Solid State Communications 147 (2008) 479–483

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Solid State Communications

journal homepage: www.elsevier.com/locate/ssc

Structural investigations and magnetic properties of cobalt ferrite nanoparticlesprepared by sol–gel auto combustion methodB.G. Toksha, Sagar E. Shirsath, S.M. Patange, K.M. Jadhav ∗

Department of Physics, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad: 431 004 (MS), India

a r t i c l e i n f o

Article history:Received 27 May 2008Received in revised form26 June 2008Accepted 27 June 2008by E.V. SampathkumaranAvailable online 8 July 2008

PACS:75.50Gg75.50Tt75.70Rf75.50.Vv33.45._x

Keywords:A. Magnetically ordered materialsB. Chemical synthesisC. Scanning and transmission electronmicroscopy

a b s t r a c t

Morphological and magnetic characteristics of cobalt ferrite nanoparticles synthesized by sol–gel autocombustion method using nitrates of respective metal ions have been studied. X-ray diffraction patternwas indexed by a Rietveld program to calculate accurate unit cell dimension. A Transmission ElectronMicroscope (TEM) confirmed the formation of single phase cobalt ferrite nanoparticles in the range11–40 nm depending on the annealing temperature and time. The size of the particles increases withannealing temperature and timewhile the coercivity goes through amaximum, peaking at around 25 nm.A very large coercivity (10.2 kOe) is observed on cooling down to 77 K while typical blocking effects areobserved below about 260 K. The high field moment is observed to be small for smaller particles andapproaches the bulk value for large particles. Mossbauer spectra recorded at room temperature is a sextetindicating that there is a strong magnetic coupling and increase in sintering temperature from 570 ◦C to800 ◦C do not affect Mossbauer parameters.

© 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Metal-oxide nanoparticles have been the subject of currentinterest because of their unusual optical, electronic and magneticproperties, which often differ from their bulk counter-part [1,2]. Nanocrystalline magnetic particles are gaining attraction dueto many important applications such as ferrofluids, magneticdrug delivery, hyperthermia for cancer treatment [3–5]. Amongthe magnetic particles, cobalt ferrite (CoFe2O4) is a well-knownhard magnetic material. The high coercivity [6] and moderatemagnetization [7] makes CoFe2O4 a good candidate for manyapplications. The remarkable properties such as high saturationmagnetization, high coercivity, strong anisotropy along with goodmechanical hardness and chemical stability are not observed in thebulk sample [8]. These properties, along with their great physicaland chemical stability, make CoFe2O4 nanoparticles suitable formagnetic recording applications such as audio and videotapeand high-density digital recording disks etc. [9,10]. The magneticcharacter of the particles used for recording media depends

∗ Corresponding author.E-mail address: drkmjadhav@yahoo.com (K.M. Jadhav).

0038-1098/$ – see front matter© 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2008.06.040

crucially on the size, shape and purity of these nanoparticles. Theparticles should be single domain, of pure phase, should have highcoercivity and mediummagnetization for using them in recordingmedia.

Techniques to produce nanoparticles include wet chemicalco-precipitation, hot spraying, evaporation condensation, matrixisolation, laser-induced vapor phase reactions and aerosols.Generally, in most types of nanoparticles prepared by thesemethods, control of size and size distribution is not possible [11].In order to overcome these difficulties, nanometer size reactorsfor the formation of homogeneous nanoparticles of cobalt ferriteare used. To protect the oxidation of these nanoparticles fromthe atmospheric oxygen and also to stop their agglomeration,the particles are usually coated and dispersed in some mediumlike sodium dodecyl sulfate (NaDS) or oleic acid [12,13]. Ingeneral, the preparation methods for CoFe2O4 nanoparticles havebeen quite complicated requiring special techniques to preventagglomeration [14]. In this paper, we have presented the synthesisof cobalt ferrite (CoFe2O4) nanoparticles by sol–gel methodfollowed by heat treatment at 570 ◦C.

The size and size distribution of the particles prepared by thismethod was studied by XRD and TEM. The dependence of theparticle size on the annealing temperature and annealing timewas

480 B.G. Toksha et al. / Solid State Communications 147 (2008) 479–483

also studied. Finally, various magnetic properties of the particleshave been studied as functions of field, temperature and size.

60CoMossbauer spectroscopy is a nuclear-probe technique thatis very well suited for the investigation of the local symmetry,the magnetic state, and the charge states of iron ions in iron-containing materials. The Mossbauer analysis of prepared samplesis also presented in this paper.

2. Experimental procedure

Nanocrystalline powder of CoFe2O4 were prepared by sol–gelauto-ignition method. The A.R. Grade citric acid (C6H8O7 · H2O),Cobalt nitrate (Co(NO3)26H2O) (>99% sd-fine) were used asstarting materials. Reaction procedure was carried out in airatmosphere without protection of inert gases. The molar ratio ofmetal nitrates to citric acid was taken as 1:3. The metal nitrateswere dissolved together in a minimum amount of double distilledwater to get a clear solution. An aqueous solution of citric acid wasmixed with metal nitrates solution, then ammonia solution wasslowly added to adjust the pH at 7. Themixed solutionwas kept onto a hot platewith continuous stirring at 90 ◦C. During evaporation,the solution became viscous and finally formed a very viscousbrown gel. When finally all water molecules were removed fromthe mixture, the viscous gel began frothing. After few minutes,the gel automatically ignited and burnt with glowing flints. Thedecomposition reaction would not stop before the whole citratecomplexwas consumed. The auto-ignitionwas completedwithin aminute, yielding the brown-colored ashes termed as a precursor. Itis known that thepure CoFe2O4 obtainedby the sol–gelmethod canbe formed at 570 ◦C and thoroughly crystallized at temperaturesabove 570 ◦C [15] The as-prepared powders of all the sampleswereheat treated separately at 570 ◦C for 4 h to get the final product.

The final product obtained was then confirmed by X-raydiffraction (XRD). The Rietveld’s powder structure refinementanalysis of XRD data is adopted to obtain the refined structuralparameter and lattice parameter. Particle size and particlesize distribution is done by transmission electron microscopy(TEM). Vibrating sample magnetometer (VSM) gave the valuesof saturation magnetization and coercivity. Mossbauer spectra ofsamples were used to identify magnetic phase of cobalt ferrite.

Mossbauer spectra were recorded at room temperature usinga constant acceleration drive and a personnel computer analyzer(PCA card with 1024 channel). The source was 60Co in Rhmatrix atroom temperature with an initial activity of 25 mCi. The optimalweight of the absorber was approximately 10 mg/cm2 of naturaliron. Metallic iron spectrum was used for the calibration of bothobserved velocities and hyperfine magnetic field.

3. Results and discussion

The analysis of X-ray diffraction pattern of the calcinedpowder synthesized using this route shows that the final productis CoFe2O4 with the expected inverse spinel structure. X-raydiffraction pattern was indexed by TREOR program to calculateaccurate unit cell dimension. The data were processed to realizethe conditions of the software program FullProof for the structurerefinement. The XRD pattern is shown in Fig. 1. The value oflattice constant is found to be 8.3731 Å ± 0.004 Å. The inversionparameter calculated from Rietveld refinement is 0.82 which is ingood agreement for values reported in literature [16,17]. The valueof discrepancy factor is 6.31 and expected value is 3.62 and thegoodness of fit index was 1.74.

The size of the particles was determined by Scherrer formulausing most incense (311) peak. The average sizes of the particlescalcined at 570 ◦C were found to be 15 nm. By annealing 15 nmparticles at 800 ◦C and 900 ◦C for 10 h, particle sizes of 24

Fig. 1. X-ray diffraction pattern (Cu Kα-radiation) of CoFe2O4 nanoparticlesprepared by sol–gel method, after calcinations at 570 ◦C for 4 h.

Fig. 2. TEM micrograph of CoFe2O4 nanoparticles prepared, after calcinations at570 ◦C for 4 h.

and 28 ± 2 nm were obtained. Finally, on further annealing, at1000 ◦C for 8 and 12 h, respectively, 32 and 40 ± 2 nm particleswere obtained. Thus, the size variation has been achieved by theannealing conditions.

The TEM images (Fig. 2) of CoFe2O4 nanoparticles calcined at570 ◦C for 4 h (with average crystallite size of about 15 nm. The sizedistribution of these nanoparticles as observed in TEM images isshown in Fig. 3. The distribution seems to be symmetric (Gaussian)about 15 nm, with particles of sizes 11–20 nm for this specimen.The maximum number lie between 14–16 nm, peaking at 15 nm,in good agreement with XRD crystallite size. Most of the parti-cles appear spherical in shape however some elongated particlesare also present. Some moderately agglomerated particles as wellas separated particles are present in the images. Agglomeration isunderstood to increase linearly with annealing temperature andtherefore agglomeration at this temperature appears unavoidable.

Fig. 4 shows the correlation between the particle size andannealing temperature. The size of the particles is observed tobe increasing linearly with annealing temperature. It appears thatthat increase in size with temperature becomes rapid between700–900 ◦C and appears to be slowing down above 900 ◦C.While annealing generally decreases the lattice defects and strains,however it can also cause coalescence of crystallites that resultsin increasing the average size of the nanoparticles [18]. The Fig. 5shows the dependence of particle size on annealing time at afixed annealing temperature of 570 ◦C. The particle size appearsto increase almost linearly with annealing time, most likely dueto the fact that longer annealing time enhances the coalescenceprocess resulting in an increase in the particle size. Thus, it appearsthat particle size may be controlled by varying either of the twoparameters i.e. annealing temperature and time.

B.G. Toksha et al. / Solid State Communications 147 (2008) 479–483 481

Fig. 3. Size distribution of CoFe2O4 nanoparticles from TEM images (570 ◦Ccalcinations for 4 h).

Fig. 4. Particle size (nm) as a function of annealing temperature (570 ◦C) forCoFe2O4 nanoparticles.

Fig. 5. Correlation between particle size and annealing time in hrs (at 570 ◦C).

Magnetic characterization of the particles was done usingvibrating sample magnetometer (VSM), at room temperature andat 77 K, with maximum applied field up to 12 kOe (Fig. 6).For the 15 nm size particles the coercivity at room temperaturewas 1215 Oe while at 77 K it had increased to ∼10.2 kOe. Thesaturation magnetization (MS) obtained at room temperature was

Fig. 6. Hysteresis loops for 15 nm CoFe2O4 nanoparticles at room temperature(300 K) and 77 K at maximum applied field of 12 kOe.

found to be 67 emu/g and remanent magnetization (Mr) was30.2 emu/g, while at 77 K the values for the same parameterswere 43 emu/g and 35 emu/g, respectively. The very largecoercivity and low saturation magnetization at 77 K are maybe related with a pronounced growth of magnetic anisotropyinhibiting the alignment of the moment in an applied field. Theremanance ratios at these temperatures indicate the same feature,rising from 0.45 to 0.81 at 77 K. The value of remanance ratioof 0.45 is close to that expected (0.5) for a system of non-interacting single domain particles with uniaxial anisotropy [19].The existence of an effectively uniaxial anisotropy in magneticnanoparticles has been attributed to surface effects [20,21] asevidencedby simulations of nanoparticles. Surface effects also tendto lead to large anisotropies. Some reports argued that due tothe disorder near the surface, the typical two sublattice picturefor antiferromagnetic or ferrimagnetic nanoparticles appears tobreak down and multiple sublattice picture appears to hold [22].There appears to be a situation where several different spinconfigurations e.g. 2, 4, 6 sublattice models have very similarenergies and hence multiple ground states are possible e.g. in aspinglass. The interaction between the core and surface spins leadsto a variety of effects including large anisotropy and exchange biaseffects. It is common to find higher effective anisotropy values inmagnetic nanoparticles as compared to their bulk counterpart.

In a fine particles system where the dominant mechanism isreplaced by the rotation ofmagneticmoment,with the assumptionthat the particles are randomly oriented and the strain distributionis homogeneous in the samples, b is described as [23]

b =1M2

S

(8

105K 21 +

415

K 2sh +

35λ2σ 2

+415

K 2sf

)where, K1 is the magneto-crystalline anisotropy constant, Kshthe shape anisotropy constant, Ksf the surface anisotropy, λ =

|λS | the magnetostriction constant and σ the internal strain.For simplicity and comparison with the bulk materials, here theeffective magnetic anisotropy KE is defined as

K 2E = K 2

sh +94λ2σ 2

+ K 2sf ,

so,

b =1M2

S

(8

105K 21 +

415

K 2E

).

482 B.G. Toksha et al. / Solid State Communications 147 (2008) 479–483

In the approaching saturation region, b can be determined fromthe approximationM = MS(1 − b/H2) andMS can be obtained byextrapolation of the line to the axis. From a fit of themagnetizationdata at high fields using the law of approach to saturation

χ =∂M∂H

∼=αK 2

eff

MSH3.

In the present study the value of the anisotropy constant isKeff = 3.7 × 107 ergs/cc, where α = 0.524 for uniaxialanisotropy and the value of anisotropy constant for 800 ◦C is Keff =

3.2 × 106 ergs/cc. For the samples sintered at 570 ◦C the value ofanisotropy constant is higher by one order of magnitude than thevalue for bulk cobalt ferrite [24]. The value for samples sinteredat 800 ◦C is very close to that of bulk samples. The coercivityof the nanoparticles was also studied as a function of particlesize. Fig. 7. shows the coercivity as a function of particle size atroom temperature (300 K). The Gaussian fit to the data shows thecoercivity increases with size rapidly, attaining a maximum valueof∼1215 Oe at 25 nm and then decreaseswith size of the particles.This decrease at larger sizes could be attributed to either of tworeasons. Firstly, itmay be due to the expected crossover from singledomain to multidomain behavior with increasing size. Secondlysuch an effect can arise from a combination of surface anisotropyand thermal energies. The former effect is expected in CoFe2O4particles for a size close to 40 nm [25] that is significantly higherthan the critical size of 25 nm that we observed. The latter sourceof the effect is therefore considered as a more likely explanationfor the peak. The initial increase of the coercivity with decreasingsize can be understood due to the enhanced role of the surface andits strong anisotropy, as opposed to the weaker bulk anisotropy.This rise is followed by a decline at small enough sizes when theproduct of the anisotropy energy and volume becomes comparableto the thermal energy, leading to thermally assisted jumps overthe anisotropy barriers. It is also likely that the two processes areoperating simultaneously and the single domain effects may notbe excluded, however the dominant role will be of the surfaceeffects for smaller particles. The decrease of Hc at d ≥ 40 nm mayvery well have a contribution from the development of domainwalls in the nanoparticles. Fig. 7 also shows the dependence ofsaturation magnetization on particle size. The MS values obtainedfor the samples are in the range 50–76 emu/g. Themaximumvalueof saturationmagnetization is 76 emu/g for 40 nm particles whichis close to the bulk value for CoFe2O4. The saturationmagnetizationincreases consistently with particle size. For small particles thevalue of Ms is significantly lower than the bulk value of 80 emu/gwhile for the size of ∼40 nm the magnetization value approachesto the bulk value. A very sharp increase in the magnetizationbetween the sizes of 10–25 nm is observed while there is aslower increase thereafter, as in the case of coercivity and blockingtemperature. The decrease in MS at small particle sizes is relatedwith the effects of the relatively non-reactive surface layer thathas lowmagnetization. This surface effect becomes less significantwith increasing sizes and above 40 nm seems to be no longerrelevant to the bulk magnetization.

The magnetization of different size nanoparticles is shown asa function of temperature in Fig. 8. The samples were zero fieldcooled (ZFC) to 77 K. After cooling a field of 5kOe was appliedand magnetization was recorded as function of temperature upto 300 K. A peak in the magnetization is evident in each casewith the exact position of the peak depending on the size. Itis understood that in the ZFC mode the magnetization of acollection of nanoparticles may go through a peak as the particles’moments become blocked along the anisotropy axes. The blockingtemperature (TB) is defined as the temperature atwhichmaximummagnetization is achieved. This temperature is a function ofapplied field and typical time scale of measurement. The Fig. 9

Fig. 7. The correlation between the coercivity (HC ) and mean particle diameter(nm), at room temperature and applied field of 15 kOe and saturationmagnetization(MS) as function of particle size (nm) at maximum applied field of 15 kOe.

Fig. 8. Temperature dependence of magnetic susceptibility for zero-field cooled(ZFC) CoFe2O4 nanoparticles at applied field of 5 kOe.

shows the effect of size on the blocking temperature. There isa clear increase in the blocking temperature with size, it is alsoobserved that this increase is very rapid in the beginning (atsmaller sizes) and thereafter the increase becomes very slowappearing to reach a maximum at T ∼ 272 K. The larger particlesseem to be blocked at high temperatures as compared to thesmaller particles at the same field. For larger particles, the largervolume causes increased anisotropy energy which decreases theprobability of a jump across the anisotropy barrier and hencethe blocking is shifted to a higher temperature. From the data itappears that above about 25 nm the particle blocking becomesrelatively insensitive to size.

The analysis of the Mossbauer spectra provides very importantinformation about the chemical, structural and magnetic proper-ties of ferrite samples from hyperfine interaction, isomer shift andquadrupole splitting. Mossbauer absorption spectra measured atroom temperature for cobalt ferrite powders annealed at 570 ◦Cand 800 ◦C temperatures are shown in Fig. 10. TheMossbauer spec-tra of samples are fitted with two six-line sub-patterns that are as-signed to A-ions in tetrahedral sites and B-ions in octahedral sitesof a typical spinel crystal structure. The value of average line width(mm/s) for samples sintered at 570 ◦C hereafter called as sample(a) and for samples sintered at 800 ◦C hereafter called as sample(b) is found to be 0.539 ± 0.003 and 0.548 ± 0.003. It may beconcluded from these values that increase in sintering tempera-ture from 570 to 800 ◦C do not much affect the site symmetry [26].

B.G. Toksha et al. / Solid State Communications 147 (2008) 479–483 483

Fig. 9. Dependence of blocking temperature (TB) on particle size (nm).

Fig. 10. Mossbauer absorption spectra measured at room temperature for cobaltferrite powders annealed at 570 ◦C sample (a) and 800 ◦C sample (b).

The values of average isomer shift for sample (a) and for sample(b) are 0.371 ± 0.003 and 0.362 ± 0.003, respectively. These val-ues are in agreement with values reported in literature [27]. Thereis no significant change of isomer shift values by increase in sin-tering temperature. This means that the s electron charge distribu-tion of Fe ions is negligibly influenced by temperature. The valueof quadrupole splitting is 0.069 for sample (a) and 0.071 for sam-ple (b), nomuch difference in quadrupole splitting values indicatesthat there is no electric field gradient generated with the changein sintering temperature. The hyperfine field values of tetrahedralsite for sample (a) and sample (b) are 461 and 462 kOe, respec-tively, and that is of octahedral site for sample (a) and sample (b)are 430 and 432 kOe. These values are in good agreement withthose reported in the literature [28]. In spinel lattice the values ofhyperfine fields are attributed to Fe3+ and tetrahedral (A) site andoctahedral [B] site. The hyperfine values for present study showsthat the percentage of Fe3+ at A and B site do not change by changein temperature from 570 to 800 ◦C.

4. Conclusion

The synthesis of CoFe2O4 nanoparticles in the range 11–40 nmis done successfully by sol–gel auto combustion technique. Theparticle size measured by both XRD and TEM of the nanoparticleswere in very good agreement with each other indicating that there

wasno agglomeration and that the size distribution of the preparednanoparticles was small. The Rietveld-refined inversion parametervalue shows the cobalt ferrite in the present study have partiallyinverted spinel structure. The size of the nanoparticles increaselinearly with annealing temperature and time most probably dueto coalescence that increases as annealing temperature increases.The very large coercivity and low saturation magnetization at77 K in comparison with room temperature appear to be due toa pronounced growth of magnetic anisotropy at low temperatures.The observed magnetization remanance ratio of 0.45 at roomtemperature is close to the value of 0.5 typical of a system ofnon-interacting single domain particles suggests that CoFe2O4nanoparticles exhibit an effective uniaxial anisotropy. The effectiveuniaxial anisotropy in magnetic nanoparticles has been explainedas arising from surface effects that also lead to large anisotropyenergy in nanoparticles. The coercivity shows maxima withparticle size at a value much smaller than the single domain rangeand is attributed to the thermal effects which are prominent atsmall particle sizes. For smaller particles the value of saturationmagnetization was significantly lower than the bulk value whilefor the larger size particles the values were approaching to thoseof the bulk. The smaller value ofMS in smaller particles is attributedto the greater fraction of surface spins in these particles thattend to be in a canted or spin glass like state with a smaller netmoment. The coercivity of ferrite powder strongly depended on theannealing temperatures and can be directly related to the variationof cobalt ferrite particle sizes. The Mossbauer patterns of boththe samples (a) and (b) are sextet indicating there is a magneticcoupling. Therefore, no doublet could be observed which is thesignificance of the superparamagnetic state.

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