Journal of Alloys and Compounds 566 (2013) 54–61

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Journal of Alloys and Compounds

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Influence of Co2+ distribution and spin–orbit coupling on the resultantmagnetic properties of spinel cobalt ferrite nanocrystals

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.02.163

⇑ Corresponding author. Tel.: +91 832 6519315; fax: +91 832 2452889.E-mail addresses: sal_arun@rediffmail.com, sav@unigoa.ac.in (A.V. Salker).

S.R. Naik a, A.V. Salker a,⇑, S.M. Yusuf b, S.S. Meena b

a Department of Chemistry, Goa University, Goa 403 206, Indiab Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

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

Article history:Received 11 January 2013Received in revised form 15 February 2013Accepted 24 February 2013Available online 16 March 2013

Keywords:Oxide materialsSol gel processSpin orbit effectMagnetically ordered materialsMossbauerTEM

Superparamagnetic properties of the cobalt ferrite nanocrystals have been demonstrated. The signifi-cance of the solgel autocombustion method in preparation of cobalt ferrite oxide in the nano range(30–40 nm) has been very well complimented with the structural, dimensional and morphological tech-niques, such as X-ray diffraction technique, Transmission Electron Microscopy and Scanning ElectronMicroscopy. The valence states of the metal ions and single phase formation of the polycrystalline oxidehave been confirmed with the help of X-ray photoelectron spectroscopy and Raman spectroscopy. Thedistribution of the Fe3+ ions in the tetrahedral and octahedral lattice sites has been illustrated with thehelp of the Mössbauer spectroscopy that shows five sextets, indicating occupancies of one tetrahedraland four octahedral sites by Fe3+ ions. Hyperfine fields of 51.29, 48.74, 46.78, 43.58 and 48.59 Tesla,respectively in CoFe2O4 have been found for four octahedral and one tetrahedral site respectively, atambient temperature. The magnetic measurements M–H and M–T demonstrate a change in the magneticmoment and a superparamagnetic–ferrimagnetic transition at 235 K in the ferrite system.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Magnetic spinel ferrites are of great technological interest andhave grabbed the attention of many researchers due to their fasci-nating electromagnetic properties. The ease of preparation and thestability of these materials under various conditions have added lotof significance for their use in the industry. The novelty observed inthe physico-chemical and the electromagnetic properties due tochanges in the particle dimension and composition has encouragedresearchers around the globe to synthesize these materials [1–7].The exotic magnetic, electronic and magneto-optical propertiesof the cobalt ferrite (CoFe2O4) have prioritized them in the manu-facturing of magnetic recording devices, magnetic refrigeration,catalysis, sensors and magnetic fluids [8,9]. Magnetic nanocrystalshave been of great interest over the past several years for the fun-damental understanding of nanomagnetism and for their techno-logical and biomedical applications [8–12]. Single phasemagnetic ferrites and their nanocomposites find tremendous appli-cation as antimicrobial agents and vehicle for drug delivery, thusproving their vitality even in the non-technological field, therebyexpanding their horizon of application [13–15]. The magneticproperties of nanocrystals vary greatly with changing dimensionsand superparamagnetism is a typical example of such a size-

dependent behavior at the nanometer scale. Different applicationssuch as technical and medical, demands different set of magneticproperties. Therefore, approaches aiming at the synthesis of ferriteoxides with uniform particle size are essential for controlling themagnetic properties to satisfy the requirements for such applica-tions. Systematic studies on the correlations between magneticproperties and the chemical compositions of nanocrystals will gen-erate invaluable insight towards the fundamental understanding ofmagnetic properties and consequently enable to identify suitablecandidates of magnetic nanocrystals for various applications. Overthe years, the magnetic nanoparticles that have been studied aremostly the iron containing oxides, particularly magnetite (Fe3O4)which could be due to its easy availability. Now, with the advance-ment in nanoscience and nanotechnological research, new types ofmagnetic nanocrystals have become available.

CoFe2O4 belongs to the crystal family of spinel ferrites (AB2O4).The magnetic properties of these oxides are mainly dominated bythe distribution and the magnetic interaction of the cations in thetwo lattices, i.e., tetrahedral (A) and the octahedral (B). Because ofthe difference in the strengths of magnetic interactions at thesetwo lattices of the spinel oxides, the magnetic properties possessedby CoFe2O4 nanocrystals vary with change in the cation distribu-tion, thereby generating scope for the various applications. CoFe2-

O4 crystallizes as random spinel which can be represented as(Co2þ

x Feþ31�x)½Co2þ

1�xFe3þ1þx�O4, where x depends on the thermal history

and the preparation conditions. A systematic study of the

S.R. Naik et al. / Journal of Alloys and Compounds 566 (2013) 54–61 55

significance of the method used in the preparation of CoFe2O4

nanocrystals would provide insights towards better understandingof the correlations between the particle dimension, spin orbit (L–S)coupling and the magnetic properties. Consequently, the desirablemagnetic properties for technological/biomedical applications canbe optimized through chemical changes which are sensitive to thepreparation issues such as method, reaction conditions, particlesize distribution and the occupancy of the two lattices by the metalions.

Sol–gel autocombustion method is among the best method andused due to its effectiveness in obtaining the desired monophasicproducts with greater homogeneity and finer particle size at verylow sintering temperatures [16–18]. However to achieve the com-positional homogeneity of the final oxide powder, the preparationof a homogenous gel with respect to the distribution of cations iscrucial. Therefore, it is essential to prepare a suitable precursorsolution which can be converted to a gel without any cation segre-gation. The usage of malic acid and ethylene glycol as complexingand gelling agents at a neutral pH for the sol–gel assisted combus-tion synthesis of CoFe2O4 is reported herein. The effect of pH onthe chelating action of the organic acid is demonstrated, which isvital in preventing the precipitation and aggregation of the ionsand maintaining the homogeneity of the solution [19]. A detailedinvestigation about the influence of distribution of the metal ions(Co2+ and Fe3+) in the crystal lattice and the dimensions of theferrite oxide on the resultant magnetic properties is reported in thisarticle. The contribution of L–S coupling, from Co2+ ions, towardshigher magnetic anisotropy and hence the magnetic properties isinvestigated. The results provide an insight on the inter-relationshipof the particle dimension, the L–S coupling and the resultingsuperparamagnetic property. The conclusions on structural andmagnetic properties have been aptly supported by the magneticand spectral data obtained from various characterization tech-niques and the results prove CoFe2O4 nanoparticles to be a suitablecandidate in the technological and biomedical applications.

2. Experimental

2.1. Preparation

For the synthesis of CoFe2O4, stoichiometric amounts of analytical gradeCo(NO3)2�6H2O (Sigma–Aldrich) and Fe(NO3)3�9H2O (Sigma–Aldrich) were utilizedand were brought into solution by dissolving in double distilled water. The solutionwas maintained around 100 �C with continuous stirring. Calculated amount ofmalic acid was than added. pH of the solution was then increased (around neutral)with the slow addition of 30% ammonia solution. A considerable change in colorfrom light pink to dark wine red was observed which confirmed the chelating actionof malic acid. The dependence of chelating action on pH, seems to be very high asthere was no chelation and hence gel formation observed at lower pH values. Itcan be said that the higher pH (around neutral), forces the organic acid to releasethe acidic proton and in turn bind with the available metal ions. After confirmingthe pH, ethylene glycol in the ratio 1:4 (with respect to the malic acid) was added.The solution was then allowed to concentrate with continuous stirring and the gelso obtained was heated in an oven at a temperature of about 200 �C for 3 h. Forma-tion of a voluminous foamy precursor was observed which was then crushed intofine powder with the help of an agate mortar and pestle. The precursor was thancalcined at 400 �C for 4 h. It was again ground with acetone and after drying wassintered at 600 �C for 6 h. The as-obtained powder was than subjected to variouscharacterization techniques.

2.2. Characterization

The crystallinity, crystal structure and the phase purity of the powders wereinvestigated by X-ray diffraction (XRD) technique using Cu Ka radiations of wave-length 1.5418 Å (filtered through Ni), in steps of 0.02� on a RIGAKU ULTIMA IV X-raydiffractometer. SHIMADZU FTIR PRESTIGE-21 spectrophotometer was put into useto record the FTIR spectrum of the oxide calcined at 600 �C. A comparative studyof the Raman and IR actives modes was done. Raman spectrum was recorded inthe backscattering geometry in the range 100–1000 cm�1 using a HORIBA JOBINYVON HR-800 Raman spectrometer with an Olympus microscope (objective 50x)attachment and equipped with a CCD detector. A 488 nm Ar+ ion laser with10 mW powers was used as the excitation source for a spot of about 1 mm in diam-

eter. Morphology analysis of the compound was carried out on JEOL JSM-6360 LVScanning Electron Microscope (SEM). Transmission Electron Microscopy (TEM)images were recorded on PHILIPS CM 200 Transmission Electron Microscope oper-ating at an accelerating voltage of 200 kV and providing a resolution of 2.4 Å. Par-ticle size distribution study of the as prepared nanoparticles was carried out byemploying a DELSA NANO S, BECKMAN COULTER, USA. The valence state and bind-ing energies of various chemical species were determined by the X-ray photoelec-tron spectroscopy (XPS) employing VSW SCIENTIFIC INSTRUMENT with Al Ka as theincident source having energy of 1486.6 eV with a resolution of 0.9 eV. The vaccummaintained in the sample analyzer chamber was 1.4 � 10�8 Torr. 57Fe Mössbauermeasurements were performed in transmission mode with a 57Co radioactivesource in constant acceleration (triangular wave) mode using a standard PC basedMössbauer spectrometer (Nucleonix Systems Pvt. Ltd., Hyderabad, India). Velocitycalibration of the spectrometer was done with an enriched a-57Fe metal foil absor-ber at room temperature and the isomer shift values were given relative to Fe foil.The outer line width of calibration spectrum was 0.29 mm s�1. The recordedMössbauer spectrum was fitted by using the WinNormos fit programme. A QUAN-TUM DESIGN PPMS-VSM magnetometer was used for magnetic characterization ofthe pelletized compounds. The variation of the DC-susceptibility of each samplewith temperature was measured from 5 K to 300 K in the ZFC (zero field cooling)and FC (field cooling) modes using a magnetic field of 10 KOe. The magnetizationwith varying magnetic field of up to 50 KOe was also measured at 5 K and 300 K.

3. Results and discussion

3.1. Structural analysis

Fig. 1a shows the X-ray diffractogram of CoFe2O4 sintered at600 �C for 6 h. All the diffraction peaks observed for the oxide cor-responds to the cubic spinel ferrite structure. The phase analysiswas carried out by matching the obtained diffractogram with thestandard ICDD card number 22-1086. The XRD pattern revealsthe monophasic formation of the polycrystalline compound andalso signifies its greater crystallinity. A higher peak width whichis obvious from the XRD peaks signifies that the compound com-prises of finer particles. The broadening of the peaks also indicatesthe decrease in the density and an increase in the surface to vol-ume ratio of the CoFe2O4 nano oxide. The unit cell parameter (a)is calculated for the CoFe2O4 system as a Voigt function for the(311) peak. Crystallite size and the lattice strain present in the sys-tem are calculated from the X-ray diffractograms. Williamson–Hallextrapolations as a Lorentzian function are very well utilized in cal-culating these parameters. Williamson–Hall plot for CoFe2O4 isshown in Fig. 1b. A microdistortion of 2.7 � 10�2 is present inthe CoFe2O4 structure. This may be accompanied with the migra-tion of Co2+ ions from the tetrahedral to the octahedral lattice. Thisprocess of migration decreases the induced lattice strain, therebystabilizing the crystal structure [16]. The average crystallite sizeand the lattice constant (a) obtained for the CoFe2O4 system fromthe X-ray diffraction is 8 nm and 8.390 Å. The value for the latticeconstant is in agreement with the reported value [8].

3.2. Morphology and chemical analysis

The SEM micrograph of CoFe2O4 sintered at 600 �C is presentedin Fig. 2. The image displays a flaky and porous morphology for theoxide ferrite. The micrograph confirms the formation of secondaryparticles or lumps of aggregates which may be due to the agglom-eration of the primary particles. Consistency in the ferrite phaseformation is seen for the sample which is equally supported bythe results obtained from XRD.

3.3. Particle size analysis

The TEM images of CoFe2O4 along with the electron diffractionpattern are presented in Fig. 3a–c. The images reveal the partiallyfused nature of the hexagonal particles with an average particlesize of 40 nm. The variation in particle size is prominently seenfor the oxide. The well crystallized nature of CoFe2O4 along with

Fig. 1. (a) X-ray diffraction pattern of cobalt ferrite nanoparticles sintered at 600 �C for 6 h, and (b) Williamson–Hall plot of cobalt ferrite.

Fig. 2. SEM micrograph of cobalt ferrite heated at 600 �C.

56 S.R. Naik et al. / Journal of Alloys and Compounds 566 (2013) 54–61

particle broadening is obvious from the electron diffraction patternfrom which the position of the individual planes for the spinelCoFe2O4 can be clearly seen. This proves the calcination conditionsto be appropriate in yielding nanosized polycrystalline CoFe2O4

particles.

Fig. 3. Images depicting TEM (a) and (b), and ele

3.4. Raman spectroscopy

Raman spectroscopy is a highly sensitive tool for many latticeeffects, such as structure transition, lattice distortion, charge-lattice and spin-lattice couplings, local cation distribution, andmagnetic ordering. CoFe2O4 has a cubic inverse/mixed ferritestructure with O7

h (Fd3m space group) which gives rise to 39 nor-mal vibrational modes, out of which five are Raman active.Room-temperature Raman spectrum of CoFe2O4 nanoparticles inthe region 100–1000 cm�1 is presented in Fig. 4a. The spectrum re-veals the single phase formation of the polycrystalline compound.All the 5 (1 A1g + 1 Eg + 3 T2g) Raman active modes which are ex-pected for the cubic spinel ferrite system are visible in the spec-trum. The deconvoluted spectra (using suitable peak fittingsoftware) are shown in Fig. 4b–d. The spectra reveal the presenceof seven peaks in the Raman spectrum at 308, 357, 469, 577,617, 658 and 693 cm�1, most of which are the typical modes ofthe cubic inverse-spinel ferrite structure. The T2g mode expectedat �190 cm�1 for the compound is not clear from the spectrum,but we do observe a peak at 308 cm�1 which can be attributed tothe Eg mode [20]. The literature survey gives a precise idea aboutthe articles that reports a similar Raman spectrum for CoFe2O4

nanoparticles confirming our results [21]. Raman peak at469 cm�1 is related to the octahedral site mode that reflects thelocal lattice effect in the octahedral sublattice of CoFe2O4 [20].The other intense peak at 693 cm�1 (A1g mode) and a small peak

ctron diffraction pattern (c) of cobalt ferrite.

Fig. 4. Graphs representing (a) comparison of the Raman and IR spectra for cobalt ferrite along with the deconvoluted patterns (b), (c) and (d).

S.R. Naik et al. / Journal of Alloys and Compounds 566 (2013) 54–61 57

at 617 cm�1 are the symmetry vibrations of the metal in the tetra-hedral site. A peak is observed in the region of 658 cm�1 which canbe attributed to cobalt ions residing at defect sites that are proba-bly located near the surface of the nanoparticles [20]. This peak ispronounced due to the higher surface-to-volume ratio of the nano-particles in this sample. Fe2O3 is the most common impurity phaseformed during the synthesis of spinel ferrites. It gives two sharpsignals at �240 cm�1 and 300 cm�1 with high intensity. The240 cm�1 peak is absent in the spectrum and a protrusion is ob-served at 308 cm�1. This is comparatively weaker in intensityand can therefore be said as the property of CoFe2O4 and notFe2O3 [22]. All together there are two different lattices in the spinelferrite structure, and an attempt is made over here to assign themodes for the various signals observed in the spectrum based onthe idea obtained from the literature.

The difference between Raman and IR vibrational modes forCoFe2O4 in the range 275–1000 cm�1 is displayed in Fig. 4a. Thereare four different modes (m1, m2, m3 and m4) assigned for the tetrahe-dral lattice. All the four modes are Raman active while only 2 (m3

and m4) are IR active. From the figure, IR stretching modes are evi-dent for the compound at 400 cm�1 and 574 cm�1, which are theFe–O symmetric and asymmetric stretching vibrations, along with2 other signals at 279 cm�1 and 302 cm�1. On the contrary the sig-nal evident at 393 cm�1 which is highly intense in the IR region isabsent in the Raman spectrum proving it to be either a m3 or a m4

mode which are strong IR active modes and are weakly observedin the Raman spectrum. A weakly intense signal in the Ramanspectrum is observed at 574 cm�1 (asymmetric Fe–O stretching)indicating that m3 and m4 must be strong IR active modes only.

3.5. XPS analysis

XPS is a powerful tool in determining the oxidation state andbinding energy of the various chemical species present in a com-pound. A change in the oxidation state brings along a change inthe number of unpaired electrons thereby changing the propertiesof a system. The spectra of all the elements are deconvulated usingpeak fitting software. The full scan (FS) spectrum is presented inFig. 5a. From the figure, we can clearly observe the respectivepeaks for O 1s, Fe 2p and Co 2p which confirms the elements tobe present in the system. We could verify the absence of carbonin the material based on the absence of the carbon peak which nor-mally originates at 285 eV. The valence state of iron and cobalt de-cides the overall magnetic moment of the system. The compoundunder study is prepared in an oxygen rich atmosphere and there-fore there is possibility of redox reaction to occur in which electronexchange can take place between Co2+ and Fe3+ to form Fe2+ andCo3+ ions. This might bring along a change in the overall magneticmoment. XPS spectrum of Fe 2p is presented in Fig. 5b. The twosignals at 725.46 eV and 711.94 eV represent the Fe 2p1/2 and Fe2p3/2 for the Fe3+ state, confirming the valence state of iron to beFe3+and not Fe2+. This also rules out the possibility of metal ironto be present. The corresponding spectrum for Co 2p is presentedin Fig. 5c. From the figure, signal for Co 2p1/2 and Co 2p3/2 are ob-served at 795 eV and 779 eV respectively which are characteristicbinding energies for the Co2+ ion. The binding energy of the oxygenhaving value of 529.4 eV corresponding to the O 1s is presented inFig. 5e. Another peak at 531.6 eV is also observed which might beassigned to the defect oxide or the surface oxygen ions with low

Fig. 5. Deconvulated XPS spectra showing (a) full scan (b) Fe 2p (c) Co 2p and (d) O 1s of cobalt ferrite.

58 S.R. Naik et al. / Journal of Alloys and Compounds 566 (2013) 54–61

coordination situation [23,24].The values obtained for the bindingenergies of all the elements are in agreement with the reportedones [25]. Studies confirm the existence of the metal ions in the re-quired valency thereby enhancing the overall magnetic moment.

Fig. 6. Room temperature Mössbauer spectrum of cobalt ferrite.

3.6. Mössbauer spectroscopy

Apart from particle dimensions of the compound and valencestate of the metal ions, magnetic properties are also governed bythe distribution of the metal ions in both the sublattices (tetrahe-dral and octahedral) of spinel oxides. The degree of interaction be-tween metal ions is fully dependent on the type of metal ions thatoccupy these two sublattices. As discussed earlier, an increase ordecrease in the magnetic property is a result of such interactionand therefore it is necessary to probe this distribution of metalions. Co2+ ion has the electronic configuration of 3d7. In CoFe2O4,Co2+ preferentially enters the octahedral site. Since the oxygenatoms surrounding Co2+ ion exert a weak field environment, thedegenerate d-orbitals of Co2+ splits to give the resultant high spinelectronic configuration of t5

2g e2g . With this electronic configura-

tion, Co2+ ion possesses the crystal field stabilization energy of�0.8 Do and preferentially occupies the octahedral site [26,27]. Re-ports in the literature reflect on the random nature of CoFe2O4 andtherefore application of Mössbauer spectroscopy becomes neces-sary to investigate the degree of inversion [8,11,28–30]. Mössbauerspectrum of CoFe2O4 sample, recorded at room temperature is pre-sented in Fig. 6. The presence of Mössbauer nuclei in five differentchemical environments is confirmed from the five six lines hyper-fine patterns (Zeeman splitting or sextet) shown in the figure. This

also confirms the ferrimagnetic nature of the compounds. TheMössbauer spectrum is not showing any superparamagneticbehavior at room temperature.

Mössbauer parameters obtained after fitting the spectrum arepresented in Table 1. The values obtained for the isomer shifts(d) of all the five sextets are 0.206 mm s�1 for tetrahedral and0.331–0.377 mm s�1 for octahedral site respectively, which con-firm the presence of iron in the Fe3+ high spin state [31]. These re-sults are in compliance with the ones obtained from the XPS study.

Table 1Mössbauer fitting parameters of CoFe2O4 obtained using Win NORMOS fittingprogramme. Isomer shift (d), quadrupole splitting (D), line width (C), hyperfinemagnetic field (Hhf) and relative areas (RA) are shown in the table. All the results arerelative to Fe metal foil.

Fe sites d (mm s�1) D (mm s�1) C (mm s�1) Hhf (T) RA (%)

Sextet 1 (tet) 0.206 �0.019 0.232 48.59 22.63Sextet 2 (oct) 0.377 0.010 0.391 51.29 23.17Sextet 3 (oct) 0.331 �0.071 0.148 46.78 12.53Sextet 4 (oct) 0.349 �0.061 0.394 43.58 13.54Sextet 5 (oct) 0.346 0.006 0.192 48.74 28.13

Fig. 7. Variation of magnetization with field cooling (FC) and zero field cooling(ZFC) from 5 to 300 K. A magnetic field of 10 kOe is maintained for the FCmeasurement.

S.R. Naik et al. / Journal of Alloys and Compounds 566 (2013) 54–61 59

Isomer shift values are in the range of 0.90–0.95 mm s�1 for tetra-hedral and 1.05–1.10 mm s�1 for octahedral site, respectively forFe2+ ions [32–34]. So the hyperfine structure does not evidenceof the presence of Fe2+ ions. The quadrupole splitting (D) valuesare found to be low indicating the overall presence of cubic sym-metry at both the sites. The magnetic hyperfine field results con-firm the distribution of the Mössbauer nuclei in all the fivesublattices (four octahedral and one tetrahedral) of the compound.The magnetic hyperfine fields for a tetrahedral and four octahedralsublattices are 48.59, 51.29, 48.74, 46.78, and 43.58 Tesla respec-tively. The A and B sites were assigned to the sextets based on iso-mer shift and the magnetic hyperfine field values as reported[31,35,36]. Since the bond separation between Fe3+–O2� is largerfor octahedral ions as compared to that of the tetrahedral ions,smaller overlapping of the orbitals of Fe3+ ions and oxygen anions(low s electron density compared to tetrahedral site) and the smal-ler covalency lead to larger isomer shift at the octahedral site. Sothe sextet with a lower isomer shift corresponds to the tetrahedralsite, and the sextet having a higher isomer shift represents theoctahedral sites [35]. The percent distribution of the Fe3+ in the tet-rahedral and the octahedral sublattices is calculated from the rela-tive areas of the hyperfine sextets. The results display thedistribution to be 22.6% (tetrahedral site) and 77.4% (octahedralsite), respectively showing the degree of inversion to be 0.45 withthe formula (Fe0.45Co0.55)[Co0.45Fe1.55]O4. This proves the possibil-ity of the cation exchange taking place in the nanocrystallineCoFe2O4 reported herein (CoFe2O4 is an inverse spinel in bulkform).

3.7. Magnetic measurements

The dependency of the magnetic property on temperature is re-ported herein. The ZFC and FC susceptibility curves of CoFe2O4

nanoparticles are given in Fig. 7. From the figure, a net irreversibil-ity is observed between the ZFC and FC curves i.e. the ZFC curveshows a clear maximum at a critical temperature and decreasesrapidly to zero with decreasing temperature, while FC remains al-most constant below this critical temperature, which is typical of asuperparamagnetic behavior. The as-synthesized nanoparticles cantherefore be considered as magnetic single domains with a block-ing temperature (TB) of 235 K, corresponding to the maximum va-lue of the ZFC curve.

It is well known that CoFe2O4 can deviate from the thermody-namically stable structure and can be described by the appropriateformula as (Co2þ

k Feþ31�k)½Co2þ

1�kFe3þ1þk�O4, where parentheses and

square brackets correspond to the tetrahedral and octahedral lat-tices of the spinel structure, respectively. The degree of structureinversion is measured by k < 1. For instance, k is found to be about0.4 [8] and 0.84 [11] in 8 nm and 5.5 nm sized CF particles respec-tively, prepared by the coprecipitation method and polyol method.It is estimated to be about 0.69 [28] and 0.8 [29] in 20 nm and 5 nmsized particles respectively prepared by the micellar technique. Inall cases, the magnetic properties appear to be sensitive to the cat-

ion distribution between the octahedral and tetrahedral sites. Acertain number of Co2+ cations migrate from the octahedral sitesto the tetrahedral ones, accompanied by a reverse transfer of Fe3+

ions from the tetrahedral to the octahedral ones in order to relievethe strain. The population of Co2+ cations in the octahedral spinelsites decreases, which affects the magnetic properties of CoFe2O4

by lowering the single ion anisotropy of Co2+ ions present in thecrystal lattice and thereby affecting the TB value. Mössbauer spec-troscopy is employed to confirm the structural deviation from aninverse-spinel-like lattice of the particles reported herein. The de-gree of inversion is found to be 0.45.

The major factor for determining the superparamagnetic prop-erties of nanocrystals is magnetocrystalline anisotropy. It origi-nates from the L–S coupling at the crystal lattices. According toStoner–Wohlfarth single domain theory [30], the magnetocrystal-line anisotropy energy, EA, of a single domain nanocrystal isapproximated by EA = KV sin2h where K is the magnetocrystallineanisotropy constant, V is the volume of nanocrystal, and h is the an-gle between the easy axis of nanocrystal and the direction of field-induced magnetization. The magnetocrystalline anisotropy servesas an energy barrier to block the spin relaxation, which changesthe magnetic state from ferrimagnetic to superparamagnetic. Theheight of EA determines the blocking temperature at which thethermal activation can overcome EA and the nanocrystals trans-form into the superparamagnetic state. Both the magnetocrystal-line anisotropy constant K and the volume of nanocrystalscontrol the magnetic anisotropy. The magnitude of K is closely re-lated to the strength of L–S coupling. As a result, the superpara-magnetic properties of nanocrystals can be directly correlatedwith the variation of L–S coupling strength.

The magnetic anisotropy is the result of the accumulative con-tribution of individual magnetic cations. Therefore, the magneticcoupling at the individual lattice site can be straightforwardly cor-related to magnetic properties such as superparamagnetism. For acrystal with a cubic spinel structure, metal cations occupy the tet-rahedral (A site) and octahedral (B site) lattice sites. Since the li-gand field is weak in spinel ferrites, all cations assume high spinstates. A Fe3+ cation with 3d5 electronic configuration usually hasits orbital angular momentum quenched in a weak ligand field.Therefore, the contribution to the magnetic anisotropy should onlycome from Co2+ cations in CoFe2O4. The L–S coupling strength ofthe Co2+ determines the relative magnitude of magnetic anisot-ropy. The cubic symmetry is often lowered to a trigonal field in spi-nels due to the structural distortion from the Jahn–Teller effectand/or the nonstructural distortion from the interactions between

Fig. 8. Graphs depicting the variation in the magnetic properties of the cobaltferrite nanoparticles at 5 K and 300 K in the magnetic field of 50 kOe.

60 S.R. Naik et al. / Journal of Alloys and Compounds 566 (2013) 54–61

the central cation and the cations outside the nearest neighbor-hood of the central ion. A Co2+ cation with 3d7 electronic configu-ration at B site in CoFe2O4 has a triplet 4T1g ground state. Eventhough the trigonal field is introduced with the T1g ground statefurther splitting into A2 and E states, the Co2+ cation with a degen-erated ground state of E is still considered to have a strong L–S cou-pling, and therefore contributes greatly to the magnetic anisotropyof CoFe2O4 [13]. The strong L–S couplings at Co2+ lattice sites gen-erate a large anisotropy constant K and results in much higheranisotropy energy barrier in CoFe2O4 nanocrystals. Hence, we ob-serve higher blocking temperature for CoFe2O4 nanocrystals.

The coercivity of magnetic nanocrystals is surely related to themagnetic anisotropy. At a given temperature, the required mag-netic field strength for overcoming anisotropy to flip the magneticspin increases with increasing anisotropy energy barrier. Strong L–S couplings in Co2+ cations result in much higher coercivity inCoFe2O4. The coercivity of nanocrystals also has a contributionfrom the surface anisotropy of nanocrystals. Since the compoundunder study consist of the particles of varying sizes, it is very diffi-cult to gauge the influence of the particle size on the resultantcoercivity.

The display of superparamagnetic properties by CoFe2O4 nano-crystals facilitates the fundamental understanding of magnetism atthe nanometer scale. The studies confirm that CoFe2O4 is a bettermagnetic material than magnetite. The presence of Co2+ ions inthe octahedral lattice and the contribution of L–S coupling towardsthe resultant magnetic properties are significantly higher. The L–Scontribution of Fe2+ ions in magnetite is lower than Co2+ in CoFe2O4

and therefore CoFe2O4 nanocrystals clearly display better magneticcharacteristics than Fe3O4 nanocrystals. Clearly, the CoFe2O4 nano-particles are good candidates for technological applications interms of magnetic characteristics.

Fig. 8 represents the dependence of magnetization of CoFe2O4

nanoparticles on magnetic field at 300 K and 5 K. At 300 K thesenanoparticles exhibit hysteresis loops with coercivity, remananceand a saturation magnetization of 1326 Oe, 33 emu g�1 and74 emu g�1 respectively. At 5 K the saturation magnetization valueincreases to 80 emu g�1. This increase in the values may be be-cause of the proper ordering of the magnetic moments along thedirection of the field, which in turn are randomized at elevatedtemperatures (at 300 K) due to the thermal vibrations. The coerciv-ity and remanance are found to be 13939 Oe and 68 emu g�1

respectively. The squareness ratio (Mr/Ms) is found to increase from0.44 (300 K) to 0.86 (5 K).

4. Conclusion

Preparation of the CoFe2O4 nanocrystalline compound was car-ried out successfully by the sol–gel autocombustion method. Thestructural characterization like XRD proved the monophasic for-mation and confirmed the nanocrystalline nature of the particles.Partial migration of the cobalt ions was suspected from the micro-distortion calculated using the Williamson–Hall plots. This wasconfirmed from the Mössbauer spectrum which showed the degreeof inversion to be 0.45 and the values for isomer shift verified theexistence of iron in the Fe3+ high spin state. XPS confirmed thepresence of the metal ions in the required valence state, which en-hanced the overall magnetic moment. Raman spectroscopy effec-tively demonstrated the formation of the spinel phase onlythereby eliminating the probability of the formation of non-mag-netic a-Fe2O3 phase. The L–S contribution from Co2+ ions inenhancing the magnetic properties was effectively demonstrated.A large variation in the magnetic properties was observed withvarying field and temperature. An increase in the value of satura-tion magnetization was observed at 5 K as compared to 300 K, sig-nifying a decrease in the randomization of the magnetic momentsby the thermal vibrations. The acquisition of lattices by the metalions due to a probable migration was effectively proved by theMössbauer spectroscopy which confirmed the formation of therandom cobalt ferrite, thereby supporting the explanation pro-vided for the magnetic properties exhibited by the material understudy.

Acknowledgement

S.R. Naik and A.V. Salker would like to acknowledge Dr. A.Banerjee, Dr. V. Sathe, Dr. T. Shripathi and Dr. S. Tripathi (UGC-DAE Consortium for Scientific Research, Indore, India) for providingVSM, Raman and XPS facility. Dr. Rahul Mohan and Ms. Sahina Gazi(NCAOR, Goa, India) are acknowledged for providing SEM facility.

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