Synthesis and Properties of Magnetic Fine Particles

Magnetic fine particles with a variety of compositions and sizes have been prepared by aerosol and coprecipitation technique. Their magnetic properties are shown to be either similar to or quite different from those of their bulk counterparts. Iron oxide, barium iron oxide and neodynium iron based fine particles have been synthesized by an aerosol technique which produced particles with average size of about 100 nm. It was found that the as-received samples were usually in thermally unstable states because of their short residence time (seconds) at the high temperature when they were created. Heat treatment turned these as -received particles into more stable phases. The final product depended on temperature, environment and composition of the initial solution. Cation and anion effects and solvent effects on the formation and morphology of the final particle have been also observed. Manganese ferrite fine particles have been made by hydroxide induced coprecipitation of mixed iron and manganese salt solutions. The particle size appeared to be an unique function of the ratio of the metallic ion concentration to the hydroxide ion concentration when the digestion conditions were fixed. The system which used ferric salt created small MnFe_2 O_4 particles with size controllable between 5 and 25 nm. The digestion process could be described by an Ostwald ripening in which OH^- acts as a catalyst. Variation of cations showed that their oxidation states had a strong influence on the particle size. The system which used ferrous salt, however, produced larger ferrite particles (above 50 nm) with single ferrite phase (Mn_ xFe_{3-x}O_4 with x <= 0.7). Dissolution and renucleation/growth occurred during digestion. For larger particles, the magnetizations were the same as for the bulk of the phases present, while the coercivities were more system dependent. We have found that nanoscale particles, on the other hand, showed profound size effects. The saturation magnetization of ultrafine MnFe_2 O_4 particles was substantially lower than that of the bulk and decreased with declining particle size due to a surface effect which came from a 7 A thick nonmagnetic surface layer. Careful characterization showed that the particle size was the only independent variable. The Curie temperature in MnFe _2 O_4 was seen to increase with decreasing particle size. The behavior was in good agreement with finite-size scaling. This size dependent Curie temperature caused difficulty in determining critical exponents due to size distribution of fine particle systems. This was the first time that studies of critical phenomena in fine particles have been even attempted.