3450 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012

Mössbauer Studies of Cation Distribution inZn Co Fe O Microspheres

Yong Hui Li and Chul Sung Kim

Department of Physics, Kookmin University, Seoul 136-702, Korea

The crystal structure of Zn Co Fe O , prepared by a solvothermal reaction method, showed cubic spinelstructure with space group Fd-3 m based on Rietveld refinement. The lattice constant increased linearly with the Zn concentrationfrom to 0.5. Field emission scanning electron microscope (FE-SEM) measurements showed that the size of the monodispersedparticles was around 300–400 nm. With increasing Zn concentration, the saturation magnetization increased from 80.3 to 109.7 emu/g,while the coercivity at 293 K decreased from 893 to 46 Oe, respectively. The magnetocrystalline anisotropy constants were deter-mined as 1.62, 1.32, and erg/cm for , 0.25, and 0.5, respectively, based on the law of approach to saturations (LAS)method. We have investigated the cation distribution by Mössbauer spectroscopy, closely related to . We have analyzed the recordedMössbauer spectra as 3 sets with six-lines of tetrahedral site, and octahedral and sites both at 4.2 and 293 K. From the isomershift values, the valence states of and site were determined to be ferric (Fe ), while that at site to be ferrous (Fe ). Thecorresponding area ratio of site decreased from 40 to 30 % while that of site increased from 60 to 70% as the Zn concentrationchanged from to 0.5 both at 4.2 and 293 K. Here, the changes in the area ratios of and sites are due to the changes in thecation distributions at the and sites, being originating from the randomly substituted Zn ions in Zn Co Fe O microspheres.

Index Terms—Zn Co Fe O microspheres, Mössbauer spectroscopy, cation distribution, magnetocrystalline anisotropy.

I. INTRODUCTION

T HE 3D transition metal–oxide nano/micro particles havebeen considered to be an ideal candidate for biological ap-

plications [1], [2]. Also, ferrites in bulk and thin film forms showinteresting physical properties [3]–[7] and these strongly corre-lated electron systems exhibit rich electrical and magnetic char-acteristics suitable for applications in functional devices [5]–[8].Moreover, soft-magnetic CoZn ferrite can be further used inmagnetocaloric pumping, drug delivery, and high frequency de-vices. The requirements for any biomedical applications of themagnetic colloids include the chemical stability and biocom-patibility. Also, the systems with spinel structure are found tobe interesting because there is the possibility of introducing avariety of magnetic disorder [9], [10]. Especially, cobalt andcobalt containing ferrites are intriguing since they possess highcoercivity, high magnetization, very high positive magnetocrys-talline anisotropy and high negative magnetostriction. Further-more, what makes the investigation of the Zn Co Fe Omicrospheres noteworthy is the difference in cation distribu-tions and spin structure of and sites.In this paper, we have studied the magnetic properties of

nonmagnetic Zn-ion doped Co Fe O (Zn Co Fe O )microspheres such as magnetization, coercivity and mag-neto-crystalline anisotropy. Also, we have performed thedetailed Mössbauer spectroscopic study on the cation distribu-tion of Fe and randomly substituted Zn and Co ions along withthe investigation of crystallographic and magnetic propertiesof Zn Co Fe O microspheres using X-ray diffraction(XRD), vibrating sample magnetometer (VSM), and FE-SEM.

Manuscript received March 02, 2012; revised April 23, 2012, May 16, 2012,and May 23, 2012; accepted May 24, 2012. Date of current version October 19,2012. Corresponding author: C. S. Kim (e-mail: cskim@kookmin.ac.kr).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2012.2202274

II. EXPERIMENT PROCEDURES

The Zn Co Fe O microsphereswere prepared by a solvothermal reaction method with the rawmaterials of ZnCl xH O, CoCl 6H O, and FeCl 4H O, and7.2 g of sodium acetate. These were dissolved in 80 ml of ethy-lene glycol, with the stoichiometric ratio of (Zn, Co)/Fe 1/5.The obtained homogeneous yellow solution was transferredto a Teflon-lined autoclave, heated at 200 C for 8 h in airand cooled down to room-temperature. The obtained blackmagnetic particles were washed with ethanol several times, andthen dried in vacuum at 60 C for 24 h.The crystal structure of the samples were examined by XRD

with Cu-Ka radiation. The morphology andstructure of the prepared samples were characterized with theFE-SEM (model: JEOL-7401F).Magnetic properties weremea-sured by a VSM and Mössbauer spectrometer, using a 50 mCiCo (Rh) source in an Rh matrix for a constant acceleration

mode.

III. RESULTS AND DISCUSSION

The crystal structure of Zn Co Fe Owas determined by Rietveld refinement technique. The Znand iron ions occupy sites, while Zn and Co , and ironions occupy sites [11], [12]. Fig. 1 shows the X-ray diffractionmeasurement of Zn Co Fe O micro-spheres with representative Miller indices at room temperature.All the samples had the crystal structure of cubic spinel withspace group Fd-3 m. Table I shows the X-ray density, latticeconstants, and Bragg factor for Zn Co Fe O . The bulkdensity increases with increasing Zn concentration. The latticeconstant also increase from 8.4058 to 8.4259 with the Znconcentration, since the ionic radius of Zn (0.88 ) is largerthan that of Co (0.79 ). Fig. 2 shows FE-SEM image ofZn Co Fe O microspheres with theparticle size of around 300–400 nm.Fig. 3 shows the magnetic hysteresis loops of

Zn Co Fe O under 10 kOe

0018-9464/$31.00 © 2012 IEEE

LI AND KIM: MÖSSBAUER STUDIES OF CATION DISTRIBUTION IN Zn Co Fe O MICROSPHERES 3451

Fig. 1. Refined X-ray diffraction patterns of the Zn Co Fe O micro-spheres at room temperature for (a) , (b) , and (c) ,respectively.

measured at 4.2 K [Fig. 3(a)] and 293 K [Fig. 3(b)].Zn Co Fe O microspheres show spinel ferrimagneticbehavior. The saturation magnetization increases from48.5 to 116.0 emu/g, and 80.3 to 109.7 emu/g, with increasingZn concentration both at 4.2 K and 293 K, while coercivity

decrease from 6918 to 179 Oe, and 893 to 46 Oe withincreasing Zn concentration at 4.2 K and 293 K, as listed inTable I.The magnetocrystalline anisotropy constant and

anisotropy field of Zn Co Fe O were determinedby the LAS method [13]. The values of and the canbe obtained by fitting the initial magnetization curve at 293 K.The LAS method can be described as

(1)

where A is the inhomogeneity parameter, B and C are theanisotropy parameters, and is the paramagnetic suscep-tibility at high field, and the saturation magnetization isfitting of obtained from curve. Here, the fitting rangeof the initial curve was between range is 8–18 kOe. The relationbetween the saturation magnetization and magnetizationdependent parameter B is given by [13], [18]

(2)

where the is anisotropy constant, and the is theanisotropy field. Table I shows the values of andof Zn Co Fe O with various Zn concentrations. Withincreasing Zn concentration and decreased from

to erg/cm , and 37.0 to 24.5 kOe,respectively.

Fig. 2. FE-SEM images for the Zn Co Fe Omicrospheres.

To determine the distribution of iron ions inZn Co Fe O , we have obtained the Mössbauerspectra of Zn Co Fe O at 4.2 K[Fig. 4(a)–(c)] and 293 K [Fig. 4(d)–(f)]. The recorded Möss-bauer spectra of the samples were analyzed with three six-linehyperfine patterns for Zn Co Fe Ocorresponding to tetrahedral and octahedral and sites.The Fe ions at the site are trivalent cations, and Fe ions atthe site have a mixed-valence state between trivalent and di-valent cations [14]. We have further analyzed Zn ions occupingand sites [12] based on Mössbauer spectroscopy. In spinel

ferrites, there are ferromagnetic exchange interactions betweenand strong antiferromagnetic exchange interactions

between . The Zn-doping into Zn Co Fe O leadsto the decrease in , and ions interaction.For Zn Co Fe O , the sharper pattern of the Mössbauerspectra is due to the presence of Fe ions at site while thebroader pattern is due to the Fe /Fe ions at site [15].However, it is noticeable that Mössbauer spectra absorptionarea ratio of Fe ion between and ( , and ) sites for

shows a significant change when compared to that for.

3452 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012

TABLE IX-RAY DENSITY , LATTICE CONSTANT , BRAGG FACTORS , STRUCTURE FACTORS , MAGNETOCRYSTALLINE ANISOTROPY CONSTANT

, ANISOTROPY FIELD , SATURATION MAGNETIZATION AND COERCIVITY OF ZN CO FE O AT

4.2 K AND 293 K WITH DIFFERENT ZN CONCENTRATION

Fig. 3. Magnetic hysteresis loops of as-synthesized Zn Co Fe Omicrospheres measured (a) 293 K and (b) 4.2 K.

Table II summarized Mössbauer parameters of theZn Co Fe O microspheres at 4.2 Kand 293 K. The corresponding area ratio of site decreasedfrom 40 to 30%, while that of site including both andsites, increased from 60 to 70% as the Zn concentration changedfrom to 0.25, both at 4.2 and 293 K. Especially, the arearatio of site increased from 10 to 25% with increasing Znconcentration. This cation distribution of the Zn , Co , andFe /Fe ions is consistent with X-ray diffraction refinementresult.From the analysis of the Mössbauer spectra, the isomer shift

value of and sites at 4.2 K are found to be 0.40–0.37,0.26–0.33, and 0.86–0.90 mm/s, respectively. Also, at 293 K

Fig. 4. Mössbauer spectra measured at (a)–(c) 4.2 K and (d)–(f) 293 K ofZn Co Fe O microspheres.

these values are 0.15–0.19, 0.48–0.52, and 0.65–0.68mm/s withincreasing Zn concentration.From these isomer shift values, the valence states at the ,

and sites are determined to be ferric (Fe state), andsite is to be ferrous (Fe state) [16], [17]. The values of the hy-perfine field at , and site at 4.2 K decrease from 538 to525 kOe, 511 to 506 kOe, and 512 to 487 kOe, respectively. Alsoat 293 K they decrease from 489 to 481 kOe, 473 to 446 kOeand 445 to 412 kOe, with Zn concentration. Based on the Möss-bauer spectra analysis, there are two possible reasons for suchchanges. One is the hopping between Fe and Fe ions de-pending on the amount of Co and Zn ions at the site, and theother is the changes in the interaction strength between Fe ionsat and site through the presence of the Zn ions substitutedinto the and site. These result in the Zn-concentration de-pendent Mössbauer parameters both at 4.2 K and 293 K.

IV. CONCLUSION

We have investigated the cation distribution in theZn Co Fe O microspheres as wellas its crystallographic and magnetic properties using XRD,VSM, and Mössbauer spectroscopy. From the analysis of theMössbauer spectra, Zn ions are found to occupy and sites.Also, from the X-ray diffraction measurements, they show

LI AND KIM: MÖSSBAUER STUDIES OF CATION DISTRIBUTION IN Zn Co Fe O MICROSPHERES 3453

TABLE IIMÖSSBAUER PARAMETERS OF ZN CO FE O MICROSPHERES AT 4.2 K AND 293 K

cubic spinel structure. The lattice constant increases linearlywith increasing Zn concentration from to . TheZn-doping into Zn Co Fe O increases and decreases. However, based on the detailed analysis of cation distri-

bution from Mössbauer spectra, absorption area ratio of Feion between and site for (Co Fe O )shows a significant change when compared to that for(Zn Fe O ). Also, from the isomer shift value, the Fevalence state at the and sites were determined to be ferric(Fe ), while that of site to be ferrous (Fe ).

ACKNOWLEDGMENT

This work was supported by Mid-career Researcher Programthrough the National Research Foundation of Korea (NRF)grant funded by the Ministry of Education, Science and Tech-nology (MEST) (2011-0000323).

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