Raman and magnetization studies of barium ferrite powder prepared by water-in-oil microemulsion
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Raman and magnetization studies of barium ferrite powder prepared by water-in-oil microemulsion M.S. Chena) and Z.X. Shen Department of Physics, National University of Singapore, Lower Kent Ridge Road, 119260, Singapore
X.Y. Liu and J. Wang Department of Materials Science, National University of Singapore, Lower Kent Ridge Road, 119260, Singapore (Received 28 May 1999; accepted 1 December 1999)
Micro-Raman spectroscopy was used to study the formation of BaFe12O19 (BaM) powders derived from water-in-oil microemulsion at different calcination temperatures. With increase in the calcination temperature, the Raman spectra of the BaM powders become narrower and stronger without apparent frequency shifts of the Raman bands. The calcination temperature dependence of the Raman spectra and the magnetic properties of the BaM powders result from the crystallization rather than size effect. Our results show that there is a strong correlation between the crystallinity and the magnetic properties, which could be explained in terms of the crystallization effect on the superexchange interaction between ferric ions. The ␥–Fe2O3 phase occurred in the BaM precursor and the powder calcined at 500 °C. The ␣–Fe2O3 phase was developed in the powders calcined at 500, 600, and 700 °C, which was not detected by x-ray diffraction. With increasing calcination temperature, the ␥–Fe2O3 phase can either react with oxide containing barium to form the BaM phase or transform to the ␣–Fe2O3 phase. The amount of ␣–Fe2O3 decreases due to reaction with BaCO3 to form BaM phase at higher calcination temperature. I. INTRODUCTION
Hexagonal ferrite BaFe12O19 (BaM, following Braun notation1) has been traditionally used in permanent magnets because of its high intrinsic coercivity and fairly large crystal anisotropy.2 In the past decade, the growing industrial and academic interests have emerged in their potential application as high density magnetic recording media due to its recording characteristics as well as excellent chemical stability and corrosion resistivity.3 These technological applications require the materials with a strict control in homogeneity, particle size and shape, and magnetic properties.4 Much attention has thus been paid to the development of methods for the preparation of ultrafine BaM particles. Microemulsion synthesis can offer ultrafine particles and has been successfully used to prepare ultrafine BaM particles.5–9 X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are the widely used characterization tools for BaM powders and films. Recently, the Raman spectra of BaM single crystal were first reported10 and Raman spectroscopy a)
Address all correspondence to this author. e-mail address: [email protected] J. Mater. Res., Vol. 15, No. 2, Feb 2000
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was used to investigate BaM thin films and the substituted BaM single crystal BaFe12
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