OXIDATION BEHAVIOR AND MAGNETIC-PROPERTIES OF METALLIC ULTRAFINE PARTICLES

Iron (including bcc and fee structures), iron nitride, iron carbide, a nd iron carbonitride ultrafine particles (< 100 nm in size) have been prepared by laser-induced pyrolysis of iron carbonyl, ammonia and ethy lene mixtures. Transmission electron microscopy, X-ray diffraction, ox ygen, nitrogen and carbon determiners, and a vibrating sample magnetom eter were employed to characterize the metallic particles and measure their magnetic properties. It is found that the presence of carbon in the metallic particles not only significantly affects the oxygen conte nt, but also affects the state of the oxygen at the particle surface, and thereby affects the magnetic properties of the ultrafine particles . In addition, taking advantage of the paramagnetism of face-centered- cubic structured iron (gamma-Fe) ultrafine particles, the morphologies and magnetic properties of the iron oxide layers formed on the partic le surfaces were determined. Convincing evidence was obtained indicati ng that the iron oxide layers formed on the metallic particles are com posed of very fine crystallites and provide no contribution to the sat uration magnetization of the particles. The structure of the iron oxid e layers on the iron particle surfaces is also discussed and we conclu de that the oxide layer formed at room temperature is a single Fe3O4 l ayer, rather than a mixture of Fe3O4 and gamma-Fe2O3.