Neutron diffraction and Mössbauer studies of CoAl x Fe 2−x O 4Sam Jin Kim, Bo Ra Myoung, and Chul Sung Kim Citation: Journal of Applied Physics 93, 7504 (2003); doi: 10.1063/1.1557955 View online: http://dx.doi.org/10.1063/1.1557955 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/93/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The crystal structure and magnetic properties of Ba2−xSrxCo2Fe12O22 J. Appl. Phys. 115, 17A523 (2014); 10.1063/1.4866892 Effect of Co-Zr doping in the M-Type barium hexaferrite BaCo x Zr x Fe(12-2x)O19: A Möss AIP Conf. Proc. 1447, 1243 (2012); 10.1063/1.4710462 A structural, magnetic, and Mössbauer spectral study of the TbCo 4 − x Fe x B compounds with x = 0 , 1, and 2 J. Appl. Phys. 105, 113908 (2009); 10.1063/1.3138808 Magnetic irreversibility in ultrafine ZnFe 2 O 4 particles J. Appl. Phys. 87, 8005 (2000); 10.1063/1.373487 Atomic migration in MgFe 2−x Cr x O 4 J. Appl. Phys. 87, 6238 (2000); 10.1063/1.372666

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Neutron diffraction and Mo ¨ ssbauer studies of CoAl xFe2ÀxO4a…

Sam Jin Kim, Bo Ra Myoung, and Chul Sung Kimb)

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

~Presented on 13 November 2002!

Al substituted CoAlxFe22xO4 powders were fabricated using the sol-gel method, and their magneticand structural properties were studied with thermal analysis, x-ray, neutron diffraction, Mo¨ssbauerspectroscopy, and magnetization measurements. The crystals of the samplesx50.1 and 0.2 werefound to have a cubic spinel structure with lattice constantsa058.3864 and 8.3784 Å, at roomtemperature, respectively. Neutron diffraction patterns on CoAl0.1Fe1.9O4 were obtained at varioustemperature ranges from 10 to 816 K. Neutron diffraction at 10 K revealed a cubic spinel structureof ferrimagnetic ordering, with the effective magnetic moments of Fe31(A)( 24.18mB),Fe31(B)(4.81mB), and Co21(B)(2.98mB), respectively. The temperature dependence of themagnetic hyperfine field in57Fe nuclei at the tetrahedral~A! and octahedral~B! sites was analyzedbased on the Ne´el theory of magnetism. For the sample CoAl0.1Fe1.9O4 , the intersublattice A–Binteraction and intrasublattice A–A superexchange interaction were antiferromagnetic withstrengths ofJA–B5223.3kB and JA–A5218.0kB , respectively, while the intrasublattice B–Bsuperexchange interaction was found to be ferromagnetic with a strength ofJB–B55.6kB . It isinterpreted that the unusual reduction of magnetic moment in Fe31(A) and a noticeable strength ofthe A–A interaction are closely related to the covalency effects. ©2003 American Institute ofPhysics. @DOI: 10.1063/1.1557955#

I. INTRODUCTION

Cobalt spinel ferrites are extensively studied for theirrecently observed promising application as recording media,for their large magneto-optical effects due to the chemicalstability.1 For the practical application of Co ferrites such asmagnetic recording and magneto-optic recording media, asol-gel pyrolysis method is introduced as an alternative tosome of the other deposition techniques that have been usedto grow Co ferrite thin films.2 In recent years, various kindsof Co ferrites have been studied for controlling the size ofthe particles.3,4 For the understanding of magnetic interactionbehavior, nonmagnetic ions (Al31, Y31) were employed todilute the magnetic materials.5,6 When one tries to obtain asmall size of particles and to control its magnetic behaviors,the systematic understanding of superexchange interactionsbetween magnetic ions is essential. Studies on the superex-change interaction of ferrites have been reported by a num-ber of authors.7–9 Two sets of data, namely, magnetic mo-ment and paramagnetic susceptibility, can be used todetermine accurate superexchange strengths. However, thetotal spontaneous magnetic moment near the Ne´el tempera-ture cannot be correctly determined in a high-external mag-netic field due to the induced magnetic moment. Thus, itwould be desirable to obtain two series of data unaffected byan external magnetic field. The magnetic hyperfine fields canbe measured without the help of an external magnetic fieldusing the Mo¨ssbauer spectroscopy. When one obtains super-exchange parameters, one can only deduce the individual

sublattice moments of A and B sites from the net magnetiza-tion, because there is no way to measure the magnetic mo-ments of sublattice separately. However, Mo¨ssbauer experi-ments give the individual magnetic hyperfine fields of A andB sites separately. Therefore, it would be reasonable and in-teresting that one calculates the exchange parameters fromindividual sublattice magnetizations that were measured di-rectly by Mossbauer experiments.

In this article, a study of their structural and magneticproperties of Al substituted Co ferrites are presented by usingthermal analysis, x-ray, neutron diffraction, Mo¨ssbauer spec-troscopy, and magnetization measurements.

II. EXPERIMENT

CoAlxFe22xO4 (x50.1, 0.2! spinel powders aresynthesized by using a sol-gel method. Appropriateportions of Al(NO3)2"9H2O, Fe(NO3)3"9H2O, andCo(CH3CO2)2"4H2O were dissolved in 2-methoxyethanoland diethanolamine@HN(CH2CH2OH)2#. The solution wasadded with acetic acid and refluxed at 70 °C for 48 h andthen dried at 120 °C in a dry oven for 24 h. The dried powderwas ground and annealed at 1000 °C for 24 h in air.

The crystal structures of the samples were examined byx-ray diffractometer with CuKa radiation and neutron dif-fractometer at Korea Atomic Energy Research Institute HA-NARO HRPD ~high-resolution powder diffractometer,l51.8348 Å) reactor. The Mo¨ssbauer spectra were recordedusing a conventional spectrometer of the electromechanicaltype with a57Co source in a rhodium matrix. The magneti-zation and hysteresis curves were obtained using a vibratingsample magnetometer~VSM!.

a!No proof corrections received from author prior to publication.b!Author to whom correspondence should be addressed; electronic mail:

cskim@phys.kookmin.ac.kr

JOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 10 15 MAY 2003

75040021-8979/2003/93(10)/7504/3/$20.00 © 2003 American Institute of Physics

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III. RESULTS AND DISCUSSION

The x-ray~Cu–Ka radiation! patterns for samples exhib-ited cubic spinel phase for all our cobalt-aluminum ferritesamples. The lattice parameters of the samplesx50.1 and0.2, at room temperature, were found to be 8.3864~3! and8.3784~3! Å, respectively. In order to examine crystallo-graphic and magnetic structure in CoAl0.1Fe1.9O4 , we ob-tained neutron diffraction patterns from 10 to 816 K. Crystalstructure is determined to be cubic spinel ofFd3m by Ri-etveld refinement of theFULLPROFprogram. Figures 1~a! and1~b! show the results of refined neutron diffraction patternsfor CoAl0.1Fe1.9O4 , at 10 and 816 K, respectively. The de-termined lattice constants, oxygen parameteru, Bragg factorRB , RF , magnetic factorRM , and magnetic moments ofatoms are listed in Table I. Figure 1~a! shows diffractionpatterns of nucleus and developed magnetic phase and Fig.1~b! shows the diffraction pattern of paramagnetic nucleuspeaks. For all the temperature range below 816 K, it wasobserved, as one lowered the temperature, the intensity of theinner Bragg reflections, namely,~111!, ~220!, ~222!, and~331!, increased significantly, denoting important magneticcontribution to these reflections. As can be seen in Fig. 1, the

diffraction peaks were found to remain very sharp indicatingthe presence of magnetic long-range order in this sample.The observed magnetic moments of Fe31(A), Fe31(B), andCo21(B) as well as net magnetic moments ofCoAl0.1Fe1.9O4 , have been plotted as a function of tempera-ture in Fig. 2. Neutron diffraction at 10 K revealed a cubicspinel structure of ferrimagnetic ordering, with magnetic mo-ments of Fe31(A)( 24.18mB), Fe31(B)(4.81mB), andCo21(B)(2.98mB), respectively. It is seen from Table I thatthe magnetic moments of Fe31(B), and Co21(B) is close tofree-ion value, while the magnetic moment of Fe31(A) issubstantially smaller than its estimated free-ion value. Tem-

FIG. 1. Refined neutron diffraction patterns of CoAl0.1Fe1.9O4 ~a! 10 K and~b! 816 K. Solid circle and continuous lines represent the observed, calcu-lated, and difference profiles, respectively. Tick marks indicate nuclear~up-per! and magnetic~lower! reflections.

FIG. 2. Observed magnetic moments of Fe31(A) and Fe31(B), Co21(B)and the net magnetic moments for CoAl0.1Fe1.9O4 .

FIG. 3. Mossbauer spectra of the CoAl0.1Fe1.9O4 at various temperatures.

TABLE I. Results of refinement parameters of neutron diffraction onCoAl0.1Fe1.9O4 $Fd3m; Fe~8a!, Fe, Co, Al (16d), O@32e; (u, u, u)] %.

10 K 816 K

a0 /Å 8.3599~3! 8.4260~3!u ~S! 0.3819~3! 0.3843~3!

Fe~A!/mB 24.18Fe~B!/mB 4.81Co~B!/mB 2.98

RB 4.37% 9.28%RF 2.89% 6.36%RM 3.70%

7505J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003 Kim, Myoung, and Kim

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perature dependence of magnetization for CoAlxFe22xO4

(x50.1, 0.2! was observed by VSM and gave Ne´el tempera-ture,TN to be 816 and 765 K, respectively.

Mossbauer spectra of CoAlxFe22xO4 (x50.1, 0.2! weretaken at various absorber temperatures from 4.2 K to theTN

~816 K!. Figure 3 illustrates some of the spectra of thesample CoAl0.1Fe1.9O4 , which were composed of two six-line hyperfine patterns A and B. The isomer shifts at roomtemperature for the A and B patterns were found to be 0.26and 0.37 mm/s relative to the metallic iron, which were con-sistent with high-spin Fe13.10 The smaller value of A siteisomer shift was due to a larger covalency at the A site. In thesamplex50.1, the magnetic hyperfine fields of A and B at13 K are 51662 and 55362 kOe, which are typical valuefor Fe31 ions.

Figure 4 shows the reduced magnetic hyperfine fieldsH(T)/H(0) for the A and B sites of the sampleCoAl0.1Fe1.9O4 as a function of the reduced temperaturet5T/TN (TN5816 K). Since both magnetic moment andmagnetic hyperfine field of the ferric ion are proportional toits spin, the reduced magnetic hyperfine fields shown in Fig.4 should be equal to the reduced sublattice magnetizationsA

andsB . To extract information on the superexchange inter-actions from Fig. 4, we applied the Ne´el theory of ferrites tothe two sublattices of CoAl0.1Fe1.9O4 . The detailed theoreti-cal procedure was published in elsewhere.11 The good agree-ment between experimental data and theoretical values ofsA

andsB was obtained, and finally exchange parametersJA–B

andJA–A , were calculated to be antiferromagnetic with thestrengths of223.360.2, and218.060.2kB , respectively,

JB–B was ferromagnetic with the strength of 5.660.2kB .Here,kB is the Boltzman constant. According to the previousAnderson and Kanamori approaches, the B–B interactionhas two terms, one ferromagnetic and the other antiferromag-netic part. Positive value of B–B interaction could be under-stood of a prevailing of ferromagnetic contribution.7 We no-tice that the strength of the A–A superexchange interactionparameter is comparable to the strongest A–B interaction. Itcan be explained that, when diamagnetic Al ions are substi-tuted, A–A and B–B superexchange interaction give rise to acompetition,12 finally leads to enlarged A–A interaction.Also, increasing of A–A interaction is interpreted qualita-tively well related to the magnetic moments of individualatoms, which were Fe31(A)( 24.18mB), Fe31(B)3(4.81mB), and Co21(B)(2.98mB), at 10 K, respectively.It is noticeable that the magnetic moment of Fe31(A) isfairly low compared with the purely ionic moment of5.0mB . There could be two important factors, bond lengthand angle, affecting the magnetic moments. In a point ofbond length it can be explained as follows. From Table I, theatomic bond lengths corresponding to Fe31(A) – O22 andFe31(B) – O22 were calculated to be 1.81 and 2.09 Å, re-spectively. Therefore, one can easily expect the larger cova-lence effects of the A site than that of the B sites. Simulta-neously electron density of the A site would be reducedlargely compared to that of the B sites due to larger cova-lency and results in the reduction of localized magnetic mo-ment of the A site.13,14 A similar procedure was carried outfor the sample CoAl0.2Fe1.8O4 and yielded the strengths su-perexchange interactionsJA–B5221.360.2kB , JA–A

5219.660.2kB , andJB–B54.860.2kB , respectively.

ACKNOWLEDGMENT

This work has been supported by Korea Research Foun-dation Grant No. KRF-2002-C00037.

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FIG. 4. Reduced magnetic hyperfine fieldsH(T)/H(0) for tetrahedral~A!and octahedral~B! sites in slowly cooled CoAl0.1Fe1.9O4 as a function ofreduced temperatureT/TN . The solid line represents calculated reducedmagnetization.

7506 J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003 Kim, Myoung, and Kim

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