A study on barium ferrite particles prepared by chemical coprecipitation
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Chemical coprecipitation was employed to prepare fine particles of barium ferrite with high coercivity (450 kA/m). Magnetic properties of the bonded barium ferrite magnet were measured at different temperatures. The results were found to be fairly close to the theoretical values based on the Stoner–Wohlfarth model. Mechanical milling was utilized to prepare ultrafine dispersed barium ferrite particles. NaF was introduced as a dispersing agent during milling and subsequent heat treatment. The dispersed particles were compacted and then subjected to die upsetting at room temperature. A weak anisotropy in the coercivity and remanence was found in the directions parallel and perpendicular to the compaction direction.
I. INTRODUCTION
It is well known that barium ferrite (BaFe12O19) is widely used as permanent magnets and magnetic recording media as it possesses good magnetic properties and good chemical stability. Various kinds of methods for synthesis of barium ferrite powder have been reported such as glass crystallization, hydrothermal precipitation,1,2 sol-gel,3 aerosol,4 chemical coprecipitation,5,6 and mechanical alloying.7–9 The conventional method for producing barium ferrite magnets is firing stoichiometric mixtures of hematite (Fe2O3) and barium carbonate (BaCO3) at a high temperature of approximately 1473 K and sintering after magnetic alignment. The coercivity of the barium ferrite produced is much less than that predicted by the Stoner–Wohlfarth theory.10,11 This is due to the high sintering temperature used which causes crystal growth, and as a result, the coercivity falls well below the theoretical value. Particle size smaller than the single-domain particle size (≈1 m) is required for high-coercivity barium ferrite.12 In order to reduce the crystal size, grinding is necessary but the lattice strains induced by the grinding process will affect unfavorably the magnetic properties.6 In this work, chemical coprecipitation has been employed to produce barium ferrite as this method is relatively simple and the intimate mixture of the precursors formed during the coprecipitation can avoid high firing temperature and hence postfiring grinding is unnecessary. This study can be divided into four main parts. The first part is the study on the variation of roomtemperature coercivity of barium ferrite precursor with respect to the heat treatment temperature and the microstructure studies of the specimens. The next will be the study on the coercivity and remanent magnetization of a J. Mater. Res., Vol. 15, No. 10, Oct 2000
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bonded sample at various temperatures, from room temperature down to the liquid nitrogen temperature. The third part is the comparison between the ultrafine dispersed and the agglomerated barium ferrite particles. It has been reported that ultrafine high-coercivity barium ferrite could be produced by the glass crystallization method13 or microemulsion mediated process.14 In this work, chemical coprecipitation followed by mechanical milling
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