Materials Research Bulletin 48 (2013) 1506–1511

Temperature dependent neutron diffraction and Mossbauer studies inzinc ferrite nanoparticles

Jeevan Job Thomas a, A.B. Shinde b, P.S.R. Krishna b, Nandakumar Kalarikkal a,c,*a School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala 686 560, Indiab Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, Indiac Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala 686 560, India

A R T I C L E I N F O

Article history:

Received 5 June 2012

Received in revised form 31 October 2012

Accepted 22 December 2012

Available online 2 January 2013

Keywords:

A. Magnetic materials

A. Nanostructures

B. Sol–gel chemistry

C. Mossbauer spectroscopy

C. Neutron scattering

A B S T R A C T

The micro-level magnetic alignments of zinc ferrite nanoparticles synthesized by sol–gel method were

investigated using magnetization study, neutron diffraction and Mossbauer spectroscopy. Magnetiza-

tion behavior of the sample clearly indicates the presence of superparamagnetism. The neutron

diffraction was performed at seven different temperatures between 6 K and 300 K. The cation

distribution is found to have a variation from the from normal spinel structure. Both A and B site

magnetic moment values found to vary with temperature which is found to correlates with variations in

bond lengths and lattice constants. The Mossbauer spectrum at room temperature shows typical doublet

while the spectrum 5 K shows well defined sextet. The two sub-spectra in the sextet resolves well by the

application of 5 T magnetic field and the noncollinear nature of magnetic alignment with in the

tetrahedral and octahedral sites of zinc ferrite are deduced.

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Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Spinel ferrite nanoparticles have been a subject of considerableresearch interest because of their unusual optical [1,2], electrical[3,4] and magnetic properties [5,6] which often differ from thebulk. The variation in these properties make them more viable topotential applications in the permanent magnets, microwaveabsorbers, chemical sensors [7,8] high speed recording media, andmagnetocaloric refrigeration systems [9,10]. Spinel ferrites havethe general formula (A2+)[B2

3+]O42� where A2+ and B3+ are the

divalent and trivalent cations occupying tetrahedral (A) andoctahedral (B) interstitial positions of the fcc lattice formed by O2�

ions. The magnetic and electrical properties of these ferritesdepend on several factors such as method of preparation, chemicalcomposition, sintering temperature, and distribution of cations atA and B sites [11,12]. It is well established that in zinc ferrite theparticle size reduction may produce a partial inversion in thecation occupancy [13,14]. The inversion parameter which have astrong dependence on the synthesis method, play a major role inthe magnetic property of zinc ferrite [15].

* Corresponding author at. School of Pure and Applied Physics, Mahatma Gandhi

University, Kottayam, Kerala 686 560, India. Tel.: +91 481 2731043;

fax: +91 481 2731669.

E-mail address: nkkalarikkal@mgu.ac.in (N. Kalarikkal).

0025-5408/$ – see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2012.12.058

Neel’s mean-field model predicts a collinear ferrimagneticordering for inverted zinc ferrites [16]. It was observed from theneutron diffraction study of ZnFe2O4 synthesized by mechanicalactivation method that the antiferromagnetic order changes toferrimagnetic order near the d value of 0.1–0.2 [17]. From theMossbauer studies of ZnFe2O4, it was concluded that the spinstructure is even more complicated when the two interactionscompete. Goya et al. [18] claimed that only the spins at the B siteare ferrimagnetically ordered, while Chinnasamy et al. [19]claimed that the canted ferrimagnetic order exists for both thespins at the A site and the B site. Oliver et al. [20] made no clearstatement about ordering but reported that the spin at the B site,which is either parallel or antiparallel to an external field is locallycanted. Recently there were reports that zinc ferrite, which is anormal spinel in the bulk regime shows noncollinear magneticstructures in the nano regime [21]. A canting of 08 and 298 for A siteand B site were reported. Even though the material is extensivelystudied, the exact spin structure remains controversial because ofthe influence of synthesis route on these properties. This invites athorough investigation, which could give a more accurate pictureof magnetic alignments in the tetrahedral and octahedral sites ofzinc ferrite. The neutron diffraction technique is particularlyhelpful to get microscopic exchange interaction and magneticordering on the individual sublattice. Correlating the results ofRietveld analysis on neutron diffraction with that of high fieldMossbauer spectroscopy will help us to build a vivid picture on themicro-level magnetic alignments in zinc ferrite nanoparticles.

Fig. 1. X-ray diffraction pattern of ZnFe2O4 nanoparticles.

Fig. 2. M–H plot of ZnFe2O4 nanoparticles at room temperature.

J.J. Thomas et al. / Materials Research Bulletin 48 (2013) 1506–1511 1507

2. Experimental

ZnFe2O4 nanoparticles have been synthesized by the sol–geltechnique using polyvinyl alcohol (PVA) as the chelating agent.0.4 M solutions of zinc nitrate [Zn(NO3)2

�6H2O] and 0.8 M solutionof ferric nitrate [Fe(NO3)2

�9H2O] were taken as precursors. Thesesolutions were mixed thoroughly and PVA is added in a weight ofPVA to weight of metal ions ratio 3. After complete dissolution ofPVA, the solution is evaporated at 80 8C as it gradually turned into aviscous sol and then to a dark brown gel. The dried gel was calcinedat 350 8C for 2 h. The basic structural characterization was carriedout using a PANalytical, X’PertPRO X-ray diffractometer (Cu Ka)and the spinel structure of zinc ferrite was identified. The basicmagnetization at room temperature and magnetization versustemperature studies were carried out using a vibrating samplemagnetometer in a Physical Property Measurement System (PPMS)of Quantum Design at UGC-CSR, Indore, India. The neutrondiffraction study has been carried out at seven differenttemperatures, 6 K, 50 K, 100 K, 150 K, 200 K, 250 K and 300 K inthe scattering angle range 8–1378 using a PSD based mediumresolution powder diffractometer at the Dhruva reactor, BhabhaAtomic Research Centre, India. The wavelength of the incidentneutrons is 1.249 A and the data has been deduced using theavailable standard software. A closed cycle refrigerator is used togo to lower temperature. Structural and magnetic analysis werecarried out by Rietveld method [22] using FULLPROF program in theWINPLOTER [23] suite. The Mossbauer spectra were recorded atUGC-CSR, Indore, India using a conventional constant accelerationMossbauer spectrometer with 57Fe in an Rh matrix as theMossbauer source at room temperature and at a low temperatureof 5 K with and without 5 T magnetic field. Least squares fittingwere performed using the WINNORMOS-SITE program.

3. Results and discussion

The X-ray diffraction pattern of ZnFe2O4 nanoparticles is shownin Fig. 1. All the reflections in the patterns correspond to the cubic

spinel structure (JCPDS pdf no: 82-1042). The particle size of thesample was calculated from the XRD line broadening of (3 1 1)plane using the Scherrer equation [24] and is obtained as 9 nm.

The magnetization study of nano ZnFe2O4 sample at roomtemperature is given in Fig. 2. The magnetization curve clearlytraces an ‘S’ shape, which shows the tendency for saturation by theapplication of an applied field of 1 kOe. The curve also showsperfectly zero hysteresis behavior with no coercivity, which clearlyindicates superparamagnetism. The maximum magnetizationachieved at an applied magnetic field of 1 kOe is 4 emu/g. Thisindicates a difference in cation distribution from the normal spinelstructure of ZnFe2O4 which generally shows completely paramag-netic behavior with very feeble magnetization and no trend tosaturate. The presence of Fe3+ ions in both A and B site is expectedfrom this. Thus the Fe–Fe interaction between A–B sublatticesdominates and is much stronger than Fe–Fe ions interaction in B

Fig. 3. ZFC and FC curves obtained from the M–T study of ZnFe2O4 nanoparticles at a

magnetic field of 1 kOe.

J.J. Thomas et al. / Materials Research Bulletin 48 (2013) 1506–15111508

site. This A–B exchange interaction due to the unequal number ofFe3+ ions on the two sites will give rise to a ferrimagnetic oruncompensated moment which in effect enhances the saturationmagnetization [25]. Neutron diffraction as well as Mossbauerspectroscopic studies will spread more light into this aspect. The

Fig. 4. Neutron diffraction profile for ZnFe

ZFC and FC curves of magnetization versus temperature at aconstant magnetic field of 1 kOe are given in Fig. 3. The curvesclearly show the presence of superparamagnetic behavior down to16 K, which is the blocking temperature.

Fig. 4a and b shows the plots of the profile matching result ofthe Rietveld refinement of the total neutron diffraction data in the2u range 8–1398 comprising both nuclear and magnetic contribu-tions for two different temperatures, 6 K and 300 K. The agreementfactors and crystallographic parameters of the refinement havebeen summarized in Table 1. The site occupancies of the cations ontetrahedral and octahedral sites were determined from theanalysis of the neutron diffraction data and are summarized inTable 2.

The degree of inversion, d can be calculated from the occupancyand multiplicity values of atoms obtained from the analysis of theneutron diffraction data. From this analysis the value of d isobtained as 0.2. Therefore the cation distribution of the sample isgiven as (Zn0.8

2+Fe0.23+)[Zn0.2

2+Fe1.83+]O4. The Fe3+ ions occupy 20%

of the tetrahedral sites and 90% of the octahedral sites. Thedifferent characteristic bond lengths were deduced from theRieltveld analysis and are presented in Table 3.

According to the Neel’s two sublattice model of ferrimagnetism,the magnetic moment per formula unit in mB is expressed by thedifference between the magnetic moments of tetrahedral andoctahedral sublattices [26]. The magnetic moment of tetrahedraland octahedral sites of zinc ferrite for each of the temperatureswere deduced by the FullProf program. The values are given in

2O4 nanoparticles at (a) 6 K (b) 300 K.

Table 1The Rietveld agreement factors as obtained from Rietveld refinement of neutron diffraction data.

Temperature 6 K 50 K 100 K 150 K 200 K 250 K 300 K

x2 2.58 2.54 2.59 2.57 2.62 2.54 1.74

Rp 5.50 5.44 5.47 5.46 5.49 5.41 4.39

Rwp 6.95 6.91 7.00 6.98 7.13 7.02 5.81

Rexp 4.33 4.34 4.35 4.36 4.41 4.40 4.40

Bragg R-factor 11.3 12.9 12.1 11.7 14.0 14.2 14.5

Rf-factor 8.62 9.33 8.78 9.57 11.8 10.6 10.7

Mag. R-factor 23.8 27.1 32.4 32.3 31.0 31.9 30.1

Lattice Parameter (A) 8.4482(4) 8.4507(4) 8.4508(4) 8.4523(4) 8.4544(5) 8.4553(5) 8.4578(4)

Oxygen position parameter 0.2601(2) 0.2600(2) 0.2601(2) 0.2602(2) 0.2599(2) 0.2601(2) 0.2600(2)

Table 2Site occupancies of the cations on tetrahedral and octahedral sites as determined

from Rietveld analysis of the neutron diffraction data.

Atom (site) Occupancy Multiplicity

Zn (Tetra) 0.300 8

Fe (Tetra) 0.075 8

Fe (Octa) 0.675 16

Zn (Octa) 0.075 16

O 1.500 32

J.J. Thomas et al. / Materials Research Bulletin 48 (2013) 1506–1511 1509

Table 4. A considerable decrease in the value of magnetic momentcan be observed from this data. In spinel ferrite systems it isreported that the distance between the magnetic ions willinfluence various physical properties [27]. It is found that thebond lengths are slightly varying as the temperature goes fromroom temperature to 6 K. This smaller change in the bondparameters will affect the superexchange interactions and thusthe magnetic moment. Another important factor which caninfluence the magnetic moment of the system is the canting ofthe magnetic moments. Mossbauer spectrum will lead us to createa more vivid picture on the orientation of internal tetrahedral andoctahedral magnetic moments in this sample.

The Mossbauer a spectrum of ZnFe2O4 at room temperature isgiven in Fig. 5. The spectrum was fitted with a single doublethaving a quadruple splitting of 0.44 mm/s and an isomer shift of0.14 mm/s. The presence of single doublet is the typicalcharacteristic of zinc ferrite at room temperature.

The usual six line absorption pattern is not observed in the caseof zinc ferrite at room temperature. The spectrum can bedeconvoluted into a pure Lorentian doublet. The doublet indicatesthe paramagnetic part in the zinc ferrite sample.

Mossbauer spectrum of ZnFe2O4 nano particles at 5 K shows awell defined sextet, typical of a magnetically ordered state. It isalready established that the Neel temperature of ZnFe2O4 is 10 K.Below 10 K, the paramagnetic contributions will be completelysuppressed and the system behaves antiferromagnetic. In the bulkstate, ZnFe2O4 forms to be normal spinel and there will be no Fe3+

ions in the tetrahedral sites. But in case of zinc ferritenanoparticles, the variation in the normal structure is expected[28]. The perfect sextet structure of the Mossbauer spectrum is not

Table 3Bond lengths obtained by Rietveld analysis of neutron diffraction data.

Bond type Bond length (A)

6 K 50 K 100 K

Zn (Tetra)–Zn (Octa) 2.9721 2.9722 2.9721

Zn (Octa)–Fe (Octa) 2.9721 2.9722 2.9721

Zn (Tetra)–O 1.9585 1.9586 1.9585

Zn (Octa)–O 2.0248 2.0249 2.0248

Fe (Octa)–Fe (Octa) 2.9721 2.9722 2.9721

Fe (Octa)–O 2.0248 2.0249 2.0248

Fe (Tetra)–O 1.9585 1.9586 1.9585

enough to establish the presence of Fe3+ ions in the tetrahedral site.However, the least square fitting of the zero field Mossbauerspectrum at 5 K shows that it composed of two sub-spectra. Thesetwo sub-spectra strongly overlap and thus unresolvable. The onlyway to sufficiently separate these sub-spectra is to apply strongexternal magnetic field. 5 T external magnetic field is enough tomake this possible. It is also sufficiently strong to suppress fastrelaxation processes in these powders at low temperatures,allowing the determination of the underlying spin structures.

The Mossbauer spectra of ZnFe2O4 nanoparticle at a tempera-ture of 5 K, recorded without applying any magnetic field and withthe application of a magnetic field of 5 T are given in Fig. 6a and brespectively.

When the 5 T field is applied the separation of two sub-spectrais more evident in Fig. 5b. This splitting of the outer lines as well asthe broadening of the sextuplet lines were arising from theopposite effects of the applied field on the hyperfine fields in A andB sites [29,30]. The second or fifth line in both A and B sub-spectradoes not vanishes and the ratio of intensities of second (or fifth)line to the first (or sixth) lines are 0.185 and 0.478 for A and B sub-spectra respectively. It is noted that this ratio for A sub-spectrum isconsiderably lower than B. The second or fifth line in the A sub-spectrum has much lower relative intensity than the second or fifthline in the B sub-spectrum. From this we can infer that the A sitemoment is more inclined to the applied magnetic field than B sitemagnetic moment.

The degree of inversion, d, was calculated from the Mossbauersub-spectral intensities using the equation

IA

IB¼ f A

f B

d2 � d

� �

where (fA/fB) is the ratio of the recoilless fraction. The value of ratioof recoilless fractions is assumed to be 1 at 5 K and 0.94 at roomtemperature [31] and the best fitted value of d is obtained as 0.2.This value is in good agreement with the value obtained from theanalysis of neutron diffraction.

The total magnetic field is the vector sum of the magnetichyperfine field and the applied field of 5 T [32]. In the presence ofexternal magnetic field applied parallel to the g-ray direction, the

150 K 200 K 250 K 300 K

2.9721 2.9723 2.9728 2.9760

2.9721 2.9723 2.9728 2.9760

1.9586 1.9586 1.9590 1.9611

2.0249 2.0249 2.0253 2.0275

2.9721 2.9723 2.9728 2.9760

2.0249 2.0249 2.0253 2.0275

1.9586 1.9586 1.9590 1.9611

Table 4Magnetic moments of A and B sites of ZnFe2O4 at different temperatures obtained

by Rietveld analysis of neutron diffraction data.

Temperature (K) Magnetic moments (mB)

A site B site

6 2.036(2) �1.906(4)

50 1.986(2) �1.950(4)

100 1.932(2) �1.910(4)

150 1.784(2) �1.973(4)

200 1.805(2) �1.923(4)

250 1.796(2) �1.812(4)

300 1.639(2) �1.907(4)

Fig. 6. Mossbauer spectra of ZnFe2O4 nanoparticles taken at (a) 5 K (b) 5 K and an

applied magnetic field of 5 T. (For interpretation of the references to color in this

figure legend, the reader is referred to the web version of this article.)

J.J. Thomas et al. / Materials Research Bulletin 48 (2013) 1506–15111510

effective magnetic field, Heff, at the 57Fe nucleus is expressed as avector sum of hyperfine magnetic field, Hhf, and the externalmagnetic field, Hext. If the effective field is considered to be inclinedat the angle u to the g-ray direction, we can express the hyperfinemagnetic field value of A site as

H2hfðAÞ ¼ H2

effðAÞ þ H2ext � 2HeffðAÞHextcos u

In the case of spinel ferrites with octahedral and tetrahedralpositions posses the antiparallel spins, the hyperfine field of theother site will be

H2hfðBÞ ¼ H2

effðBÞ þ H2ext þ 2HeffðBÞHextcos u

The line intensities (i.e. the areas of the peaks in a magneticsextet), proportional to the Clebsh–Gordan coefficients [33] arethen commonly given by

A1;6ð’Þ ¼ 3ð1 þ cos2uÞ;A2;5ð’Þ ¼ 4 sin2u;A3;4ð’Þ ¼ 1 þ cos2u;

where Aij is the intensity of the line i or j (i,j = 1, 6; 2, 6; 3, 4). Theintensities of the lines in the magnetically split spectrum are thusin the ratio of 3:x:1:1:x:3 with x ¼ 4sin2u=ð1 þ cos2uÞ. The value ofw is usually calculated from u ¼ arcsin

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi6r=4 þ 3r

por in the

equalent form u ¼ arcsinffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi4 � s=4 þ s

pwhere r = A2,5/A1,6 and

s = A2,5/A3,4, respectively. When some spins in the particle are

Fig. 5. Mossbauer spectrum of ZnFe2O4 recorded at room temperature. (For

interpretation of the references to color in this figure legend, the reader is referred

to the web version of this article.)

not fully align in a direction of the external magnetic field, A2,5 6¼ 0and it implicates that u 6¼ 0. The actual canting angle betweenhyperfine magnetic field and the external magnetic field will be

’A=B ¼ sin�1 HextðA=BÞsinuA=B

HhfðA=BÞ

!

The hyperfine fields evaluated and the canting angles calculatedusing the above equations and are given in Table 5.

From these observations it is reasonable to conclude that boththe spins in tetrahedral and octahedral sites of zinc ferrite arecanted. The canting angle of the spins in the B site depends on thenumber of nearest-neighbor Zn ions in the A site and the ratio ofthe A–B to B–B interaction strengths [34]. Thus contradicting to theNeel’s mean field model, these zinc ferrite nanoparticles found topossess noncollinear spin orientation in which the A site and B siteare canted with an angle 358 and 428 respectively. Sinceferrimagnetism in zinc ferrite is observed only in nanocrystalline

Table 5Calculated values of hyperfine magnetic field and canting angles.

Site Heff (T) u (degrees) Hhf (T) w (degrees)

A 45.9 30 41.6 35

B 40.6 47 44.2 42

J.J. Thomas et al. / Materials Research Bulletin 48 (2013) 1506–1511 1511

samples, it is possible that the nanometer scale influences not onlythe macroscopic magnetic property of zinc ferrite but also the spincanting angle. The B–B interaction is responsible for the presenceof the antiferromagnetism in the bulk ZnFe2O4. But in the case ofZnFe2O4 nanoparticles which have Fe3+ ions in the A site, A–Binteraction will come into existence and a specific ferrimagneticnature will be exhibited. In case of ZnFe2O4 nanoparticles it wasshown that ferrimagnetic as well as the antiferromagneticcoupling coexist above 10 K [35]. Thus the spin canting in thezinc ferrite can also arises from the competition between theantiferromagnetic and ferrimagnetic spins.

4. Conclusion

The results of the investigations clearly indicate the presence ofnoncollinear magnetic alignments in nano zinc ferrite systemsynthesized through PVA sol–gel route. The cationic distribution inthe system is found to have a deviation from that in the bulk formwhich contributed to the magnetization behavior. The latticeparameter, oxygen position parameter is found to have variationsas the temperature moves down to 6 K from the room temperaturewhich in effect contributed to the variations in bond lengths. Thevariation in crystal parameters is clearly reflected in the micro levelmagnetic contributions. Contradicting to the Neel’s mean fieldmodel, these zinc ferrite nanoparticles found to possess noncollinearspin orientation in which the A site and B site are canted with anangle 358 and 428 respectively. The coexisting of ferrimagnetic aswell as antiferromagnetic spins found to have influence in theoverall magnetic spin structure.

Acknowledgements

We thank UGC-DAE Consortium for Scientific Research, Indorefor extending the VSM and Mossbauer facility. The financialassistance from UGC-Govt. of India through SAP and DST-Govt. ofIndia through FIST, PURSE schemes and Nanomission program aregratefully acknowledged.

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