Magnetic and structural properties of ultrafine CoFe 1.9 RE 0.1 O 4 ( RE = Gd,Nd )powders grown by using a sol-gel methodWoo Chul Kim, Sam Jin Kim, and Chul Sung Kim Citation: Journal of Applied Physics 91, 7607 (2002); doi: 10.1063/1.1452214 View online: http://dx.doi.org/10.1063/1.1452214 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/91/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magnetic and structural properties of Co 0.8 Mn x Fe 2.2 − x O 4 ( x = 0.2 , 0.4, 0.6, 0.8) polycrystalline powderssynthesized by sol-gel process J. Appl. Phys. 105, 07A519 (2009); 10.1063/1.3072441 Magnetic properties of Zn 2+ substituted ultrafine Co-ferrite grown by a sol-gel method J. Appl. Phys. 91, 7610 (2002); 10.1063/1.1452215 Mössbauer studies of BaFe 11.9 Mn 0.1 O 19 by a sol–gel method J. Appl. Phys. 87, 6244 (2000); 10.1063/1.372668 Magnetic and structural properties of ultrafine Ni–Zn–Cu ferrite grown by a sol–gel method J. Appl. Phys. 87, 6241 (2000); 10.1063/1.372667 Growth of ultrafine Co–Mn ferrite and magnetic properties by a sol–gel method J. Appl. Phys. 85, 5223 (1999); 10.1063/1.369950

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

141.212.109.170 On: Tue, 25 Nov 2014 13:42:33

Magnetic and structural properties of ultrafine CoFe 1.9RE0.1O4 „REÄGd,Nd …powders grown by using a sol-gel method

Woo Chul Kim, Sam Jin Kim, and Chul Sung Kima)

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

Ultrafine CoFe1.9RE0.1O4 (RE5Gd, Nd) powders have been fabricated by a sol-gel method.Magnetic and structural properties of the powders were investigated by x-ray diffractometer,Mossbauer spectroscopy, and vibrating sample magnetometer. The CoFe1.9Gd0.1O4 powders thatwere fired at and above 923 K contained only a single spinel phase and behaved ferrimagnetically.The grain diameters were estimated to be 11–30 nm for the Co-Gd ferrite powders fired in 773–1123 K. Mossbauer spectra measurements showed that the CoFe1.9Gd0.1O4 powders fired at 723–823 K and the CoFe1.9Nd0.1O4 powders fired at 523–1023 K had a spinel structure and were mixedparamagnetic and ferrimagnetic in nature. Mo¨ssbauer spectra of the Co–Gd ferrite powder fired at923 K were taken at various temperatures ranging from 14 to 875 K. The iron ions at bothA~tetrahedral! and B ~octahedral! sites were found to be in ferric high-spin states. The Ne´eltemperatureTN was found to be 87562 K. Debye temperatures forA andB sites were found to beQA564065 K and QB521765 K, respectively. The magnetic behaviors of the CoFe1.9Gd0.1O4

powders fired at and above 723 K, and CoFe1.9Nd0.1O4 powders fired at and above 923 K,respectively, showed that an increase of the firing temperature yielded a decrease in the coercivityand an increase in the saturation magnetization. The maximum coercivity and the saturationmagnetization wereHc51,149 Oe andMs572 emu/g in the CoFe1.9Gd0.1O4 samples andHc

5959 Oe andMs563 emu/g in the CoFe1.9Nd0.1O4 samples. ©2002 American Institute ofPhysics. @DOI: 10.1063/1.1452214#

I. INTRODUCTION

Cobalt ferrite, CoFe2O4, is a well-known hard magneticmaterial which has been studied in detail due to its highcoercivity and moderate saturation magnetization as well asits remarkable chemical stability and mechanical hardness.1,2

These magnetic and physical properties of Co ferrite enablethe development of high density magnetic recording media.Although the nanometric Cobalt ferrite magnetic particleshave the saturation magnetization much lower than bulk ma-terial, the large coercive forces and the still reasonable mag-netization make them promising hard materials for high den-sity recording.3 Recently, a few sol-gel methods has beenintroduced for ultrafine cobalt and cobalt-mangane ferrite asmagnetic recording media.4,5 However, rare earth elements~RE!-doped ultrafine CoFe2O4 spinel have not yet been in-vestigated to a great extent.

In this study, the growth of ultrafine CoFe1.9RE0.1O4

(RE5Gd, Nd) powders is introduced by a sol-gel methodand their magnetic and structural properties were character-ized by using an x-ray diffactometer, Mo¨ssbauer spectros-copy, and vibrating sample magnetometer. X-ray diffractionmeasurements provide information about the formation ofphases, the crystallization temperature, and particle sizes.Mossbauer spectroscopy measurements were used to identifymagnetic phases of ultrafine ferrite powders. Vibratingsample magnetometer~VSM! measurements gave the satura-tion magnetization and coercivities of the spinel ferrite pow-ders. The magnetic and structural properties of ultrafineCoFe1.9RE0.1O4 (RE5Gd, Nd) powders as a function of an-nealing temperature are presented in this article.

II. EXPERIMENT

Ultrafine CoFe1.9RE0.1O4 (RE5Gd, Nd) powdersare synthesized by using a sol-gel method. Weightedamounts of Co(CH3CO2)2•4H2O, Gd(CH3CO2)2•H2O,Nd(CH3CO2)2•H2O, and Fe(NO3)3•9H2O were first dis-solved in 2-methoxyethanol and water for 30 min using anultrasonic cleaner. The solution was refluxed at 343 K for 12h to allow the gel formation and then dried at 373 K in a dryoven for 24 h. The dried powder was ground and fired atvarious temperatures for 6 h in air. These compositions ofsamples fired at various temperatures were identified by anx-ray diffractometer with CuKa radiation. Mossbauer spec-tra were recorded using a conventional Mo¨ssbauer spectrom-eter of the electromechanical type with a 30 mCi source in aRh matrix.

III. RESULTS AND DISCUSSION

The x-ray diffraction measurement shows that the forma-tion of CoFe1.9Gd0.1O4 powder starts at 723 K and all peaksfired at and above 923 K are consistent with those of a typi-cal spinel structure of a cobalt ferrite powder prepared by aconventional solid-state reaction.6 However, the diffractionpeaks for the powder fired at 723 K are fairly broad, indicat-ing that the sample consists of quite fine crystallites. Thisline broadening for the other samples decrease with an in-crease in the firing temperature corresponding to the growthof larger particles of CoFe1.9Gd0.1O4 spinel powders. Themean particle diameters were obtained by the Scherrermethod7 from broadening of x-ray diffraction peaks and areestimated to be 11–30 nm for the powders fired in 773–1123

a!Author to whom correspondence should be addressed; electronic mail:cskim@phys.kookmin.ac.kr

JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 10 15 MAY 2002

76070021-8979/2002/91(10)/7607/3/$19.00 © 2002 American Institute of Physics

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

141.212.109.170 On: Tue, 25 Nov 2014 13:42:33

K. In the case of the CoFe1.9Nd0.1O4 samples, some traces ofa-Fe2O3 phase at 2u533.5°, in addition to the spinel peaksat and above 1123 K, are shown.

Figures 1 and 2 show the room-temperature Mo¨ssbauerspectra for CoFe1.9Gd0.1O4 and CoFe1.9Nd0.1O4 powdersmeasured as a function of firing temperature. Spectra of allsamples at and above 923 K in the CoFe1.9Gd0.1O4 powdersand at 1023 K in the CoFe1.9Nd0.1O4 powders are fitted withtwo six-line subpatterns that are assigned toA ions in tetra-hedral sites andB ions in octahedral sites of a typical spinelcrystal structure. The isomer-shift value at room temperaturefor A andB patterns are found to be (0.12– 0.14)60.01 and(0.18– 0.20)60.01 mm/s in the CoFe1.9Gd0.1O4 powders andto be 0.1260.01 and 0.1960.01 mm/s in the CoFe1.9Nd0.1O4

powders relative to the Fe metal, respectively, which are con-sistent with the high spin Fe31 charge states.8 The hyperfinefield values forA andB patterns of the ferrites are found tobe ~469–483! and ~484–504! kOe in the CoFe1.9Gd0.1O4

powders and ~462–469! and ~485–503! kOe in theCoFe1.9Nd0.1O4 powders.

The spectrum for the samples fired at 723–823 K in theCoFe1.9Gd0.1O4 powders and at 523–923 K in theCoFe1.9Nd0.1O4 powders consists of two six-line subspectraand a doublets as shown in Figs. 1 and 2. It is suggested thatthe samples fired at 723–823 K in Fig. 1 and at 523–923 Kin Fig. 2 are both paramagnetic and ferrimagnetic in nature.

In case of the sample fired at 723 K in Figs. 1 and 2, 95%and 40% of the particles are too small to maintain the ferri-magnetic properties at room temperature, respectively andthe remaining portion has a particle size large enough tobecome ferrimagnetic. The Mo¨ssbauer spectrum measured atroom temperature for the powder fired at 623 K in Fig. 1 andat 423 K in Fig. 2 is attributable to Fe ions in the superpara-magnetic state9 of CoFe1.9Gd0.1O4 and CoFe1.9Nd0.1O4 pow-der. That is, most of the spinel particles fired at 623 K in theCoFe1.9Gd0.1O4 powder and at 423 K in the CoFe1.9Nd0.1O4

powder are too small to become ferrimagnetic. The magneticphase change of the ferrite powders can be explained withthe variation of the particle sizes as a function of firing tem-perature.

Mossbauer spectra of the CoFe1.9Gd0.1O4 powder fired at923 K were taken at various temperatures ranging from 14 to875 K. The Ne´el temperatureTN was found to be 87562 K. The magnetic hyperfine field values at 14 K are 528and 537 kOe for theA and B patterns, respectively, whichare typical values for ferric ions. Figure 3 shows lnF vs T2

for CoFe1.9Gd0.1O4 fired at 923 K, whereF stands for thetotal resonance absorption areas of Mo¨ssbauer spectrum attemperature (T). F is proportional to the recoil-free fractionf . The curves are close to straight lines, and the results areQA564065 K and QB521765 K, suggesting a larger in-teratomic binding force for theA site than for theB site. Alarge difference in Debye temperatures between theA site

FIG. 1. Room temperature Mo¨ssbauer spectra of CoFe1.9Gd0.1O4 powders asa function of firing temperature.

FIG. 2. Room temperature Mo¨ssbauer spectra of CoFe1.9Nd0.1O4 powders asa function of firing temperature.

7608 J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Kim, Kim, and Kim

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

141.212.109.170 On: Tue, 25 Nov 2014 13:42:33

andB site is similar to previous reports on ferrite.10,11

The magnetic properties of fired powders forCoFe1.9RE0.1O4 (RE5Gd, Nd) have been determined atroom temperature using a VSM. Figure 4 shows the firingtemperature dependence of saturation magnetization (Ms)and coercivity (Hc) of CoFe1.9Gd0.1O4 ~a! andCoFe1.9Nd0.1O4 ~b! powders at a maximal field of 15 kOe.The saturation magnetization increases drastically with firingtemperature. However, the coercivity decreases when the fir-ing temperature is higher than 723 K in the CoFe1.9Gd0.1O4

samples and 923 K in the CoFe1.9Nd0.1O4 samples. Thismagnetic behavior is related to the variation of the particlesize.12 The samples fired at 523 K in the CoFe1.9Gd0.1O4 andCoFe1.9Nd0.1O4 samples shows only a small amount of satu-ration magnetization. With the Mo¨ssbauer spectroscopy mea-surement, it is known that most of the CoFe1.9Gd0.1O4

and CoFe1.9Nd0.1O4 spinel powders have particle sizessmaller than the critical size to maintain their magnetic prop-erties. The maximum coercivity and the saturation magneti-zation were Hc51149 Oe andMs572 emu/g in theCoFe1.9Gd0.1O4 samples andHc5959 Oe andMs563 emu/gin the CoFe1.9Nd0.1O4 samples.

ACKNOWLEDGMENTS

This research was sponsored by the Korea Science andEngineering Foundation through the Research Center for Ad-vanced Magnetic Materials at Chungnam National Univer-sity, by the Korea Science and Engineering Foundation~R01-1977-00109!.

1E. S. Murdock, IEEE Trans. Magn.28, 3078~1992!.2R. Valenzuela,Magnetic Ceramic~Cambridge University Press, Cam-bridge, 1994!.

3F. E. Luborsky, J. Appl. Phys.32, 171S~1961!.4J. G. Lee, J. Y. Park, and C. S. Kim, J. Mater. Sci.33, 3965~1998!.5C. S. Kim, Y. S. Yi, K. T. Park, H. Namgung, and J. G. Lee, J. Appl. Phys.87, 6241~2000!.

6G. A. Sawatzky, F. Vander Woude, and A. H. Morrish, Phys. Rev.187, 747~1969!.

7B. D. Cullity, Elements of X-ray Diffraction~Addison-Wesley, Reading,MA, 1978!, p. 102.

8S. W. Lee, D. H. Kim, S. R. Hong, S. J. Kim, W. C. Kim, S. B. Kim, andC. S. Kim, IEEE Trans. Magn.35, 3418~1999!.

9M. Grigorova, H. J. Blythe, V. Blaskov, V. Rusanov, V. Petkov, V. Ma-sheva, D. Nihtianova, Ll. M. Martinez, J. S. Munoz, and M. Mikhov, J.Magn. Magn. Mater.183, 163 ~1998!.

10C. S. Kim, S. W. Lee, S. I. Park, J. Y. Park, and Y. J. Oh, J. Appl. Phys.79,5428 ~1996!.

11W. C. Kim, S. J. Kim, S. W. Lee, S. H. Ji, and C. S. Kim, IEEE Trans.Magn.35, 3399~2000!.

12K. V. P. M. Shafi, A. Gedanken, R. Prozorov, and J. Balogh, Chem. Mater.10, 3445~1998!.

FIG. 3. Natural logarithm of the absorption area, lnF vs T2 for theA andBsubspectra of CoFe1.9Gd0.1O4 fired at 923 K.

FIG. 4. Changes of saturation magnetizations (Ms) and coercivities (Hc) ofCoFe1.9Gd0.1O4 ~a! and CoFe1.9Nd0.1O4 ~b! spinel powders as a function offiring temperature.

7609J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Kim, Kim, and Kim

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

141.212.109.170 On: Tue, 25 Nov 2014 13:42:33