IV.5 Conclusions 88
Bibliography IV 91
V. Contributions to the study of ceramic magnetoelectric systems
consisting of barium titanate with cobalt-zinc ferrites 95
V.1 Composite preparation, structural characterization (XRD)
and microstructural (SEM) 96
V.2 Weak field dielectric properties as a function of temperature
and frequency 102
V.3. Magnetic properties, nonlinear dielectric character and
magnetoelectric coupling 107
V.4 Conclusions 112
Bibliography V 115
VI. Study of the laminar composite 0.33BaTiO3 – 0.33Co0.8Zn0.2Fe2O4 –
0.33BaTiO3 119
VI.1 Microstructural characterization 119
VI.2 Weak field electrical properties 121
VI.3 Electrical properties at different temperatures 125
VI.4 Magnetic properties 127
VI.5 Conclusions 129
Bibliography VI 131
VII. General conclusions 132
List of original publications 138
International conferences 138
National conferences 139
4
Acknowledgements
I would like to especially thank Mrs. Prof. Univ. Dr. Liliana
Mitoșeriu, my scientific coordinator, who showed a lot of understanding,
patience and guided me throughout my doctoral studies.
I also thank the ladies Dr. CS II Cristina Ciomaga, lect. univ. dr. hab.
Lavinia Petronela Curecheriu and dr. CS III Felicia Gheorghiu, who are part
of the guiding commission, for the support provided during the period of
doctoral studies and thesis elaboration.
Last but not least, I would like to thank the members of the doctoral
committee present at the public presentation and Mrs. Prof. Univ. Dr. Diana
Mardare for the time given and for the honor of reviewing this paper.
I also express my gratitude to Mr. assistant. univ. Dr. Leontin
Pădurariu and Dr. Mirela Airimioaei and Dr. Nadejda Horchidan from the
group of "Dielectrics, Ferroelectrics and Multiferoics" without which this study
would not have taken shape.
The financial costs were borne by the projects PN-II-PT-PCCA-2013-
4-1119 (MECOMAP) and UEFISCDI PN-III-P4-ID-PCCF-2016-0175
(HighKDevice).
5
I. Introduction
Magnetoelectric materials are those materials in which the ferroelectric
and magnetic order coexist simultaneously, these being a field of current interest
both from a theoretical point of view and for technological applications such as
sensors, actuators, transducers, data storage devices (memories, in which
writing data could be done with an electric field, and reading them with a
magnetic field), etc. It is known that the electrical polarization of a material
changes following the application of an electric field, and magnetization by the
action of a magnetic field. In the case of magnetoelectric materials, by applying
an electric field (respectively a magnetic field) a variation of the magnetization
(respectively of the electric polarization) is observed.
The studies performed on this type of systems aim at obtaining
magnetoelectric materials having simultaneously dipolar and magnetic order
(ferroelectric and ferro, feri- or antiferomagnetic properties) in the same
structure and high magnetoelectric coefficient, in the field of temperatures of
interest for applications, with resistance to high corrosion and mechanical
hardness, and can be made at relatively low prices. There are very few single-
phase magnetoelectric materials, and existing ones usually have these properties
at cryogenic temperatures and as such, there is a permanent interest in finding
new single-phase or composite materials that sum up these properties at ambient
temperature.
The research in this paper focused on the study of magnetoelectric
composites consisting of a magnetostrictive oxide material and a piezo /
ferroelectric material, having two types of arrangements of the constituent
phases: (i) structure multilayered and (ii) structure with randomly mixed phases.
In these composites, none of the component materials taken separately possess
magnetoelectric properties, but together, the magnetoelectric effect may occur
as a product property, through the mechanical coupling between them.
The analyzed composites are composed of oxide materials, namely,
BaTiO3 (BT) the best known ferroelectric oxide with the perovskite structure
ABO3, in combination with spinel ferrites type CoFe2O4 (CF) and
Co0.8Zn0.2Fe2O4 (CZF). It is known that ferroelectric has electrical insulating
properties, is characterized by high permittivity and low losses, while ferrites
are usually semiconductor materials with low permittivity and high dielectric
losses. Usually, compositions are chosen in which the dielectric phase is
predominant, in order to limit the conduction and dielectric losses and to obtain
a better magnetoelectric coupling, but too little ferrite leads to the weakening of
6
the magnetic characteristics (decreased saturation magnetization and remnants
of composite, due to a “dilution” effect due to mixing with a material without
magnetic ordering, BaTiO3).
In this paper, combinations were chosen between the two materials in
which the ferrite concentration is at the limit of the percolation region, ie 33%
to maintain a strong magnetic response and also trying to maintain the dielectric
character in the composite. In order to understand the role of microstructure on
their dielectric and magnetic properties, combinations were made with the same
composition of the two phases (66% ferroelectric - 33% ferrite), but distributed
differently in the volume of the composite. Thus, the magnetoelectric materials
studied in this paper are mixed composites with the formula 0.33CF - 0.66BT
and 0.33CZF - 0.66BT respectively with random phase mixing, as well as
laminated composites (triple-layer) type 0.33BT - 0.33CZF - 0.33BT and
0.33BT - 0.33CF - 0.33BT, respectively. The properties of these composite
ceramics, having the same composition but with the constituent phases placed
in different ways (random mixing or in laminar structures) were analyzed
comparatively and described by finite element modeling. It was also
investigated for the same composition and how the sintering method modifies
the microstructural characteristics (porosity, granulation) and functional
properties when using two different sintering methods: traditional method
sintering and plasma arc sintering, having phases mixed randomly.
I.1 Introductory notions
I.1.1 Magnetoelectric systems
The magnetoelectric effect was first observed by Röntgen in 1888 and
by Pierre Curie in 1894 [2] in two independent studies. Pierre Curie identified
the magnetoelectric effect by analyzing crystalline symmetry criteria. The term
"magnetoelectric" was first used by Debye in 1926 [3], and the first single-phase
material with magnetoelectric switching (having hysteresis M(E) and P(H),
respectively), discovered was Cr2O3, but which had low values of polarization
and field-induced magnetization. Subsequently, the research was extended to a
large number of materials and it was established that more than 80 categories of
single-phase materials (including Ti2O3, GaFeO3, phosphates, boracites) as well
as a large number of their combinations have a magnetoelectric effect.
For a given material, it is important to describe and understand the
relationships between complex electrical, mechanical and magnetic properties.
7
These cause-effect (stimulus-response) relationships, in which experimentally
determinable material constants are involved, are schematically illustrated in
the Heckmann diagram [1] (Figure I.1).
From a mechanical point of view, it is interesting the interdependence
between the deforming forces X and the deformations x, which describe the
elasticity as the principal effect. The electrical properties describe the response
of the polarization P to the application of an electric field E, and the magnetic
ones represent the response of the magnetization M to the application of the
magnetic field H. Practically, in the case of simple relations, each property is
independent, an electric field E can determine the polarization P, and the
deforming force X can control the deformation x of the environment.
Figure I.1: The Heckmann diagram shows the relationship between the
electrical, mechanical and magnetic properties of the material [1].
When we can control the polarization by the action of a magnetic field
or reciprocal, the magnetization by the action of an electric field, we are talking
about the existence of a magnetoelectric effect (ME) in the material.
The magnetoelectric effect (ME) represents the variation of an electric
quantity when applying a magnetic field and vice versa; it can be primary or
8
secondary. The primary magnetoelectric effect consists in the appearance of an
electric polarization under the action of a magnetic field P(H):
𝑀𝐸 =𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐
𝑚𝑒𝑐𝑎𝑛𝑖𝑐×
𝑚𝑒𝑐𝑎𝑛𝑖𝑐
𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 (I.1)
or in the occurrence of a magnetization when applying the electric field M(E)
(electromagnetic effect) described schematically as follows:
𝐸𝑀 =𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐
𝑚𝑒𝑐𝑎𝑛𝑖𝑐×
𝑚𝑒𝑐𝑎𝑛𝑖𝑐
𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 (I.2)
The secondary magnetoelectric effect consists in the variation of the permittivity
under the action of a magnetic field ɛ(H) or the variation of the permeability
to the application of a magnetic field µ(E).
I.1.2 Ferroelectrics - general properties
Ferroelectric materials represent a special class of polar dielectrics,
which have the property of reversing their polarization P(E) or electrical
induction D(E) in the presence of an external electric field. This property of
ferroelectrics underlies many applications based on the controlled and
reversible change in the electrical state of the material.
These substances, unlike linear dielectric media, in which there is a
linear variation of the electric polarization/induction with the applied field (and
whose permittivity is a constant with respect to the electric field), are nonlinear
dielectric media for which the permittivity is a function of the applied field ɛ(E).
Ferroelectric media have hysteresis and electrical remanence properties. Unlike
linear dielectric media, ferroelectric substances also have special mechanical,
thermal and optical properties and have a coupling between them. Ferroelectric
materials can vary their electrical polarization under the action of temperature
variations and therefore have a pyroelectric character and also can vary their
electrical polarization following the application of mechanical actions, so they
are piezoelectric. Consequently, ferroelectrics are multifunctional materials
with memory (hysteresis), being at the same time pyro- and piezoelectric.
The main characteristics of ferroelectric materials are [4-8]:
(1) Spontaneous polarization (PS) is defined as the maximum value of
9
the polarization of a single ferroelectric domain in the absence of
an external electric field and an external mechanical deformation.
(2) Ferroelectric hysteresis The main property of ferroelectric
materials is the hysteretic character of the polarization depending
on the applied external electric field.
(3) Dependence of the permeability on the applied electric field
("tunability") Ferroelectric materials have the property of changing
the value of the permittivity according to the value of the intensity
of the applied electric field ("tunability"), a very important property
in various applications.
I.1.3 Magnetic materials. Ferrites
Ferrites are complex oxides that usually contain M2+ divalent metals and
have the general chemical formula 𝑀2+𝐹𝑒23+𝑂4
2−. Ferrites are a class of
materials characterized by weak ferrimagnetism and/or ferromagnetism, having
electrical, dielectric or semiconductor properties and are widely used in technical
applications especially for their combined properties.
Ferromagnetism is a property specific to certain materials that consists
in the presence of a spontaneous magnetization in the absence of the external
magnetic field. Any ferromagnetic material has a Curie magnetic temperature
above which the material loses its ferromagnetic properties, becoming
paramagnetic. Examples of such ferromagnetic materials are Fe, Co, Ni, Mg, Zn
and combinations thereof in alloys or oxide compounds.
I.1.3.1 Classification of ferrites
Ferrites were classified according to the shape of the hysteresis curve
M(H) and according to the values of the main magnetic characteristics (coercive
field Hc and residual induction Br) into two broad categories:
→ Soft ferrites - are characterized by a high saturation magnetization and
a low coercive field, small cycle area;
→ Hard ferrites - usually have a hexagonal crystal structure and have the
following magnetic properties: very high coercive field and high residual
induction, large M(H) cycle area.
10
I.1.3.2 Spinel structure
The spinel-type crystalline structure is characteristic of ferrites, and the
specific chemical formula is of the form: 𝐴2+𝐵23+𝑂4
2−, where A is a divalent ion
and B is a trivalent ion. The spinel structure has ions placed in a cubic grid with
compact packing, and depending on the number of neighboring oxygen ions,
cations have two types of interstices: tetrahedral and octahedral. The spinel
elementary cell contains 96 ion-filled interstices or vacancies.
I.2 Magnetoelectric multiferroics
Multiferoic materials are those systems that have in the same phase two
or more types of ferric order (at least two parameters of different order that are
switchable). Magnetoelectric multiferroics (ME) are simultaneously ferro-/feri-
or antiferromagnetic and ferro-/feri- or antiferoelectric in the same phase, and
between the magnetic and ferroelectric order parameter there is a magnetoelectric
coupling [9-12].
The necessary condition for a material to be magnetoelectric is the
coexistence of magnetic and electric dipoles in the same phase, and if they are
also switchable, it is a multiferoic.
I.2.1 Magnetoelectric composite materials
Magnetoelectric composites are made of at least two different materials,
which separately do not possess magnetoelectric properties, but when combined
in the composite, magnetoelectric properties result. One of the materials that
make up the magnetoelectric composite is piezoelectric, and the other is
magnetostrictive. When a magnetic field is applied, the magnetostrictive
component changes its physical dimensions, this deformation being transmitted
to the piezoelectric phase, having as effect the appearance of induced electric
charges. The phenomenon can also occur in reverse: when applying an electric
field there is a change in the physical dimensions of the piezoelectric component,
the effect being the change in the magnetization of the magnetostrictive phase.
I.2.2 Applications of magnetoelectric composites
Based on the type of magnetoelectric coupling and the mechanisms used
to control various parameters, the variety of applications of ME materials
11
includes: magnetic sensors, high frequency inductors, storage devices and high
frequency signal processing devices.
II. Description of the experimental methods used
In this thesis were studied several oxide magnetoelectric composite
ceramic systems consisting of magnetostrictive material (ferrite) and
ferro/piezoelectric material (barium titanate). In these systems, the arrangement
of the constituent phases was different, they being prepared either in the form of
multilayer (laminar structures) and in the form of ceramics with a random phase
mixing. In these systems, the magnetoelectric effect appears as a product
property, none of the component materials taken separately having distinct
magnetoelectric properties.
The research in this doctoral thesis is dedicated to understanding the
relationship between preparation, microstructural characteristics and electrical
and magnetic properties, their description through theoretical models, and testing
for possible applications. The studied systems contain the same
ferroelectric/ferrite volume ratio (composition at the percolation limit), namely
66% ferroelectric BaTiO3 and 33% cobalt ferrites: pure ferrite CoFe2O4 (CF) and
doped with Zn: Co0.8Zn0.2Fe2O4 (CZF). Composites have two types of constituent
phase arrangements: randomly mixed phase composites (type 0-3 or more
complex connectivity) and triple layer structures in which the ferrite layers are
framed between the two dielectric layers of BaTiO3 (type 2-2 connectivity).
Experimental characterization methods
II.1 Structural and phase analysis by X-ray diffraction
To characterize the systems chosen for the study, diffractograms were
recorded using a Shimadzu LabX 6000 diffractometer from the AMON platform,
Faculty of Physics, with CuKα radiation (λ=1.5405Å), with a scanning increment
of 0.02° and a counting time of 1s/step in the range 2θ = 20-80°.
II.2 Microstructural analysis
The microstructures of the samples from this paper were investigated
with a Hitachi S-3400N II scanning electron microscope from the RAMTECH
center (collaboration with Dr. Sorin Taşcu).
II.3 Impedance spectroscopy
12
Complex impedance was measured in the frequency range (20Hz -
2MHz) in the temperature range (20 ÷ 250)°C using an impedance analyzer
precision RLC type Agilent E4980A and for low frequencies Solartron 1260A
Impedance Analyzer (10µHz - 32 MHz) from the endowment of the AMON
platform of the Faculty of Physics. An Agilent E4991ARF impedance analyzer
was used for high frequency dielectric measurements (1MHz-1GHz) performed
at room temperature. Dielectric measurements at low temperatures (-150 ÷
150)°C in the frequency range (1Hz-1MHz) were performed using a dielectric
spectrometer Concept 40 Novocontrol Tehnologies in collaboration with the
Institute of Macromolecular Chemistry of the Romanian Academy „P. Poni ".
II.4 Determination of the ferroelectric hysteresis loop P(E) and
nonlinear dielectric properties ("DC tunability")
The hysteresis cycles of the P(E) polarization were recorded using a
modified Sawyer-Tower circuit, at room temperature using a sinusoidal
waveform of amplitude E0 in the range (1.5-3.5) kV/mm to ensure sample
saturation and different frequencies f = (1-10) Hz. The resistivity of the samples
was checked with a High Resistance Meter (HP 4329A) before the measurements
to check if they are good insulators and if they will withstand cycles of hysteresis
under high voltages.
To determine the nonlinear dielectric properties, ie the dependence ɛ(E)
at high voltages, a circuit designed and built in the Laboratory of Dielectrics,
Ferroelectrics and Multiferoics, of the Faculty of Physics was used.
II.5 Determination of magnetic and magnetoelectric properties
The magnetic properties of the composites were measured at room
temperature under magnetic fields in the (0-14) kOe range using a vibrating
sample magnetometer (VSM, Lake Shore7410, USA) from AMON.
Thermomagnetic analysis (magnetization temperature dependence) was
determined in a magnetic field of 10 kOe, at low temperatures (5÷300 K), using
a QD PPMS-9 system, while measurements above room temperature in the range
(300÷900 ) K were made with the VSM vibrating sample magnetometer model
LakeShore VSM 7410 in collaboration with the National Research Institute for
the Development of Technical Physics Iași, within the research grant in
partnership PN-II-PT-PCCA-2013-4-1119.
13
III. Preparation of composite ceramics
The ferroelectric BaTiO3 (BT) powders were prepared by solid phase
reaction using as precursors titanium oxide TiO2 (Sigma Aldrich, 99.5%) and
barium carbonate BaCO3 (Merck, 99%) mixed in stoichiometric proportions.
The magnetic nanoparticles of cobalt ferrite CoFe2O4 (CF) and cobalt
ferrite substituted with zinc Co0.8Zn0.2Fe2O4 (CZF) in this thesis were prepared
by a simple method, which involves relatively low precursor costs and thermal
budget, using as precursors Fe(NO3)3x9H2O (purity > 99.9%, Sigma Aldrich),
Co(NO3)2x6H2O (Merck, purity > 99.5%), ZnO and HNO3. The synthesis
technique used combines the sol-gel method with self-combustion and involves
the use of a combustion and complexing agent, in this case citric acid C6H8O7
(Sigma Aldrich, purity > 99.5%), whose decomposition generates high
temperatures during the combustion process and determines the formation of the
cubic spinel phase [13,14].
The prepared magnetic and ferroelectric powders were used to obtain
layered (2-2) magnetoelectric composites but also with random phase mixing.
These composites were obtained both by the classical sintering method and by
using plasma arc sintering (SPS). For the realization of dense magnetoelectric
ceramics with fine granulation, in this work was used for densification a plasma
arc sintering system SPS Model FCT- (FAST) HPD5 existing at the National
Research and Development Institute of Technical Physics Iași, in a collaboration
from within the research grant in partnership PN-II-PT-PCCA-2013-4-1119.
IV. Study of the role of the type of interconnectivity on the
macroscopic properties of 0.66BT-0.33CF composites
In this chapter we described the results obtained in the comparative study
of ferroelectric-ferrite-phase composites, having a composition in the vicinity of
percolation: 0.66BaTiO3-0.33CoFe2O3, sintered by SPS, but with different phase
arrangements (randomly mixed phases and structured triple-layer). The aim of
this study was to understand and describe the effect of the type of phase
interconnectivity on the electrical and magnetic macroscopic properties of the
analyzed composites.
IV.1 Preparation, structural and microstructural characterization
of samples
14
Co (CF) ferrite powders were synthesized by the sol-gel method
combined with self-combustion according to the method described in Chapter
III. These, in the case of mixed composite, were mixed in a humid environment
with barium titanate (BT) nanopowders obtained by the hydrothermal method
(Sigma Aldrich), having the characteristics described in Chapter III.
To obtain the layered ceramic composites, a sequence of 0.33BT-
0.33CF-0.33BT was poured into the cylindrical carbon die of the SPS device.
20 30 40 50 60 70 80
0
100
200
300
(311)
(310)
(300)
(220)(211)
(210)(200)
(111)
(110)
(100)
(622)
(533)
(620)
(440)
(511)
(422)
(400)
(222)
(311)
(22
0)
CF
Inte
ns
ity
(u
.a.)
2 (degrees)
BT
0.33BT-0.66CF phase mixture
Figure IV.1: X-ray diffractograms for CoFe2O4 powder, for the BaTiO3 layer
in the laminated ceramic composite and for the ceramic composite with a mixture of
random phases having the composition 0.66BaTiO3 - 0.33CoFe2O4.
The crystalline structure was determined by Rietveld structural
refinement of the entire diffractogram using the GSAS (General Structure
Analysis System) software package developed by Larson and Von Dreele [15].
20 30 40 50 60 70 80
2 (degrees)
Inte
nsit
y (
a.u
)
experimental
calculating
BTO
BTT
BT ceramic
20 30 40 50 60 70 80
2 (degrees)
Inte
ns
ity
(a
.u)
experimental
calculating
BTO
BTT
CF
0.66BT-0.33CF
(phase mixture)
15
Figure IV.2: The results of the Rietveld refinement for the BaTiO3 layer in the
triple-layer ceramic composite and for the mixed composite 0.66BaTiO3 - 0.33CoFe2O4.
SEM microstructure for mixed composite 0.66BaTiO3 - 0.33CoFe2O4:
Figure IV.3: SEM micrograph performed in fracture for the mixed composite
0.66BaTiO3 – 0.33CoFe2O4
In the randomly mixed phase composite, the corresponding BaTiO3 areas
are white and compact, with ultrafine granulation (approximately 150 nm), and
the corresponding dark-colored CoFe2O4 areas are inhomogeneously distributed
in the ceramic, forming large, irregularly shaped areas consisting of large-grained
crystalline agglomerates (~ 1μm). There was a diffusion of Fe and Co ions in
BaTiO3 at the contact interfaces, this doping occurring mainly on the Ti4+
positions, due to the compatibility of their ionic dimensions [16]. This diffusion
of Fe and Co ions in BaTiO3 was also highlighted by the SEM-EDX technique,
confirming the Rietveld structural calculations for this type of composite.
Figure IV.4: SEM micrograph made in fracture for laminated ceramics
16
0.33BaTiO3 – 0.33CoFe2O4 – 0.33BaTiO3
In the case of the laminar structure 0.33BT-0.33CF-0.33BT, sintered in
SPS plasma, SEM micrographs (made in fracture) indicate the obtaining of a
compact, well-densified ceramic, with a clear and regular interface between the
two phases, without pores, obtaining thus a perfect lamination between the two
oxide components through a transition zone with nanometric granules, achieved
by using the plasma sintering technique.
The analysis of the chemical elements performed by SEM - EDX in the
transition region for the laminar structure 0.33BT-0.33CF-0.33BT, indicates the
doping of BaTiO3 with very small amounts of magnetic ions at the interfaces in
the case of the laminar composite.
IV.2 Estimation of the effective permittivity
A numerical estimation of the effective permittivity for the two types of
composites was performed by a technique implemented within the group of
Dielectrics, ferroelectrics and multiferoics, Finite Element Method (FEM). The
simulation results show major differences in the electric field configurations in
the two cases, demonstrating that microstructure and phase connectivity play a
major role in the effective dielectric response. The simulations show that the
composite ceramic with random phase mixture is characterized by an effective
intrinsic permittivity of ~525, almost an order higher than that of the laminated
composite (~58), due to the contribution of the ferroelectric phase which is
subject to a fairly electric field great for such a composition close to the
percolation limit.
IV.3 Dielectric and ferroelectric properties
IV.3.1 Weak field electrical properties
The dielectric properties measured at room temperature as a function of
frequency for the two types of composites analyzed are presented in comparison
in Figure IV.5. Both types of structures show a monotonous decrease in
permittivity as a function of frequency, with a tendency to saturate at a high
frequency above 10 kHz, from giant values corresponding to the frequency of 1
Hz (15000 and 9000 for mixed composite and laminate ) up to 1000 and 60 at 1
17
MHz, respectively.
100
101
102
103
104
105
106
0
3000
6000
9000
12000
15000
BT-CF (composite with
random mixture)
BT-CF (laminate composite)
Th
e r
ea
l p
art
of
perm
itti
vit
y
Frequency (Hz)
100
101
102
103
104
105
106
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0 BT-CF (composite with
random mixture)
BT-CF (laminate composite)
tg
Frequency (Hz) (a) (b)
Figure IV.5: Dependence of the real part of the permittivity (a) and of the
dielectric losses (tgδ) of frequency at room temperature for the two types of structures
BT-CF analyzed composites
The dielectric losses are quite high, indicating a semiconductor character
of the dielectric, rather than an insulating one, with maximums due to the
relaxation phenomena at low frequencies of 10 Hz, especially in the mixed
structure, but also around 1 kHz for both structures. The losses tend to decrease
and reach values corresponding to a dielectric behavior only above 10 kHz, when
these relaxation phenomena cease (fact observed by linearizing the real part of
the permittivity in Figure IV.5 (a).
The experimental dielectric response contains a strong influence of
extrinsic phenomena that are discussed comparatively.
High losses, DC conductivity and the thermally activated relaxation
mechanism, with an activation energy of less than 0.3 eV compared to 0.5 eV are
present in mixed ceramics, compared to layered ones, in which the ferrite layer
with low conductivity is completely isolated from those of BaTiO3.
IV.3.2 Ferroelectric properties
In order to verify the existence of the ferroelectric character in the two
types of composites, the hysteresis cycles of the P(E) polarization in dynamic
regime were measured.
18
-20 -15 -10 -5 0 5 10 15 20-4,5
-3,0
-1,5
0,0
1,5
3,0
4,5
BT-CF randomly
mixed
P (
C
/cm
2)
E (kV/cm)-20 -15 -10 -5 0 5 10 15 20
-40
-30
-20
-10
0
10
20
30
40
P (
C
/cm
2)
E (kV/cm)
BT-CF layered
(a) (b)
Figure IV.6: Hysteresis cycles of polarization at room temperature measured in
dynamic mode for composition ceramics 0.66BaTiO3-0.33CoFe2O4:
(a) randomly mixed, (b) layered structure.
The composite with random phase mixture has a linear dielectric
character (the permittivity is invariable as the field increases), unlike the
laminated composite which is characterized by a nonlinear hysteresis cycle (the
behavior of a nonlinear capacitor over which a leakage resistive component
overlaps).
IV.4 Magnetic and magnetoelectric properties
IV.4.1 Magnetic and thermomagnetic properties
In a composite made of material with magnetic order (ferrite) together
with one without magnetic order (dielectric), the magnetic properties will be
derived from those of ferrite, ie they should have a typical ferrimagnetic magnetic
order determined by uncompensated antiparallel spines from the pure system
CoFe2O4.
The results of the magnetic characterization of the two types of studied
composites are presented comparatively in Figure IV.7, when applying a parallel
magnetic field, respectively perpendicular to the layers.
19
-20 -15 -10 -5 0 5 10 15 20-30
-20
-10
0
10
20
30
-5.0 -2.5 0.0 2.5 5.0
-20
-10
0
10
20
H (kOe)
M (emu/g)
BT-CF (randomly
mixed)
BT-CF (layered):
H
H⊥
M (
em
u/g
)
H (kOe)300 400 500 600 700 800 900
0
5
10
15
20
25
30
400 500 600 700 800 900-0.30
-0.25
-0.20
-0.15
-0.10
-0.05
0.00
0.05
632 K
745 K
T (K)
720 K
dM/dT (emu/g/K)
BT-CF (randomly
mixed)
BT-CF (layered)
Temperature (K)
M (
em
u/g
)
Figure IV.7: (a) The M(H) curves corresponding to the studied BT-CF
composite systems (randomly mixed and laminated (triple-layer)) under the action of a
magnetic field applied parallel/perpendicular to the layers (the area of low fields in the
insect is also highlighted); (b) Temperature magnetization dependence for the two
composites (the field is applied perpendicular to the layer); Inset: temperature
dependence of the dM/dT derivative.
The magnetization in both composites is low compared to the known
values for cobalt ferrite [17], as a consequence of the sum property. Both ceramic
composites have saturation magnetizations in the (23-29) emu/g range, values
that fit very well with the expected values of magnetization as an effect of the
sum property (e.g., one-third reduction from typical values for pure phase of
ferrite characterized by a saturation magnetization of 82 emu/g) [17].
It turns out that the interfaces and possible doping, as well as the
arrangement of the phases play a minor role in the magnetic properties of these
magnetoelectric composites. Small magnetization is typical of mixed composites
(23 emu/g) for a coercive field of ~ 320 Oe and a saturation field of ~ 4kOe.
In the case of laminated composites, the magnetization shows a weak
anisotropy when the magnetic field has been applied perpendicular or parallel to
the ceramic layers. Figure IV.7 (b) shows the temperature dependence of the
magnetization for mixed and layered composites on a field cooling sequence,
when applying a magnetic field H┴ = 10 kOe. One method of accurately
determining magnetic anomalies is to derive magnetization from temperature.
The curve dM/dT = f(T) is inserted in Figure IV.7 (b) and shows two well-
pronounced minima, one at 720 K for mixed ceramics and another at 746 K for
laminated ceramics, corresponding to the ferromagnetic-paramagnetic phase
transition, and another at 632 K for both ceramic systems.
IV.4.2 Magnetoelectric coupling properties
Next, the magnetoelectric response in composites was determined, in
20
dynamic regime, measuring the electric potential induced by the action of a small
AC variable magnetic field (Hac = 10 Oe), while the ceramic sample is
simultaneously subjected to the action of a large DC continuous magnetic field
of bias (Hdc), in a configuration in which both magnetic fields are applied parallel
to the ceramic electrodes (so perpendicular to the direction of the electric
polarization field). Due to the very high dielectric losses, it was practically not
possible to complete the polarization of the sample with a mixture of random
phases and therefore the magnetoelectric response could be recorded only for
ceramics with laminar structure.
The dependence of the transverse magnetoelectric coefficient depending
on the applied static magnetic field presents a complex but completely
reproducible hysteretic nonlinear variation, with many maxima and minima
depending on the frequency of the applied field. This behavior can be caused by
the complex coupling phenomena of electric and magnetic fields through
mechanical stresses, but also the nonlinear nature of the properties of
magnetostriction, permittivity and conductivity that can distort the shape of
curves describing the field dependence of the ME coefficient [18]
The original results presented in this part of the doctoral thesis were
published in the paper: A. Guzu, C.E. Ciomaga, M. Airimioaei, L. Padurariu,
L.P. Curecheriu, I. Dumitru, F. Gheorghiu, G. Stoian, M. Grigoras, N. Lupu, M.
Asandulesa, L. Mitoseriu, Functional properties of randomly mixed and layered
BaTiO3 - CoFe2O4 ceramic composites close to the percolation limit, J. Alloys
& Compds. 796, 55-64 (2019) [19].
V. Contributions to the study of ceramic magnetoelectric systems
consisting of barium titanate with cobalt-zinc ferrites
This chapter presents the results of a comparative study of feroelectric-
ferrite composite ceramic composite systems, which have the same composition
(close to the percolation limit): 0.66BaTiO3-0.33Co0.8Zn0.2Fe2O4, but they were
sintered differently: (i) by the classical method, respectively (ii) by SPS plasma
arc sintering. The aim was to investigate the effect of the sintering method used
and the resulting microstructures on the macroscopic properties of the
composites.
V.1 Composite preparation, structural (XRD) and microstructural
characterization (SEM)
21
Magnetic powders of Co0.8Zn0.2Fe2O4 (CZF), were prepared using as
precursors: Fe(NO3)3·9H2O, Co(NO3)2·6H2O, ZnO and HNO3. The synthesis
method combines sol-gel and self-combustion procedures, consisting in the use
of a combustion agent (citric acid C6H8O7) [20, 21]. Self-ignition was initiated
by heating in the first stage at 350 °C, and complete formation of the spinel phase
(CZF) took place after a heat treatment at 500 °C for 3 hours.
Commercial BaTiO3 (BT) nanopowders produced by hydrothermal
synthesis (Sigma Aldrich, purity > 99%, average particle size of 60 nm) were
chosen as the ferro/piezoelectric phase. After wet mixing in suitable
compositions, the mixture was either:
(a) pressed into tablets, then sintered by the conventional method at 1200
°C for 2 hours, or
(b) sintered by SPS at 1000 °C for 5 min under a pressure of 50 MPa.
The SPS sintered ceramic was subsequently reoxidized at 800 °C for 72
hours, then slowly cooled to reduce the amount of oxygen vacancy.
Figure V.1 shows the diffractograms of the constituent powders BT and
CZF obtained by the two sintering methods (CM and SPS).
Figure V.1: Diffractograms made for: (a) BT, CZF powders and ceramic
composites sintered by CM and SPS; (b) detailed representation in the field
2θ ~ (44÷58)°
Regardless of the sintering method, only the constant phases are present
in the composite, i.e. a pure di-phase composite was formed after sintering,
without secondary phases. The major difference between the XRD
diffractograms of the two types of ceramics is related to a difference in the crystal
structure. The composite ceramic (CM) shows a separation of the diffraction
maxima corresponding to the planes (200) and (210) of the perovskite phase of
22
BT, which indicates that in the composite composite (CM) the BT phase has a
tetragonal structure (T). In SPS sintered composite ceramics, the maximum (200)
is not split, similar to the initial BT powder (Figure V.1 (b)). This indicates a
pseudo-cubic structure, which is a typical feature of nanocrystalline BT particles
and nanostructured ceramics [22-25].
The relative density measured by the Archimedes method is quite low,
85% for BT-CZF (CM) and much higher, 98% for BT-CZF ceramics (SPS).
Attempts to increase density by increasing the temperature and sintering time in
the case of the CM method have led to the formation of secondary phases [26].
Figure V.2 shows the SEM microstructural image made in fracture of the
sintered ceramic composite by the classical method.
Figure V.2: SEM image in fracture of composite ceramics
0.66BaTiO3-0.33Co0.8Zn0.2Fe2O4 sintered by the classical method (CM)
The microstructures are relatively porous and the co-existence of two
phases with distinct morphologies is observed: BT has a smaller ceramic granule
size (average granulation of ~ 700-800 nm), while CZF ferrite formed larger
ceramic granules, with faceted appearance (about 1-2 μm medium grain), which
are agglomerated in areas extending to tens of μm.
The SEM-EDX elemental chemical analysis performed in different areas
of the pottery indicates a mixture of phases and/or mutual doping.
The microstructure of plasma arc sintered ceramics indicates a lower
degree of homogenization than in the case of sintering by the traditional method,
23
with the presence of distinct areas corresponding to the two phases: dense areas
with ultrafine granulation (~300 nm) corresponding to BaTiO3 containing
elongated clusters corresponding to ferrite, having ceramic granules with
dimensions of approximately 1μm for the CZF phase.
Figure V.3: Overview of SEM in SPS sintered ceramic fracture
It turns out that the SPS sintering method ensures an almost perfect
densification (porosity of only 2%), and the contact between the two phases is
perfect, although overall no ideal homogenization of the two phases has been
achieved, i.e. ferrite is present in the form of clusters elongated with large
granulation inside the ultra-dense and fine BT ferroelectric matrix. The SEM-
EDX elemental analysis shows in each of the analyzed areas a mixture of ions in
relative quantities, one of the phases being predominant.
V.2 Weak field dielectric properties as a function of temperature
and frequency
Weak field dielectric properties measured with LCR (Concept 40
Novocontrol Technologies) in the frequency range (1÷106) Hz and temperature
range (-150 ÷ 200)°C.
20 μm
24
-150 -100 -50 0 50 100 150 2000
250
500
750
1000
1250
TR-O
(-67oC)
TO-T
(17oC)
Re
al p
art
of
pe
rmit
tiv
ity
Temperature (oC)
BT-CZF (CM)
500kHz
100kHz
50kHz
10kHz
TC
(126oC)
(a)
-150 -100 -50 0 50 100 150 2000
250
500
750
1000
1250
(b)
Real p
art
of
perm
itti
vit
y
Temperature (oC)
BT-CZF (SPS)
500kHz
100kHz
50kHz
10kHz
TC
(104oC)
-150 -100 -50 0 50 100 150 2000,0
0,1
0,2
0,3
0,4
0,5
0,6
(c)
Temperature (oC)
BT-CZF (CM)
Die
lec
tric
lo
ss
500kHz
100kHz
50kHz
10kHz
-150 -100 -50 0 50 100 150 2000,0
0,1
0,2
0,3
0,4
0,5
0,6
(d)
Temperature (oC)
Die
lectr
ic lo
ss
BT-CZF (SPS)
500kHz
100kHz
50kHz
10kHz
Figure V.4: Temperature dependence of the real part of the permittivity (a), (b) and
of dielectric losses (c), (d) for composite ceramics sintered by
CM and SPS at several selected frequencies.
In the case of the compound sintered by the classical CM method the
existence of three peaks from the structural phase transitions of the BaTiO3
component at temperatures of 127°C, 17°C and -62°C (Fig.V.4 (a)), while in BT-
CZF (SPS) ceramics, only the Curie temperature could be identified by a flat
maximum around 104°C, the other structural transitions not being localized,
although some anomalies are still observed below 0°C (Figura V.4 (b)).
Permittivity and losses increase at high temperatures, above the Curie
range, mainly at low frequencies, due to slow thermally activated relaxations
generated by space tasks (Maxwell - Wagner relaxation). This phenomenon
overlaps with the decrease of the Curie - Wiess permittivity, which is usually
observed in ferroelectrics, in their paraelectric state, and in the studied ceramics
it can be observed at high frequencies. The permittivity of SPS sintered ceramics
is characterized by a remarkable thermal stability in a wide temperature range.
25
101
102
103
104
105
106
2000
4000
6000
8000
10000
101
102
103
104
105
106
700
800
900
1000
1100
Frequency (Hz)
Re
al
pa
rt o
f p
erm
itti
vit
y
T=230C
Frequency (Hz)
Re
al p
art
of
pe
rmit
tiv
ity BT-CZF (CM)
-1450C
2000C
(a)
101
102
103
104
105
106
0
1000
2000
3000
4000(b)
101
102
103
104
105
106
300
400
500
600
700
800
Frequency (Hz)
Rea
l p
art
of
perm
itti
vit
y
T=230C
-1450C
Frequency (Hz)
Real p
art
of
perm
itti
vit
y BT-CZF (SPS)
2000C
100
101
102
103
104
105
106
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
(c)BT-CZF (CM)
Frequency (Hz)
Co
nd
uc
tiv
ity
(S
/cm
)
-1450C
2000C
100
101
102
103
104
105
106
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
BT-CZF (SPS)
Frequency (Hz)
Co
nd
ucti
vit
y (
S/c
m)
2000C
-1450C
(d)
Figure V.5: Dependence of dielectric constant and conductivity vs. frequency
for different temperatures: (a) - (b) the real part of the permittivity, (c) - (d) the ac-
conductivity for the two types of composites
In both types of ceramics, the permittivity decreases monotonically with
increasing frequency, with a pronounced increase at ultra-low frequencies
(Figure V.5 (a-b)), especially at high temperatures due to the Maxwell Wagner
effect, causing increased losses dielectric and permittivity. It is also observed the
presence of thermally activated relaxation phenomena in the range of
intermediate frequencies, which are similar in the two types of composites.
In the same temperature range, the two types of ceramics indicate a
similar dependence of conductivity as a function of frequency (Figure V.5 (c-d)).
Based on the Arrhenius analysis, the following were found:
(i) an activation energy Ea = 0,62 eV and relaxation time
characteristic τ = 6x10-13s for the sample sintered by the classical method
and respectively Ea = 0,64 eV and τ = 3,6x10-13s for SPS sintered
ceramics;
26
(ii) the plasma sintered sample also shows a second dispersion process,
specific to low temperatures, i.e. in the field (-145, -50)°C, characterized
by a lower activation energy, Ea = 0,31 eV and the characteristic
relaxation time τ = 2,8x10-14s.
V.3. Magnetic properties, nonlinear dielectric character and
magnetoelectric coupling
The values of magnetization are similar in the two types of ceramics,
having slightly higher values (by about 13%) in the case of ceramic sintered
ceramics due to the higher densification of this sample compared to the one
sintered by the traditional method.
0 100 200 300 400 500 600 700 800 9000
4
8
12
16
20
24
28
32
-10 -5 0 5 10-30
-20
-10
0
10
20
30
H (kOe)
Magnetisation
(emu/g)
BT-CZF (CM)
BT-CZF (SPS)
M (
em
u/g
)
Temperature (K)
T = 300K
Figure V.6: Magnetic properties of ceramic composites: magnetization as a
function of temperature. Inset: cycles of hysteresis M(H) at room temperature
Magnetic Curie temperature, determined as the temperature where
magnetization vanishes in the M(T) dependence, has the value of ~ 637°C for
both samples, being quite close to that of single-phase ferrite with the same
composition (reported values ~ 623° C [27]). The observed temperature
difference can be interpreted as being determined by a slight doping of the ferrite
with Ba or Ti ions at the interfaces between the two phases.
The hysteresis loops of the composites, recorded at room temperature
(inset in Figure V.6) show a ferrimagnetic character, with very low coercivity (
132 Oe), saturation magnetization of 24-27 emu/g and residual magnetization of
27
4 emu/g, both values being lower than those found in pure ferrite, as a result of
the "sum property", i.e. the "dilution" of the ferrite with 66% BaTiO3, a material
that has no magnetic order.
-25 -20 -15 -10 -5 0 5 10 15 20 25200
300
400
500
600
700
800
900
Perm
itti
vit
y
E (kV/cm)
BT-CZF (SPS)
BT-CZF (CM)
before
(a)
-25 -20 -15 -10 -5 0 5 10 15 20 25
200
300
400
500
600
700
800
900
(b)
Perm
itti
vit
y
E (kV/cm)
BT-CZF (SPS)
BT-CZF (CM)
the remaining state (after 10kOe)
0 5 10 15 20 25
1,0
1,2
1,4
1,6
1,8
2,0
(c)BT-CZF (CM)
before M
after M (10kOe)
Tu
nab
ilit
y, n
E (kV/cm)
0 5 10 15 20 25
1,0
1,2
1,4
1,6
1,8
2,0
(d)
before M
after M (10kOe)
BT-CZF (SPS)
Tu
na
bil
ity
, n
E (kV/cm)
Figure V.7: (a-b) Permittivity depending on the electric field dc applied to a
complete cycle of increase/decrease of the field for the virgin sample and in a state of
magnetic remanence, after applying a 10 kOe field and reducing it to zero;
(c-d) Tunability in electric field for the virgin sample and in a state of
magnetic remanence for composites BT-CZF (CM) și BT-CZF (SPS)
It can be observed (Figure V.7 (a)) that, in the virgin state, the BT-CZF
(MC) ceramic has a hysteretic ɛ(E) dependence symmetrical on the E = 0 axis,
still unsaturated at the maximum value of the applied field, while in the case of
the sintered plasma sample, the dependence is nonlinear but reversible (non-
hysteretic), almost linear, without a tendency to saturation.
After the application of a static magnetic field of 10 kOe and its reduction
28
to zero, under remanence conditions, the ceramics maintained their nonlinear
dielectric character, i.e. the variation of the permittivity with the applied electric
field, but the permittivity value itself and the tunability were considerably
reduced (Figure V.7 (b)) in the state of magnetic remanence.
Figure V.7 (c-d) shows how the tunability in the electric field is affected
by the application of the magnetic field in the case of the two types of samples.
It is observed that for both types of ceramics, in the state of magnetic remanence,
the tunability is strongly reduced and in particular, for the sintered plasma
sample, it is almost canceled (Figure V.7 (d)).
Next, to complete the magnetoelectric characterization of the ceramics
investigated in this chapter, the S11 reflection coefficients in the microwave range
in the range (2-6) GHz were measured. Resonant structures containing composite
ceramics as active material were made, using a vector analyzer, in two situations:
(i) without applying a magnetic field;
(ii) under the action of a magnetic field of 1.9 kOe (about 10 times larger
than the coercive field).
1 2 3 4 5 6-35
-30
-25
-20
-15
-10
-5
0
H=0
Hdc=1.9kOe
BT-CZF
(CM)
S1
1 (
dB
)
Frequency (GHz)
(a)1,0 1,5 2,0 2,5 3,0
-35
-30
-25
-20
-15
-10
-5
0
(b)
H=0
Hdc=1.9kOe
BT-CZF
(SPS)
S1
1(d
B)
Frequency (GHz)
Figure V.8: Variation of the coefficient S11 with the frequency for sintered
composite ceramics by the classical method: BT-CZF (CM) and BT-CZF (SPS)
respectively in the absence and in the presence of a magnetic field dc of 1.9 kOe
In both types of composite ceramics, the resonance can be shifted under
the action of a magnetic field dc having values higher than the coercive one. The
displacements of the resonance curves towards higher values, respectively lower
in the two cases, can be explained by the combined effect of the variation of
permittivity and tunability with the magnetic field, i.e. by a bi-tunable character
and by the opposite sign of magnetocapacity in the two types of composite
ceramics investigated.
29
VI. Study of the laminar composite
0.33BaTiO3 – 0.33Co0.8Zn0.2Fe2O4 – 0.33BaTiO3
In this last chapter, the properties of a laminar system having the
composition 0.33BT-0.33CZF-0.33BT, which was densified by SPS sintering,
are presented. Its properties can be compared with those of the system prepared
under similar conditions: 0.33BT-0.33CF-0.33BT which was presented in
Chapter IV, in which the observed differences will be due to the compositional
difference of the ferrite in the intermediate layer, but also with those of the
magnetoelectric composite with the same composition, but having a random
mixture of phases, which was presented in Chapter V.
VI.1 Microstructural characterization
The microstructure of this type of ceramic can be seen in Figure VI.1,
which was performed by scanning electron microscopy in cross section in the
fresh fracture of multi-layer ceramics.
Figure VI.1: Microstructures obtained by SEM microscopy of the laminar composite
0.33BT-0.33CZF-0.33BT, made in fresh fracture, in which a region of the interface and
the microstructures of the constituent phases are observed: BT and CZF.
30
A very good densification of the composite ceramic is observed, having
distinct areas characteristic of the two phases, BT and CZF respectively. In the
ferrite and ferroelectric layers, respectively, the compaction is very good, but at
the interfaces that separate them there are still areas with a certain degree of
porosity.
The micrograph of the interface also shows very well the dimensional
contrast of the two oxide phases, which have very different average granulations,
namely: 150 nm for the corresponding BT regions and respectiv ~ 1.15 µm for
the corresponding CZF region, respectively. Both in the area corresponding to
ferroelectric and CZF spinel, the ceramic granules are faceted, well crystallized
and compact, without intragranular porosity and having perfect triple points,
which indicates a very good sintering of composites and a good compatibility of
the two oxide phases.
The SEM-EDX elemental chemical analysis performed in the three areas
of the studied composite, indicates a clear separation of the component phases;
no more detailed analysis was performed at the interface, as it has a slightly
irregular structure.
VI.2 Weak field electrical properties
The electrical properties were studied by the method of impedance
spectroscopy, which allows the explanation of dielectric and conduction
properties in relation to the microstructure and composition, taking into account
the contributions of different ceramic components (grains, their boundaries, the
interface between ceramic and electrode, etc.) .
The complex impedance diagram at room temperature for the BT-CZF
layered composite (SPS) shows two components.
The electrical properties at room temperature are shown in Figure VI.2.
100
101
102
103
104
105
106
102
103
104
BT-CZF (layered)
SPS
Real p
art
of
perm
itti
vit
y
Frequency (Hz)
(a)
10
010
110
210
310
410
510
610
0
101
102
103
104 (b)BT-CZF (layered)
SPS
Imag
inary
part
of
perm
itti
vit
y
Frequency (Hz)
31
100
101
102
103
104
105
106
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
(c)
104
105
106
0.00
0.05
0.10
0.15
0.20
0.25
BT-CZF (layered)
SPS
tg
Frequency (Hz)10
010
110
210
310
410
510
60,000
0,001
0,002
0,003
0,004
0,005
0,006
(d)
M''
BT-CZF (layered) SPS
Frequency (Hz)
100
101
102
103
104
105
106
10-7
10-6
10-5
10-4
(e)
Co
nd
ucti
vit
y (
S/m
)
BT-CZF (layered) SPS
Frequency (Hz)
=A f n
n=0.4
Figure VI.2: Dependence of dielectric properties as a function of frequency at
room temperature for BT-CZF laminated composite sintered in plasma arc (SPS):
(a) the real part of permittivity; (b) imaginary part of permittivity;
(c) dielectric loss; (d) imaginary part of dielectric module; (e) electrical conductivity.
.
A monotonous decrease in permittivity can be observed from giant
values of 1.2x104, for a frequency of 1 Hz, with a saturation tendency at a value
of ~ 55 for a frequency of 1 MHz (Figure VI.2 (a)).
The imaginary part of the permittivity shows a decrease with frequency,
from 14000 (f = 1Hz) to 1.4 for a frequency of 1 MHz (Figure VI.2 (b)). The
frequency dependence of the imaginary part provides information on the charge
transport mechanisms and conductivity relaxations, allowing the distinction
between dielectric relaxation and conductivity processes.
The conduction is indicated by the presence of a maximum in M"(f),
which is not accompanied by a maximum in ɛ"(f), while a dielectric relaxation
would determine maximums in both dependencies.
32
Dielectric losses show quite high values in the frequency range (10,103)
Hz, with a maximum due to relaxation phenomena (Figure VI.2 (c)), of 3.84 at
a frequency of 37 Hz, which indicates a semiconductor character of the
dielectric in this frequency range.
The frequency conductivity dependence is represented in Figure VI.2
(e), a curve that is subject to the universal law of Jonsker's dielectric relaxation
[29]: σ = Afn, where n is a frequency and temperature dependent exponent, in
general [28,29] with values between 0 and 1, which in this case has the value n
= 0,4.
Permittivity and dielectric losses at high temperatures increase above
the Curie temperature (Figure VI.3), especially at low frequencies, due to the
relaxation of thermally activated slow species, generated by space charges
(Maxwell Wagner relaxation). Comparatively, at a fixed frequency of 500 kHz,
the permittivity of the BT-CZF laminated composite (SPS) varies between
(55÷135) and the permittivity of the randomly mixed BT-CZF (SPS) composite
between (210÷397). A greater variation of the temperature permittivity is
presented by the laminated composite BT-CZF (SPS) at the frequency of 1kHz,
between (55÷4783), essentially feeling the contribution of the ferroelectric
phase of BT.
-150 -100 -50 0 50 100 150 200
102
103
(a)
Perm
itti
vit
y
BT-CZF (layered) SPS
Temperature (0C)
1MHz
500kHz
100kHz
50kHz
10kHz
1 kHz
-150 -100 -50 0 50 100 150 200
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
(b)
tg
1MHz
500kHz
100kHz
50kHz
10kHz
1kHz
BT-CZF (layered)
SPS
Temperature (0C)
Figure VI.3: Temperature dependence of permittivity (a) and dielectric loss
(b) for BT-CZF laminated ceramic composite sintered in plasma arc (SPS)
Dielectric losses (Figure VI.3 (b)) increase with increasing temperature,
their maximums shifting to high temperatures as the frequency increases.
33
VI.3 Electrical properties at different temperatures
Figure VI.4 shows the frequency dependencies of the electrical
properties at the temperature variation in the range (-145C, 198C). It can be
observed for the laminated ceramic composite BT-CZF (SPS) a monotonous
decrease of the permittivity with the increase of the frequency from the room
temperature to T = 198 ° C, in the range of low frequencies with very high
values. This behavior is mainly due to the Maxwell-Wagner effect, which
causes an increase in permittivity and dielectric loss. Comparatively, at room
temperature, the permittivity of the BT-CZF laminated composite (SPS) varies
in the range of 90 ÷ 6400, and in the case of the randomly mixed BT-CZF (SPS)
composite between 350 ÷ 800, in the analyzed frequency range.
100
101
102
103
104
105
106
0
5000
10000
15000
20000
25000
30000
T=-145C
T=-126C
T=-101C
T=-76C
T=-51C
T=-25C
T=-1C
T=23C
T=48C
T=73C
T=98C
T=123C
T=148C
T=173C
T=198CRe
al p
art
of
pe
rmit
tiv
ity
Frequency (Hz)
BT-CZF (layered) SPS
(a)
100
101
102
103
104
105
106
0
2000
4000
6000
8000
10000
(b)
BT-CZF (layered) SPS
Frequency (Hz)
Ima
gin
ary
pa
rt o
f p
erm
itti
vit
y
C
T=-126C
T=-101C
T=-76C
T=-51C
T=-25C
T=-1C
T=23C
T=48C
T=73C
T=98C
T=123C
T=148C
T=173C
T=198C
Figure VI.4: Frequency dependence of the real part of the permittivity (a),
the imaginary part of the permittivity (b) and the conductivity ac (c)
100
101
102
103
104
105
106
10-14
10-13
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
(c)
Frequency (Hz)
BT-CZF (layered) SPS
Co
nd
uc
tiv
ity
(s
/cm
)
T=-145C
T=-126C
T=-101C
T=-76C
T=-51C
T=-25C
T=-1C
T=23C
T=48C
T=73C
T=98C
T=123C
T=148C
T=173C
T=198C
34
High dc conductivities were obtained, as in the case of the laminated
compound BT-CF (SPS), in the range (10-13, 10-8) S/cm, which indicates the
existence of uncompensated electrical charges located at the interfaces between
the two ferroelectric and magnetic phases (Maxwell-Wagner phenomena), due
to the polarization difference between the two types of oxide materials. The
frequency dependence of the permittivity and conductivity, corroborated with
high dielectric losses in the analyzed frequency range, indicates a more
semiconductor than dielectric character of the analyzed laminated composite.
Using the law Arrhenius ln(τ) vs. 1/T, we obtained from the maxima of
the tangent of the loss angle an activation energy of Ea = 0,67 eV, and from the
maxima of the imaginary part of the dielectric module, an activation energy Ea
= 0,58 eV, values comparable to the activation energy determined in the case of
the randomly mixed composite BT-CZF sintered in SPS plasma arc, Ea = 0,64
eV (chapter V of this thesis).
In the case of BT-CF laminate composite (SPS), two thermally activated
processes were obtained, the relaxation process identified by activation energies
in the range (0,47÷0,50) eV and another process corresponding to a higher
activation energy, (0,55÷0,63) eV.
Activation energies in the field (0,5÷1) eV, are attributed in the
literature [30,31] to the presence of oxygen vacancies or the phenomenon of
hopping conductivity Maxwell-Wagner.
VI.4 Magnetic properties
The maximum value obtained for magnetization in the case of the BT-
CZF laminate compound (SPS), rather high (~ 39 emu/g) (Figure VI.5 (a)), is
due to the very good densification achieved by the sintering method used. For
the magnetic Curie temperature (the temperature at which the magnetization is
canceled in the dependence M(T)) the value of 639 K was obtained (Figure VI.5
(b)).
35
0 150 300 450 600 750 9000
10
20
30
40
BT-CZF layered
(SPS) (a)
T (K)
M (
em
u/g
)
400 500 600 700 800 900-0,30
-0,25
-0,20
-0,15
-0,10
-0,05
0,00 BT-CZF layered
(SPS) (b)
T (K)
dM
/dT
(e
mu
/g/K
)
639K
Figure VI.5: Magnetic properties of ceramic composites: (a) magnetization
as a function of temperature; (b) the temperature dependence of the dM/dT
Figure VI.6 comparatively represents the temperature dependencies of
the magnetization and the dM/dT, corresponding to the samples with the same
composition but with different phase arrangement, the two being densified by the
same SPS method. It can be seen that in both composites, the magnetization of
(30-39) emu/g is reduced compared to the known values for pure ferrite
Co0.8Zn0.2Fe2O4 [32], this being a consequence of the sum property in composites
(Figure VI .6 (a)). For both composites the same magnetic Curie temperature of
~ 639 K was found, which is relatively close to that of the single-phase ferrite
with the same composition for which values of ~ 623 K were reported [33]. The
temperature difference obtained may be due to a slight doping of the ferrite with
Ba or Ti ions at the interfaces between the two phases.
Due to the large losses in the laminated composite BT-CZF (SPS), it
was not possible to measure the nonlinear dielectric properties below the high
dc field (tunability) and it was not possible to perform measurements of
ferroelectric hysteresis P(E). In conclusion, this system also needs to be
optimized in order to reduce losses and to be able to support the application of
intense electric fields, in order to meet the conditions of laminated
magnetoelectric material with ferroelectric character and nonlinear dielectric
at room temperature.
VII. General conclusions
The research carried out in this doctoral thesis focused on the study of
magnetoelectric composites consisting of a magnetostrictive oxide material and
36
a piezo/ferroelectric one, having two types of arrangements of the constituent
phases: (i) in the form of multilayer and (ii) under form of solid material with
randomly mixed phases.
Studies on magnetoelectric materials aim to obtain magnetoelectric
materials having simultaneously dipole and magnetic order (ferroelectric and
ferro, feri- or antiferomagnetic properties) in the same structure and high
magnetoelectric coefficient, in the field of temperatures of interest for
applications, with corrosion resistance high and mechanical hardness, and can
be made at relatively low prices. There are very few single-phase
magnetoelectric materials, and existing ones usually have these properties at
cryogenic temperatures and as such, there is a permanent interest in finding new
single-phase materials or combining composite materials to sum up these
properties at ambient temperature.
In this paper, combinations were chosen between the two materials in
which the ferrite concentration is higher, at the percolation limit, namely 33
vol.%, to maintain a strong magnetic response and also trying to maintain the
dielectric character in the composite. In order to understand the role of
microstructure on their dielectric and magnetic properties, combinations were
made with the same composition of the two phases (66% ferroelectric - 33%
ferrite), but distributed differently in the volume of the composite. Thus, the
magnetoelectric materials studied in this paper were mixed composites with the
formula 0.33CF - 0.66BT and 0.33CZF - 0.66BT, respectively, having a random
phase mixture, as well as laminated composites (tri-layer) type 0.33BT -
0.33CZF - 0.63 BT and 0.33BT - 0.33CF - 0.63BT, respectively. The properties
of these composite ceramics, having the same composition but with the
constituent phases located in different ways in the volume of the ceramics
(random mixture or in laminar structures) were analyzed comparatively and
described by finite element modeling.
It was also investigated for the same composition and how the sintering
method modifies the microstructural characteristics (porosity, granulation) and
functional properties, in case of using two different sintering methods:
traditional method sintering and plasma arc sintering, having randomly mixed
phases.
37
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List of original publications:
40
1. Alexandra Guzu, Cristina E. Ciomaga, Mirela Airimioaei, Leontin
Padurariu, Lavinia P. Curecheriu, Ioan Dumitru, Felicia Gheorghiu,
George Stoian, Marian Grigoras, Nicoleta Lupu, Mihai Asandulesa and
Liliana Mitoseriu, Functional properties of randomly mixed and layered
BaTiO3 - CoFe2O4 ceramic composites close to the percolation limit, Journal
of Alloys and Compounds, 796, 55-64, (2019)
IF:4.175, AIS: 0.601
2. Cristina E. Ciomaga, Alexandra Guzu, Mirela Airimioaei, George
Stoian, Mihai Asanduleasa, Lavinia P. Curecheriu, Ovidiu Avadanei and
Liliana Mitoseriu, Comparative study of BaTiO3–Co0.8Zn0.2Fe2O4 ceramic
composites sintered by classical method and by Spark Plasma Sintering,
Ceramics International (2019).
IF: 3.45, AIS: 0.454.
TOTAL AIS: 1.055
International conferences:
41
1. M. Airimioaei, C. E. Ciomaga, A. Guzu, N. Horchidan, L. P. Curecheriu,
N. Lupu, F. M. Tufescu, L. Mitoseriu, Study of microstructure and functional
properties of layered BaTiO3– ferrite–BaTiO3 magnetoelectric composites
obtained by SPS method, ECerS 2017,15th Conference & Exhibition of the
European Ceramic Society Budapest, Hungary, July 9–13, 2017 (poster
presentation)
2. C. E. Ciomaga, M.Airimioaei, A. Guzu, O. Avadanei, N. Lupu, L.
Mitoseriu, Study of functional properties of ferroelectric-magnetic ceramic
composites obtained by different synthesis method, International Conference
CIEC 16, Torino, Italy, 9-11 September 2018 (poster presentation)
3. C. E. Ciomaga, M. Airimioaei, A. Guzu, F. Gheorghiu, G. Stoian, M.
Grigoraș, M. Asănduleasa, L. Pădurariu, L. Mitoșeriu, Comparative study of the
functional properties magnetoelectric composites, ISAF-ICE-EMF-IWPM-PFM
Joint Conference, July 14-19, 2019, Lausanne, Switzerland (oral presentation)
National conferences:
1. Alexandra Guzu, Cristina E. Ciomaga, Lavinia P. Curecheriu, Mirela
Airimioei, Nădejda Horchidan, Petronel Postolache and Liliana Mitoseriu,
Studies on structural, electrical and magnetic behavior of CoZn ferrite and
BaZr0.15Ti0.85O3 ferroelectric ceramic composites, FARPHYS (Fundamental and
Applied Research in Physics) 29 October, Iasi, Romania, 2016 (poster
presentation)
2. Alexandra Guzu, Lavinia P. Curecheriu, Mirela Airimioei, Nădejda
Horchidan, Petronel Postolache, Cristina E. Ciomaga and Liliana Mitoseriu,
Dielectric and magnetic properties of BaZr0.15Ti0.85O3 and Co-Zn ferrite ceramic
composites, CNFA (National Conference on Applied Physics), 26, 27 November,
Iasi, Romania, 2016 (poster presentation)
3. Alexandra Guzu, Cristina E. Ciomaga, Mirela Airimioei, Felicia
Gheorghiu and L. Mitoseriu, Comparative study of functional properties of
BaTiO3-based magnetoelectric composites, a XLVII-a FTEM (National
Conference Physics and Modern Educational Technologies), Iasi, May19-20,
2018 (poster presentation).