Journal of Alloys and Compounds 551 (2013) 72–81

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Journal of Alloys and Compounds

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Cation distribution by Rietveld technique and magnetocrystalline anisotropy of Znsubstituted nanocrystalline cobalt ferrite

Lawrence Kumar a,b, Pawan Kumar a, Manoranjan Kar a,⇑a Department of Physics, Indian Institute of Technology Patna, Patna 800013, Indiab Centre for Nanotechnology, Central University of Jharkhand, Ranchi 835205, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 August 2012Received in revised form 1 October 2012Accepted 3 October 2012Available online 12 October 2012

Keywords:Oxide materialsSol–gel processingCrystal structureX-ray diffractionMagnetic measurements

0925-8388/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jallcom.2012.10.009

⇑ Corresponding author. Tel.: +91 6122552013; faxE-mail address: mano@iitp.ac.in (M. Kar).

Zn2+ ion substituted nanocrystalline cobalt ferrite materials with the chemical formula CoZnxFe2�xO4 forx = 0.0, 0.1, 0.15, 0.2, 0.25 and 0.3 have been synthesised by standard citrate precursor method. The crys-tal structure and phase purity have been studied by powder X-ray diffraction (XRD) method by employ-ing Rietveld refinement technique. The cation distribution between the tetrahedral site (A-site) andoctahedral site (B-site) has been determined by Rietveld analysis. The distribution of cations changeswith the Zn2+ concentration. Average crystallite size and lattice constant increase with the annealingtemperature. The lattice constants decrease with the increasing Zn concentration. The vibrational modesof the octahedral and tetrahedral metal complex in the sample have been carried out using Fourier trans-form infrared spectroscopy (FT-IR). The FT-IR spectra of the sample have been studied in the wave num-ber range of 380–800 cm�1 and it shows the presence of absorption bands which are assigned totetrahedral and octahedral metal complexes. The elemental analysis has been carried out using energydispersive spectroscopy (EDS) with the help of Field Emission Scanning Electron Microscope (FE-SEM)and the results reveal that the elements are as per the stoichiometric ratio in all the samples. The Mag-netic hysteresis loop measurements were carried out at room temperature using a vibrating sample mag-netometer (VSM) over a field range of ±2 T. Annealing at 200 �C, superparamagnetic nature has beenobserved for compositions x = 0.15–0.3. The magnetic data in saturation have been analysed by employ-ing the law of approach (LA) technique. The saturation magnetisation increases with the Zn for x 6 0.15and then decreases. The coercivity decreases with the increasing Zn concentration for 600 �C annealedsamples. The magnetocrystalline anisotropy constants are found to increase for x 6 0.15 and thendecrease with the Zn concentration.

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1. Introduction Nanocrystalline cobalt ferrite exhibits superparamagnetism due

The mixed spinel ferrite with general formula ½A2þ1�xB3þ

x �½A3þ

x B3þ2�x�O

2�4 exhibits interesting physical properties which lead to

wide variety of applications such as, magnetic resonance imagingcontrast agents, data storage devices, transformer cores, magnetocaloric refrigeration etc. [1,2]. Among the various mixed ferritematerials, cobalt ferrite (CoFe2O4) has drawn considerable attentiondue to remarkable physical properties, such as high coercivity, highmagnetocrystalline anisotropy constant, moderate saturation mag-netization along with good mechanical hardness and chemical sta-bility [3–5]. It is a hard ferrimagnetic material with a high magneticordering temperature around �520 �C and crystallises in mixedspinel structure with space group Fd3m [3]. The magnetic propertiesin this ferrite are mainly due to a predominant superexchange inter-action between the cations in the A-site and B-site via oxygen ions[6–8].

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to high surface to volume ratio [9,10]. When the size of magneticparticle is decreased below a critical length, multiple domain for-mation is no longer energetically favoured due to enhanced do-main wall energy and, hence particles exist as single domain.When the size of these ferrite nano-particles becomes smaller thanthe single domain size, the spins are affected by the thermal fluc-tuations and magnetic nanoparticles are found to exhibit super-paramagnetic behaviour with vanishing hysteresis loop and, theircoercivity approaches to zero [11]. The appearance of superpara-magnetism in the nanoparticles critically depends on the particlesize and surface disorderness [12]. The superparamagnetism innanocrystalline spinel ferrite at room temperature may beachieved by substituting non magnetic ions in place of magneticions in spinel ferrites. For example Velmurugen et al [13] havereported the superparamagnetism at room temperature in nano-sized Zn substituted nickel ferrite with higher Zn concentration.The critical particle size for superparamagnetism behaviour inCoFe2O4, Fe3O4 and MnFe2O4 are 14, 25 and 50 nm respectively[14].

L. Kumar et al. / Journal of Alloys and Compounds 551 (2013) 72–81 73

The mixed spinel cobalt ferrite has large positive magnetocrys-talline anisotropy constant (�3.9 � 106 erg/cm3) [15–18]. Hence,these materials are known as hard magnetic material [19–21]. Ithas been shown by several authors that the magnetocrystallineanisotropy constant decreases with decrease with the crystallitesize for nanocrystalline spinel ferrites. Also the earlier reportsshow that magnetocrystalline anisotropy constant can be tunedby substitution of Ga, Mn, Al etc. in place of Fe site [22–26].

Mozaffari et al. [27] synthesized the Zn substituted cobalt fer-rite (Co0.5Zn0.5Fe2O4) nanoparticles by co-precipitation methodand observed that crystallite size in the range of 6–8 nm showssuperparamagnetic behaviour at room temperature. Jadhav et al.[28] found that, lattice constant increases and, magnetic orderingtemperature and magnetocrystalline anisotropy constant de-creases with the increasing substitution of Zn ions in Co1�xZnxFe2-

O4. Duong et al. [29] synthesized Zn substituted cobalt ferrites byforced hydrolysis method and the results reveal that, the magneti-zation increases but the coercivity and the anisotropy constant de-crease with the increasing Zn substitution. Most of the reports areavailable for the substitution at the Co site and/or Fe site of cobaltferrite (CoFe2O4) by the cations having the same valence state asthat of Co site and/or Fe site [23–26]. There are limited efforts havebeen made to substitute the cations at the Fe site with different va-lence state. For example Dwivedi et al. [30] have studied the sub-stitution of Mo6+ at the Fe site of cobalt ferrite (CoFe2O4) and foundthat Mo doping results the coexistence of both ferroelectricity andmagnetic ordering at room temperature. Rezlescu et al. [31] havestudied the Mo6+ and Sn4+ substitution at the Fe site of MgFe2O4.In the present work, we have substituted Zn at the Fe site of cobaltferrite (CoFe2O4) under the impression that if Zn2+ is substituted atthe Fe site, then due to size mismatch disorderness would beintroduced in the structure and surface effects are expected tocause for superparamagnetic behaviour, modification of exchangeinteraction, magnetic anisotropy, and exchange of cations betweenthe interstitial sites. Here, we have emphasized on the understand-ing of crystal structure and magnetocrystalline anisotropy ofCoZnxFe2�xO4 (x = 0.0–0.3) samples at room temperature.

2. Experimental

Nanocrystalline CoZnxFe2�xO4 for x = 0.0–0.3 were prepared by the citrate pre-cursor method [32]. Cobalt nitrate (Co(NO3)2�6H2O), Iron nitrate (Fe(NO3)3�9H2O),Zinc nitrate (Zn(NO3)2�6H2O), and Citric acid (C6H8O7�H2O) with 99.9% purity wereused as starting materials. The metal nitrates were dissolved in a deionised water(milli Q grade) to get a solution. An aqueous solution of citric acid was mixed withmetal nitrates solution. The mixed solution was heated at 80 �C with constant stir-ring using hot plate. The solution became viscous and finally forming brown gel.The molar ratio of metal nitrates to citric acid was taken as 1:3. The metal nitratesare used as metal and oxidant source in the reaction. The citric acid plays importantroles in the mixture solution because it has complexing ability with metal nitratesolution. Therefore, it forms complexes with metal ions which prevent the precip-itation of hydroxylated compounds. Due to above effect citric acid acts as a best fuelto carry out the reaction at low temperature and shorter time to form nanocrystal-line cobalt ferrite. We could express the combustion reaction as,

ð2� xÞFeðNO3Þ3 � 9H2OðaqÞ þ CoðNO3Þ � 6H2OðaqÞ þ xZnðNO3Þ2 � 6H2OðaqÞ

þ 3C6H8O7 �H2OðaqÞ þ 7þ 6x2

O2ðgÞ ! CoZnxFe2�xO4ðsÞ þ ð39� 3xÞH2OðgÞ

þ ð18� 3xÞCO2ðgÞ þ ð4�x2ÞN2ðgÞ

Enthalpy is the heat content of any reaction. The enthalpy of the above combus-tion reaction can be expressed as

DH ¼Xðn� HÞproducts �

Xðn� HÞreactants

Here n is the stoichiometric number. The nature of given reaction is an exother-mic. Hence, it provides extra energy for the reaction, hence the nucleation of nano-crystalline samples takes place at low annealing temperature.

The gel was dried overnight using an oven at 80 �C in order to remove excesswater. During the process of drying, the gel swells into the fluffy mass and eventu-ally broke into brittle flakes. The resulting materials were heat treated in air atmo-sphere at 200 and 600 �C for 2 h at each temperature. The crystalline phase of the

annealed samples was identified by the powder X-ray diffraction method (RigakuMiniflex) using Cu Ka radiation. Fourier Transform Infrared Spectra (FT-IR) wererecorded at room temperature using the Perkin Elmer (model 400) in the rangefrom 380 to 800 cm�1. The compositions of all the samples were studied by EDS(energy-dispersive spectroscopy) using Hitachi S4800 Field Emission Scanning Elec-tron Microscope (FE-SEM). The magnetic hysteresis loops were recorded at roomtemperature (300 K = 27 �C) by using a LakeShore (Model No. 7410) Vibrating Sam-ple Magnetometer (VSM). The maximum field was used ±2T.

The analysis of the crystallite size has been carried out using the broadening ofthe XRD peaks. Peak broadening comes from several sources such as instrumentaleffect, finite crystallite size and strain effect within the crystallite lattice [33]. Crys-tallite size has been calculated by using Scherrer’s formulae, Williamson–Hall plotand Rietveld method, and has been compared for all the methods. The Scherrer’sformula [34] is defined as,

D ¼ kkb cos h

ð1Þ

where constant k depends upon the shape of the crystallite size (=0.89, assuming thecircular grain), b = Full width at Half Maximum (FWHM) of Intensity (a.u.) vs. 2h pro-file, k is the wavelength of the Cu Ka radiation (=0.1542 nm), h is the Bragg’s diffrac-tion angle and D is the crystallite size. In Scherrer’s formula FWHM has beencalculated using Gaussian fit to the peaks in XRD pattern. D has been taken averageto all the peaks. According to Williamson–Hall method [33] the individual contribu-tions to the broadening of reflections can be expressed as,

b cos h ¼ kkDþ 4� sin h ð2Þ

where 4eSinh is the strain effect on the crystallites. A complete expression used inRietveld method [35] is defined as,

FWHM2 ¼ ðU þ D2STÞðtan2 hÞ þ Vðtan hÞ þW þ IG

cos2 hð3Þ

where U, V and W are the usual peak shape parameters, IG is a measure of the iso-tropic size effect, DST = coefficient related to strain.

3. Results and discussions

3.1. Structural analysis

The XRD patterns of CoZnxFe2�xO4 with x = 0.0, 0.1, 0.15, 0.2,0.25 and 0.3 are shown in Figs. 1 and 2 for the samples annealedat 200 and 600 �C respectively. All the samples are essentially insingle phase form. We have not observed any trace of impuritypeaks which conforms that, Zn2+ ions have been incorporated intothe spinel lattice. All the XRD peaks could be indexed to Fd3mspace group with cubic symmetry.

Size of the crystallites and their distributions are related to thenucleation and subsequent growth. If during reaction all the nucleiare formed at the same time, then it gives uniform size distribution.Once all the nuclei are formed, their growth occurs simultaneously.Hence, samples annealed at 200 �C have low crystallinity and highstrain in the lattice (due to large surface to volume ratio of nanocrys-talline material) which leads to large peak broadening. It appearsthat under identical synthesis condition, nucleation has occurreduniformly with the same rate. The rate of nucleation and its growthis influenced by reaction temperature. Samples annealed at 200 �Cmight not have sufficient thermal energy to grow larger crystallitesand decompose all derivatives. Hence, XRD patterns show amor-phous like behaviour i.e. very small crystallite size (short rangeordering).

The XRD pattern of the samples annealed at 600 �C showsreduction in the peak broadening compared to the samples an-nealed at 200 �C which suggests the formation of CoZnxFe2�xO4

crystals with enhanced crystallinity and reduction in internalstrain. Above discussion reveals that the growth of crystal takesplace with increasing heat treatment.

All XRD patterns were analysed employing Rietveld technique[35] with the help of Fullprof Suite 2009 release programme. Thepatterns for all the samples could be refined using the Fd3m spacegroup. Typical XRD patterns along with Rietveld refinement areshown in Figs. 3 and 4 for the sample CoZn0.1Fe1.9O4 annealed at

10 20 30 40 50 60 70

CoFe2O

4

Inte

nsity

(A

.U)

2θ (Degree)

CoZn0.1

Fe1.9

O4

CoZn0.15

Fe1.85

O4

CoZn0.2

Fe1.8

O4

CoZn0.25

Fe1.75

O4

(CoZn0.3

Fe1.7

O4)

(440

)

(511

)

(400

)

(311

)

(220

)

(111

)

Fig. 1. XRD patterns of the sample CoZnxFe2�xO4 for x = 0.0–0.3 annealed at 200 �C.

10 20 30 40 50 60 70

Inte

nsi

ty (

A.U

)

2θ (Degree)

CoFe2O

4

CoZn0.1

Fe1.9

O4

CoZn0.15

Fe1.85

O4

CoZn0.2

Fe1.8

O4

CoZn0.25

Fe1.75

O4

CoZn0.3

Fe1.7

O4

(440

)

(511

)

(422

)

(400

)

(222

)

(311

)

(220

)

(111

)

Fig. 2. XRD patterns of the sample CoZnxFe2�xO4 for x = 0.0–0.3 annealed at 600 �C.

74 L. Kumar et al. / Journal of Alloys and Compounds 551 (2013) 72–81

200 and 600 �C respectively. Here the experimental data are shownas open circles and calculated intensities are shown as solid line.The bottom line represents the difference between measured andcalculated intensities. The allowed Braggs positions for the Fd3mspace group are marked as vertical lines. All the observed peaksare allowed Bragg 2h positions. The typical fractional atomic posi-tions and isothermal parameters of the atoms for the samplesCoZn0.1Fe1.9O4 annealed at 600 �C are given in Table 1. The oxygenpositions (x = y = z) were taken as free parameters, however allother atomic fractional positions were taken as fixed. Other param-eters, such as, lattice constants, isothermal parameters, occupan-cies, scale factors and shape parameters were taken as freeparameters. Background was corrected by pseudo voigt function.

The Rp (profile factor) and refined lattice parameters for allthe samples are listed in Table 2. The Rp factor is found to belarge. A similar Rp factors have been observed by the other re-search group for nanocrystalline samples [36]. It could be dueto the low signal to noise ratio of XRD patterns for nanocrystal-line materials. We have observed a low values of v2 (goodness offit) which justifies the goodness of refinement. In the diffractionpattern of nanocrystalline materials, diffuse scattering is domi-nant than in those of bulk crystalline materials due to large ratioof surface to volume atoms. The diffuse scattering becomes sig-nificant at the nanoscale while Bragg scattering gets diminishedwhich leads to decline in crystallinity. Hence large reliability fac-tors are observed.

Fig. 3. Rietveld refined XRD pattern for the sample CoZn0.1Fe1.9O4 annealed at 200 �C. The circles represent experimental points and the solid line represents Rietveld refineddata. The bottom line shows the difference between the experimental and refined data. The marked 2h positions are the allowed Bragg peaks.

Fig. 4. Rietveld refined XRD pattern for the sample CoZn0.1Fe1.9O4 annealed at 600 �C. The circles represent experimental points and the solid line represents Rietveld refineddata. The bottom line shows the difference between the experimental and refined data. The marked 2h positions are the allowed Bragg peaks.

Table 1Fractional coordinates (x, y, z), and isothermal parameters (Biso) of different atoms forthe samples CoZn0.1Fe1.9O4 annealed at 600 �C.

Atoms x y z Biso

Co/Fe/Zn 0.1250 0.1250 0.1250 0.2502Fe/Co/Zn 0.5000 0.5000 0.5000 3.6234O 0.2518 0.2518 0.2518 7.3069

L. Kumar et al. / Journal of Alloys and Compounds 551 (2013) 72–81 75

The cation distributions among the tetrahedral and octahedralinterstitial sites were estimated qualitatively by the Rietveld

refinement of the occupancy values. The estimated cation distribu-tion from Rietveld analysis for the samples annealed at 600 �C isshown in Table 3. From occupancy variation we have observedthat, Co ion occupy both tetrahedral and octahedral sites. So, wecan infer that the present samples are in mixed spinel structure.It is known that the cations in bulk spinel ferrites have a strongpreference for a particular site such as Zn2+ have a strongpreference for tetrahedral site. However in the present sampleseries we observed the occupancy of Zn2+ ions are more on theB-sites which shows that in the present series Zn2+ ions have apreference for octahedral sites. It reveals that in case of nanocrys-talline materials, the cation preference does not hold well, any

Table 3Site occupancy of cations for the samples CoZnxFe2�xO4 (x = 0.0–0.3) annealed at600 �C.

Sample A-site (tetrahedral site) B-site (octahedral site)

CoFe2O4 Co0.110Fe0.890 Co0.890Fe1.110

CoZn0.1Fe1.9O4 Co0.089Fe0.896Zn0.015 Co0.911Fe1.004Zn 0.085

CoZn0.15Fe1.85O4 Co0.045Fe0.929Zn0.026 Co0.955Fe0.921Zn0.124

CoZn0.2Fe1.8O4 Co0.152Fe0.818Zn0.030 Co0.848Fe0.982Zn0.170

CoZn0.25Fe1.75O4 Co0.201Fe0.763Zn0.036 Co0.799Fe0.987Zn0.214

CoZn0.3Fe1.7O4 Co0.224Fe0.743Zn0.033 Co0.776Fe0.957Zn0.267

Table 2Rietveld agreement factors (Rp = profile factor, v2 = goodness of fit factor) and latticeconstant (a = b = c) of samples CoZnxFe2�xO4 (x = 0.0–0.3) annealed at 200 and 600 �C.

Sample Annealing temperature (�C)

200 600

Rp (%) v2a = b = c (ÅA

0

) Rp (%) v2a = b = c (ÅA

0

)

0.0 16.50 1.54 8.3609 (67) 16.60 0.78 8.3783 (22)0.10 16.60 0.91 8.3604 (13) 15.50 0.80 8.3775 (32)0.15 16.70 0.87 8.3599 (32) 19.70 1.08 8.3764 (48)0.20 18.40 1.28 8.3582 (12) 18.70 0.89 8.3733 (42)0.25 17.80 1.22 8.3563 (35) 18.00 1.19 8.3678 (58)0.30 17.60 1.17 8.3499 (18) 19.40 0.79 8.3595 (39)

Table 4Crystallite size (nm) calculated by Scherrer’s, Williamson–Hall and Rietveld methodsfor the samples CoZnxFe2�xO4 (x = 0.0–0.3) annealed at 600 and 200 �C.

Sample Scherrer’smethod600 �C

W–Hmethod600 �C

Rietveldmethod600 �C

Rietveldmethod200 �C

CoFe2O4 38 37 35 6CoZn0.1Fe1.9O4 31 29 28 7CoZn0.15Fe1.85O4 29 28 26 6CoZn0.2Fe1.8O4 24 23 22 6CoZn0.25Fe1.75O4 20 18 16 6CoZn0.3Fe1.7O4 15 14 13 6

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.20.0048

0.0051

0.0054

0.0057

0.0060

0.0063

0.0066 CoZn0.2

Fe1.8

O4 annealed at 600oC

Experimental Point Linear Fit

βCos

θ

4Sinθ

Fig. 5. Typical Williamson–Hall plot of CoZn0.2Fe1.8O4 annealed at 600 �C. 1% errorbar has shown for each data.

76 L. Kumar et al. / Journal of Alloys and Compounds 551 (2013) 72–81

more. The occupancy of Co ions at the B-site increases with the Znsubstitution for composition x = 0.0–0.15 and then decreases forx > 0.15. The estimated cation distribution might be explained asfollows: in spinel lattice, the radius of the octahedral site is largerthan tetrahedral site. Therefore we assumed that when Zn2+ ionsare substituted for Fe cations, it enters into the octahedral sitesand due to size mismatch it might induce a strain in the octahedralsites. So to relax the lattice strain effect in the octahedral site, someof the Zn2+ goes to tetrahedral sites and equivalent number of Coion enters into the octahedral sites to minimise the free energyof the system. This effect happens up to x� 0.15. However, furtherincreasing the Zn ions (x > 0.15), the strain in the octahedral site isno longer less than tetrahedral site, as a result more Co ions occupyin tetrahedral sites to relax the strain in tetrahedral sites. Hencethe Co occupancy increases in the tetrahedral sites and decreasesin the octahedral sites with the increase of Zn concentration forx > 0.15. The observed behaviour produces a significant effect onthe saturation magnetization and magnetocrystalline anisotropyconstant (discussed in the next section).

The variation of lattice constant with the Zn concentration forthe samples annealed at 200 and 600 �C are given in Table 2. Thelattice parameters increase with the annealing temperature. Theincrease of annealing temperature gives rise to volume expansionof the crystal which in turn increases the volume to surface atomsleading to enhanced crystallinity with the result of increase oflattice parameter. We have observed that, the lattice constant de-creases with increasing Zn2+ concentration. The observed behav-iour might be explained on the basis of cations distribution(discussed above) and charge neutrality. The ionic radius of theZn2+ ion is larger than Fe ion and in CoFe2O4; the valences of Coand Fe ions are +2 and +3 respectively. Also, we have observed thatCoZnxFe2�xO4 has been in single phase form. Therefore, one can as-sume that, when Zn2+ (ionic radius = 0.78 ÅA

0

) ion is substituted forFe cation, it might transform a part of Co2+ ions (ionicradius = 0.74 ÅA

0

) to Co3+ ions (ionic radius = 0.61 ÅA0

) and part ofFe3+ ions (ionic radius = 0.64 ÅA

0

) to Fe2+ ions (ionic radius = 0.77 ÅA0

)to maintain the charge neutrality as well as it induces rearrange-ment of cations between tetrahedral and octahedral site (discussedabove) to minimize the free energy of the system. Because of allthese changes, it might lead to decrease the lattice parameters.

Similar decrease in lattice parameters, with Mo6+ substituted spi-nel CoFe2O4 [30] and with Sn4+ and Mo6+ substituted MgFe2O4

[31] in Fe site have been observed.Crystallite sizes calculated by Scherrer’s formulae, Williamson–

Hall plot and Rietveld method are listed in Table 4. We have ob-served that the crystallite sizes of the samples annealed at 200 �Care independent of the Zinc concentration. It reveals that at200 �C, the density of nucleation is high and, due to high ratio ofsurface to volume atoms, surface strain becomes prominent, whichinhibit the growth of the crystallite. The sizes of the crystallite havebeen increased with the annealing temperature. It is due to theexpansion of volume of the crystal at high temperature. Duringthe thermal annealing soild-vapour surface of the crystals is re-placed by solid–solid interface via diffusion to reduce the overallsurface energy which leads to the expansion of volume. We haveobserved that the crystallite sizes of the samples annealed at600 �C decreases with the Zn concentration although the sampleswere prepared in similar condition. It reveals that when Zn2+ ionis substituted in place of Fe cation, it might induce a crystallineanisotropy and this crystalline anisotropy increases with Zn2+ con-centration. In addition to this with increasing Zn2+ concentration,volume strain inside the material will also increase. Therefore fora system to remain in stable equilibrium, crystal anisotropy andvolume strain should balance to each other. In order to relax thevolume strain the crystallite size decreases with the increase ofZn2+ concentration. The crystallite size obtained by Rietveld meth-od is less than that of Williamson Hall method. It is due to thecorrection of peak broadening taking into account of all instrumen-tal factors in Rietveld method. Also crystallite size obtained byWilliamson Hall method (Fig. 5.typical W–H plot for CoZn0.2Fe1.8O4

samples annealed at 600 �C) is less than those obtained by Scher-rer’s formulae. It is because, the strain correction factor has beentaken into account in case of Williamson Hall method whereas, ithas not taken into account in Scherrers’s method.

Fig. 7. FE-SEM image of the sample CoZn0.1Fe1.9O4 annealed at 600 �C.

L. Kumar et al. / Journal of Alloys and Compounds 551 (2013) 72–81 77

3.2. Spectral measurement

Typical FT-IR spectra at room temperature for the sampleCoZnxFe2�xO4 with x = 0.0, 0.1 and 0.3 annealed at 600 �C areshown in Fig. 6. The spectra indicate the presence of absorptionbands in the range of 400–600 cm�1 which is a common featureof the spinel ferrites structure [37]. In spinel ferrite structure, theabsorption band (m1) around 600 cm�1 is assigned to the vibrationof the bond between the oxygen ion and the tetrahedral metal ion(O–MTet) and the band (m2) around 400 cm�1 is assigned to thevibration of the bond between the oxygen ion and the octahedralmetal ion (O–MOct) [37]. For undoped sample (x = 0.0) the spectrashow absorption bands with peaks at 502 (m1), 582 (m1) and 435(m2) cm�1. The band positions are found to be in agreement withthe characteristic infrared absorption bands of cobalt ferrite nano-crystals [38]. When Zn2+ is substituted in place of Fe cation, we ob-served that the position of absorption bands slightly shifts. Forx = 0.1 composition, the spectra shows absorption band with peaksat 580 (m1) and 460 (m2) cm�1 and for x = 0.3 composition, spectrashows the peaks at 575 (m1) and 455 (m2) cm�1. The shift in theband positions and change in relative intensity indicates that theZn2+ ion has been incorporated into the spinel lattice which sup-ports the formation of nanocrystalline spinel CoZnxFe2�xO4 andshifting of cations from tetrahedral to octahedral site and viceversa.

3.3. Elemental analysis

Typical FE-SEM image of the sample CoZn0.1Fe1.9O4 annealed at600 �C is shown in Fig. 7. The image shows that the particles havean almost homogeneous distribution. The composition of the sam-ples CoZnxFe2�xO4 is determined using the EDS analysis. The com-positional analysis for all the samples was found to be close tostoichiometry. The typical EDS pattern of sample CoZn0.1Fe1.9O4

annealed at 600 �C is shown in Fig. 8. The EDS pattern revealsthe presence of Co, Fe, Zn, Au and O elements in the sample. One

x=0.1

% T

rans

mitt

ance

400 500 600 700

400 500 60

% T

rans

mitt

ance

wave numbe

wave number (cm-1)

Fig. 6. FT-IR spectra of the sample CoZnxFe2�xO4

can see that except the extra Au peak; no any other peaks havebeen traced. The gold peak appears due to the thin coating onthe sample surface to make it conducting. It reveals that there isno contamination in the sample.

3.4. Magnetic studies

Fig. 9 shows the room temperature magnetic hysteresis loopsfor the samples CoFe2O4 and CoZn0.1Fe1.9O4 annealed at 200 �C.For the compositions x = 0.15–0.3, the M versus H curves (Fig. 10)at room temperature depicts a ‘S’ shape behaviour and shows ab-sence of hysteresis which is due to the superparamagnetism natureof the sample. Also the magnetisation data does not saturate at thehigh field, coercivity field (Hc) is close to zero and zero remanentmagnetization for these samples. The above behaviour depictsthe signature of superparamagnetism. It can be explained byLangevin function which has been discussed below.

x=0.3

% T

rans

mitt

ance

wave number (cm-1)

400 500 600 700

0 700

x=0.0

r (cm-1)

with x = 0.0, 0.1 and 0.3 annealed at 600 �C.

Fig. 8. Typical EDS (Energy dispersive spectrum) pattern of sample CoZn0.1Fe1.9O4 annealed at 600 �C. Au (gold) peaks are due to thin coating on the sample.

-20 -10 0 10 20-4

-3

-2

-1

0

1

2

3

4

CoZnxFe

2-xO

4 annealed at 200oC X=0.1

x=0.0

Mag

netiz

atio

n (e

mu/

g)

Applied Field (kOe)

Fig. 9. Hysteresis loops for CoZnxFe2�xO4 (x = 0.0, 0.1) samples annealed at 200 �C.

-20 -10 0 10 20-15

-10

-5

0

5

10

15

Zn0.2

Zn0.25

Zn0.15

Zn=0.3

Mag

netiz

atio

n (e

mu/

g)

Applied Field (kOe)

Fig. 10. Magnetization versus applied field plot for CoZnxFe2�xO4 (x = 0.15, 0.2, 0.25,0.3) samples annealed at 200 �C.

78 L. Kumar et al. / Journal of Alloys and Compounds 551 (2013) 72–81

For an assembly of small magnetic nanoparticles exhibitingsuperparamagnetic behaviour, the magnetisation is given byLangevin function [39]

M ¼ M0LðblHÞ ¼ M0 CothðblHÞ � 1blH

� �ð4Þ

where b = 1/kbT. Here kb is the Boltzmann constant, T is the temper-ature, MO is the saturation magnetization of the sample, l is themagnetic moment of the particle. In equation (4), MO and l were as-sumed dependent variables and H is the independent variable. Thesaturation magnetization (highest magnetization) was determinedby fitting the experimental data to the Langevin function (equation4). Typical fitting to Langevin function for the sample CoZn0.15Fe1.85-

O4 annealed at 200 �C is shown in Fig. 11. We have observed that,the samples for x = 0.15–0.3 compositions are superparamagneticat room temperature, as the sizes of the crystallite for these compo-sitions are small. Similar behaviour has been reported by Velmuru-gen et al. [13] in Zinc doped nickel ferrite. The Langevin function fitto the experimental data shows a little deviation between experi-mental data and Langevin function fitting. The deviations from aLangevin function fit to the superparamagnetic magnetizationmay be attributed to the surface spin disorder [40]. The saturationmagnetization of the samples x = 0.0, 0.1, 0.15, 0.2, 0.25, 0.3 werefound to be 2.97, 3.29, 10.08, 7.52, 3.02, and 1.52 emu/g

-20 -15 -10 -5 0 5 10 15 20

-3

-2

-1

0

1

2

3 CoZn0.15

Fe1.85

O4 annealed at 200oC

Experimental point Fit to Langevin

Mag

netiz

atio

n (e

mu/

g)

Applied Field (kOe)

Fig. 11. Fit to Langevin for the sample CoZn0.15Fe1.85O4 annealed at 200 �C.

-20 -10 0 10 20-80

-60

-40

-20

0

20

40

60

80

CoZnxFe

2-xO

4 annealed at 600oC

x=0.00 x=0.10 x=0.15 x=0.20 x=0.25 x=0.30

Mag

netiz

atio

n (e

mu/

g)

Applied Field (kOe)

Fig. 12. Hysteresis loops for CoZnxFe2�xO4 (x = 0.0, 0.1, 0.15, 0.2, 0.25, 0.5) samplesannealed at 600 �C.

L. Kumar et al. / Journal of Alloys and Compounds 551 (2013) 72–81 79

respectively.The room temperature hysteresis loops for the samples CoZnx-

Fe2�xO4 (x = 0.0–0.3) annealed at 600 �C are shown in Fig. 12. Thesaturation magnetizations of all the samples were obtained fromfitting to equation ‘‘Law of Approach to Saturation’’ (discussed inthe next section). We observed that, the saturation magnetizationfor the samples annealed at 600 �C is more than that of annealed at200 �C. This increase in saturation magnetization with the anneal-ing temperature is due to the increase of crystallite size, as surface

0.00 0.05 0.10 0.15 0.20 0.25 0.3045

50

55

60

65

70

(a)

Ms (

emu/

g)

Zn Concentration

0.00 0.05 0.10 0.15 0.20 0.25 0.300.4

0.6

0.8

1.0

1.2

1.4

(b)

Coe

rciv

ity (

kOe)

Zn Concentration

Fig. 13. Variation of (a) saturation magnetization (b) coercivity with the Znconcentration of the samples CoZnxFe2�xO4 (x = 0.0–0.3) annealed at 600 �C.

effect reduces with the increase of particle size (particle size in-creases with the annealing temperature).

Fig. 13(a) shows the variation of saturation magnetization withZn concentration for the samples CoZnxFe2�xO4 annealed at 600 �C.It has been observed that, the saturation magnetization increasesto reach a maximum for x = 0.15 and then it decreases fromx = 0.15 to 0.3. The cation distribution in the nanocrystalline phaseis usually different from that in the bulk phase and this can lead toa change in the net magnetic moment [41]. The observed behav-iour of the saturation magnetization with the zinc concentrationcould be explained on the basis of cation distribution betweenthe A and B sites. Cation distributions of the present samples showthat all the atoms (Co, Fe and Zn) are present on both A and B sites(Table 3). In the cubic system of ferromagnetic spinels, the mag-netic interaction could be explained by the superexchange interac-tion mechanism between the cations in the A and B sites.According to Neel’s two sublattice model [39] of collinear ferrimag-netism, the magnetic moment per formula unit is expressed asM = MB �MA where MB and MA are the magnetic moment at theB-site and A-site respectively. For compositions x = 0.0–0.15 satu-ration magnetization increases. Cations distribution shows (Table4) that ions (Co, Zn and Fe) are randomly distributed and, maybe occupancy of Co3+ (4lb) and Fe3+ (5lb) are more in B site whichwould induce a increase in net magnetization. For compositionx > 0.15, the saturation magnetization decreases which may bedue to the redistribution of cations between the A-site and B-site(Table 4), decrease in size (crystallinity) and to an increase in thespin canting due to surface effect [42].

Fig. 13(b) shows the coercivity versus Zn concentration for thesamples (CoZnxFe2�xO4) annealed at 600 �C. The coercivity de-creases with the increasing Zn concentration. The coercivity Hc inthe single domain region is expressed as [4] Hc ¼ g � h

D2, where gand h are constants and D is the size of the crystallite. In a singledomain region, the coercivity decreases with the decrease of crys-tallite size because of the thermal effects. As the crystallite size(discussed in the previous section) decreases with the increasingZn concentration, it results in decrease of coercivity.

In order to extract the anisotropy information of the samplesCoZnxFe2�xO4 annealed at 600 �C, the ‘‘Law of Approach (LA)’’ tosaturation was employed to analyze the data in saturation region.LA describes the dependence of magnetization M on the appliedmagnetic field for H� Hc. The magnetization near the saturationMs can be written as [18],

M ¼ Ms½1�b

H2� þ jH ð5Þ

where b ¼ 8105

K21

l2o M2

s, M is the magnetization, H is the applied mag-

netic field, MS is the saturation magnetization, lo is the permeabil-ity of the free space, K1 is the cubic anisotropy constant and theterm jH is known as the forced magnetization. The forced magne-tization is caused by a linear increase in the spontaneous magneti-zation especially at high fields. The numerical coefficient 8/105applies to cubic anisotropy of random polycrystalline samples. Ingeneral, the forced magnetization term was found to be necessaryto fit the hysteresis curves at higher temperatures and higher fields,hence in the present case it has been neglected. Therefore Ms and K1

are the only fitting parameters in the equation (5). In the presentseries the experimental data [M–H curve] above than 1T (high fieldparts) is fitted to equation M ¼ Ms½1� b

H2�. Typical fitting curve to LA

is shown in Fig. 14a for the sample CoZn0.25Fe1.75O4 annealed at600 �C. The values of Ms and b are obtained from fitting and, cubicanisotropy constant (K1) was calculated using the equation,

K1 ¼ loMs

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi105b=8

qð6Þ

0 10 20

20

30

40

50

Experimental Data Fit to LA

Mag

netiz

atio

n (e

mu/

g)

Applied Field (kOe)

Fig. 14a. Fit to LA (Law of Approach) for the sample CoZn0.25Fe1.75O4 annealed at600 �C.

0.00 0.05 0.10 0.15 0.20 0.25 0.302.0x106

2.2x106

2.4x106

2.6x106

2.8x106

3.0x106

3.2x106

K1

(erg

/cm

3 )

Zn Concentration

Fig. 14b. Variation of magnetocrystalline anisotropy constant with the Zn concen-tration of the samples CoZnxFe2�xO4 (x = 0.0–0.3) annealed at 600 �C.

80 L. Kumar et al. / Journal of Alloys and Compounds 551 (2013) 72–81

Our result is comparable to the earlier reports on the nanocrys-talline cobalt ferrites [43].

Fig. 14b shows the variation of magnetocrystalline anisotropyconstant with the Zn2+ concentration of the samples CoZnxFe2�xO4

annealed at 600 �C. The anisotropy constant first increases up tox = 0.15 and then decreases with the Zn concentration. Accordingto the one-ion model, the strong anisotropy of cobalt ferrite is pri-marily due to the presence of Co2+ ions on the octahedral sites ofthe spinel structure. As the crystal field is not capable of removingthe orbital degeneracy of Co2+ at the octahedral sites, the orbitalmagnetic moment is not quenched and, therefore there is a strongspin- orbit coupling which leads to strong anisotropy of cobalt fer-rite (CoFe2O4). Cation distribution given in Table 4 shows that, forcomposition up to x = 0.15, the occupancy of Co ions increases atthe B-site and for x > 0.15, it decreases. It reveals that as the Znsubstitution at Fe site is increasing; concentration of Co2+ ionsare increasing at the B-site, which enhance the spin–orbit couplingand leads to increase in anisotropy constant. Further increase in Znconcentration (x > 0.15), decreases the anisotropy constant whichis due to the reduction of Co2+ ions at the B-site.

The substitution of Zn at the Co site in cobalt ferrite leads to de-crease in anisotropy constant with Zn concentration [28]. Howeverwe have observed that, the substitution of Zn at the Fe site in co-balt ferrite leads to increase the magnetocrystalline anisotropyconstant up to x = 0.15 and, then decrease with the Zn concentra-tion. Substituting Zn at the Co site, have a preference for tetrahe-dral sites whereas substituting Zn at the Fe site shows apreference for octahedral sites [28]. So from above discussion it

is clear that, Zn can be substituted at tetrahedral (A-site) and/oroctahedral (B-site) sites to tune the magnetic properties of cobaltferrite.

4. Conclusion

The substitution of Zn ions at the Fe site (CoZnxFe2�xO4) incobalt ferrite causes appreciable changes in the structural andmagnetic properties. The samples are in single-phase cubic spinelstructure with the Fd3m space group. The lattice constantdecreases with the increasing Zn concentration. The cation distri-bution shows the preference of Zn for B-sites. FT-IR spectra showthe absorption bands in the range of 380–800 cm�1 which con-firms the incorporation of the Zn2+ ion into the spinel lattice. Thesamples annealed at 200 �C with composition x P 0.15 showssuperparamagnetic nature. The magnetic properties of the sampleCoZnxFe2�xO4 annealed at 600 �C are strongly affected by cationdistribution. The saturation magnetization and magnetocrystallineanisotropy constant depends upon the Co concentration in B-site.Saturation magnetization and anisotropy constant increases withZn concentration up to x 6 0.15 for the sample CoZnxFe2�xO4 (an-nealed at 600 �C) and, then decrease with Zn concentration. Thecoercivity decreases with the Zn concentration for the sampleCoZnxFe2�xO4.

Acknowledgments

The authors are thankful to the Department of Physics, IndianInstitute of Technology Guwahati for extending the VSM facilities.The authors gratefully acknowledge Department of Atomic Energy,Government of India (Sanction No. 2011/20/37P/03/BRNS) for thefinancial support.

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