Oxidation behavior and magnetic properties of metallic ultrafine particles

Iron (including bec and fee structures), iron nitride, iron carbide, and iron carbonitride ultrafine particles (< 100 nm in size) have been prepared by laser-induced pyrolysis of iron carbonyl, ammonia and ethylene mixtures. Transmission electron microscopy, X-ray diffraction, oxygen, nitrogen and carbon determiners, and a vibrating sample magnetometer 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 content, 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 (γ-Fe) ultrafine particles, the morphologies and magnetic properties of the iron oxide layers formed on the particle surfaces were determined. Convincing evidence was obtained indicating that the iron oxide layers formed on the metallic particles are composed of very fine crystallites and provide no contribution to the saturation magnetization of the particles. The structure of the iron oxide layers on the iron particle surfaces is also discussed and we conclude that the oxide layer formed at room temperature is a single Fe₃O₄ layer, rather than a mixture of Fe₃O₄ and γ-Fe₂O₃.