Weak ferromagnetic component on the bulk ZnFe2O4 compound

C.B.R. Jesus a, E.C. Mendonça b, L.S. Silva a, W.S.D. Folly b, C.T. Meneses b, J.G.S. Duque b,n

a Departamento de Física, Campus prof. Aluísio de Campos, UFS, 49100-000 São Cristóvão, SE, Brazilb Departamento de Física, Campus prof. Alberto Carvalho, UFS, 49500-000 Itabaiana, SE, Brazil

a r t i c l e i n f o

Article history:Received 12 August 2013Received in revised form27 August 2013Available online 18 September 2013

Keywords:Oxide materialSolid state reactionMagnetic measurement

a b s t r a c t

Magnetization data on the bulk ZnFe2O4 antiferromagnetic compound (TNE10 K) obtained via solid statereaction at different synthesis temperatures show one weak ferromagnetic component at roomtemperature. We have related it with the cationic disorder effect present on spinel structure of our bulksamples which comes from the magnetic interaction between iron ions sit on both octahedral andtetrahedral sites. The magnetization measurements show to all samples a clear peak around 10 Kconsistent with the antiferromagnetic phase transition. On the other hand, after extracted the para-magnetic component, the hysteresis loops measured at room temperature display one weak ferromagneticcomponent. Once the T-dependence of magnetization does not fit to a Curie–Weiss law to temperatureswell above the magnetic transition we have used a combination of the Curie–Weiss law (paramagneticspins) and a typical temperature dependence of M0, M0(T)¼M0(0)[1�(T/TC)2]0.5 (ordered ferromagneticspins). We note an increase of the M0(0) as function of the synthesis temperature. This reinforce oursupposition of a cationic disorder effect driving the system to present two kinds of magnetic interactionsbetween iron ions on A and B sites.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

The occupation state of tetrahedral (A) and octahedral (B) sitesby divalent and trivalent metal ions on spinel structure com-pounds has drawn much interest and has been a subject ofintensive studies. From a theoretical point of view such materialsare generally assumed to be fully normal (divalent ions aretetrahedrally coordinated and the trivalent ions are octahedrallycoordinated by oxygen atoms) or inverse (the divalent ions occupyhalf of the B sites) structures. However, from a material growthpoint of view, spinel compounds can develop very complex sitedistributions. Generally, there are deviations of normal and inverseions distributions (cationic disorder) which can originate modifi-cations on the fundamental physics properties. For instance, to thecase of magnetic spinel ferrites, the migration of small amount ofiron from B to A sites can generate high levels of disorder andexchange frustration due the competition between superexchangeinteractions between JAB and JBB. This fact give rise to a variety ofinteresting fundamental magnetic state such as ferrimagnetism(FM), local spin canting, re-entrant spin-glass (RSG) and spin-glass(SG) [1,2].

In its bulk form ZnFe2O4 is an antiferromagnetic compoundwith Néel temperature at 10 K. The microscopy origin comesfrom antiferromagnetic coupling between iron ions on octahedral

sites. However, ZnFe2O4 nanoparticles with grain sizes of about5–20 nm presents a large magnetic moment even at high tem-peratures. This is attributed to the change in distribution of Zn2þ

and Fe3þ between the A and B sites mainly at regions close ofparticles surface [3,4] doing it to change from the normal to amixed spinel structure. In this paper we show via structural andmagnetic data that even in its bulk form ZnFe2O4 develops a weakferro or ferrimagnetic component which can be associated to smalllevel of cationic disorder of the samples.

2. Experimental procedure

Polycrystalline samples of ZnFe2O4 were grown by solid statereaction starting with Fe2O3 and ZnO. Stoichiometric amounts ofstarting oxides with purity better than 99.9% were mixed andhomogenized using an agate mortar during 5 h. The resultingpowders (around 100 mg to each single batch) were finally heatedin air at 1173, 1373, 1473, and 1573 K during 12 h using a constantheating rate of 473 K/h. The powder X-ray diffractions which wereobtained with a Rigaku diffractometer using the Bragg-Brentanogeometry in continuous mode with a scan speed of (¼)1/min in the2θ range from 251 to 901 using CuKα radiation has shown that all ofthe samples are single phase having the spinel structure. Rietveldrefinements were performed using the free software DBWS9807.Magnetic measurements as function of the magnetic field andtemperature were carried out using a SQUID magnetometer(Quantum Design MPMS evercool system).

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Journal of Magnetism and Magnetic Materials

0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jmmm.2013.09.025

n Corresponding author.E-mail address: gerivaldoduque@gmail.com (J.G.S. Duque).

Journal of Magnetism and Magnetic Materials 350 (2014) 47–49

3. Experimental results

Fig. 1 display X-ray diffraction patterns measured at roomtemperature to samples synthesized at 1173, 1373, 1473, and1573 K. We also show the difference between the experimentaland calculated patterns which have been analyzed via the Rietveldmethod for using the free software DBWS9807, as described inRefs. [5,6]. The vertical bars on the bottom of the figure means thepattern obtained from literature (PDF 22-1012) to ZnFe2O4. AllXRD patterns shown in Fig. 1 are consistent with a cubic phase(space group: fd�3m).

Fig. 2 displays the M vs H loops measured at room temperatureto the samples shown in Fig. 1. In the inset we present one as-measured curve and the paramagnetic and ferromagnetic compo-nents. One must note that the paramagnetic signal was extractedfrom high field region of the as-measured sample where themagnetization is a linear function of the magnetic field (M¼χH).

Fig. 3 shows the magnetic susceptibility, χ(T), as a function oftemperature in the range from 2 to 300 K at H¼1 kOe. It is worth tocomment that our experimental data do not fit to a Curie–Weiss lawindicating the existence of others ordered magnetic phases even in thehigh temperature region. In this sense, to fit the T-dependence of thesusceptibility we have combined the Curie–Weiss law (paramagneticspins, C/(TþΘ)) and a typical temperature dependence of M0, M0(T)¼M0(0)[1�(T/TC)2]0.5 (ordered ferromagnetic spins) [13], where TC isa magnetic transition temperature of the small ferromagnetic compo-nent andM0(0) is the magnetization at T¼0 K. To ensure that samplesare completely in paramagnetic state we have carried out the fittingsto the intervals of temperature above ordering temperature. The solidlines in Fig. 3 mean the fitting using such combined expression. In theinset of Fig. 3 we show the M0 (T¼300 K) component calculated viaM0 (T) expression and the saturation value extracted from M vs H

loops. We have obtained the following values toM0(0): 0.06, 0.31, 0.48and 0.67 emu/g to samples heated at 1173, 1373, 1473, and 1573 K,respectively. The best values of Curie constant and Θ were around3.6 emu/mol and 25 K, respectively.

4. Discussion

The analyses of the XRD patterns shown in the Fig. 1 viaRietveld method are consistent with a cubic symmetry and spacegroup: fd�3m. The results also show the success of our synthesisprocedures once no spurious phases were identified.

As commented above, due magnetic interactions between ironions on tetrahedral sites, ZnFe2O4 with normal spinel structureshould present an antiferromagnetic phase transition with Neeltemperature at 10 K. So, above this temperature,M vs H loops shouldproduce a linear behavior coming from paramagnetic contribution ofiron ions. However, as one can see in Fig. 2, after subtractingthe paramagnetic signal, there is a small saturation effect whichincreases with the synthesis temperature indicating likely additionalmagnetic phases. Besides, above Neel temperature, the T-dependenceof magnetization cannot be fitted taking account only a Curie–Weiss

28 32 36 40 44 52 56 60 64

T = 1573 K

T = 1473 K

T = 1373 K

T = 1173 K

Inte

nsity

(arb

s. u

nits

)Iobs

Icalc

difference

2θ θ (degree)

Fig. 1. XRD pattern to ZnFe2O4 samples synthesized at 1173, 1373, 1473, and 1573 K.The solid lines are the fittings using the Rietveld method and difference betweenthe experimental and calculated patterns. The horizontal bars mean the standardpattern to ZnFe2O4.

-60 -40 -20 0 20 40 60

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Mag

netiz

atio

n (e

mu/

g)

1573 K1473 K1373 K1173 K

ZnFe2O4

T = 300 K

H (kOe)

Fig. 2. M vs H loops measured at room temperature to ZnFe2O4 samples synthesizedat 1173, 1373, 1473, and 1573 K. In the inset we present one as-measured curve andthe paramagnetic and ferromagnetic components. The paramagnetic signal wasextracted from high field region of the as-measured sample where the magnetizationis a linear function of the magnetic field (M¼χH).

0 50 100 150 200 250 300

0.03

0.06

0.09

0.12

0.15

0.18

0.21

χ χ (e

mu/

mol

e)

1573 K1473 K1373 K1173 K

ZnFe2O4

H = 1000 Oe

T (K)

Fig. 3. Magnetic susceptibility, χ(T), as a function of temperature in the range from2 to 300 K at H¼1 kOe. The solid lines mean the fitting using a combination of theCurie–Weiss law [C/(TþΘ)] and M0 (T)¼M0(0)[1�(T/TC)2]0.5. In the inset we showthe M0 (T¼300 K) component calculated via M0 (T) expression (black square) andthe saturation value extracted from M vs H loops (red circle). (For interpretation ofthe references to color in this figure legend, the reader is referred to the webversion of this article).

C.B.R. Jesus et al. / Journal of Magnetism and Magnetic Materials 350 (2014) 47–4948

expression. If one assumes that a small amount of iron ions migratefrom B to A site (cationic disorder) these results can be explained as acoexistence of magnetic phases originated of interactions betweeniron ions in both sites. Indeed, further investigations via neutronsdiffraction [7] on highly disordered zinc ferrite samples have shownthat two kinds of magnetic interactions exists between iron ions on Aand B sites driving the system to present two magnetic phasetransitions at low (around 10 K) and high (around 600 K) tempera-tures. Besides, it has been showed via Mössbauer spectroscopy[10–12] that spin structures more complicated can be observed iftwo magnetic interactions compete. So, the saturation effectobserved in the M vs H loops shown in Fig. 2 is an evidence ofcationic disorder. As the saturation values are small it is reasonable tosuppose a low level of cationic disorder in our samples. It is well-known that to nanocrystalline ZnFe2O4 which shows larger levels ofcationic disorder presents a large magnetic moment even at hightemperatures [7–9]. The fact ofM vs T curves have been fitted using aM0(T)¼M0(0)[1�(T/TC)2]0.5 yields other strong evidence of magneticinteraction between iron ions on A and B sites. In the same way, onemust note that the M0(0) values increase with the synthesistemperature. If the appearance of this ordered phase comes fromthe migration of the iron ions from B to A sites the increase inM0(0) can be related with the enhance of the cationic disorder. So, wecan conclude that the experimental results shown in Figs. 2 and 3clearly show the role of the cationic disorder on the magneticproperties of magnetic materials with spinel structure. Our magne-tization results are consistent with that observed in literature [7–9],that is, despite the bulk form of our samples there is a small level ofcationic disorder giving origin to a ferromagnetic order with a highertransition temperature which coexists with an antiferromagneticorder with a transition temperature at 10 K. It is worth to mentionthat the best fittings were obtained to a TC value around 600 K whichis close to that found in Ref. [7] to the same system in thenanostructured form.

5. Conclusions

In summary, polycrystalline ZnFe2O4 samples grown via solidstate reaction presented a small ferromagnetic component which

changes as function of the synthesis temperature. From X-ray datawe can conclude that our samples are single phase and have cubicsymmetry (space group: fd�3m). The M vs H loops measured atroom temperature showed a ferromagnetic component. Once theT-dependence of magnetization do not fit to a Curie–Weiss law wehave added a typical temperature dependence ofM0,M0(T)¼M0(0)[1�(T/TC)2]0.5 in order to fit our experimental results. We havediscussed our results in terms of the cationic disorder of the spinelstructure, that is, the appearance of an ordered phase is relatedwith the migration of the iron ions from B to A sites. Such fact wasrelated with the increase of both M0(0) and the cationic disorderwith the synthesis temperature.

Acknowledgments

This work was supported by the CNPq and FAPITEC Brazilianagencies.

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