Ultrafine alpha-Fe (b.c.c) and gamma-Fe (f.c.c) particles have been prepared by laser-induced pyrolysis of Fe(CO)(5). The gamma-Fe particles, because of their paramagnetic nature, are very convenient for investigation on the attributes of iron oxide layers formed on the particle surfaces. Structures, morphologies and magnetic properties of the oxide layers covering the iron ultrafine particles have been studied using transmission electron microscopy observation, magnetic property measurement, X-ray diffraction and annealing treatment. Convincing evidences established that the iron oxide layers are not continuous and consist of very fine crystallites, and that these layers are non-ferromagnetic and have no contribution to the saturation magnetization of the iron particles. The iron oxide layers formed at room temperature was determined to be Fe3O4. Additionally, a brief annealing of the iron particles in air were performed to examine magnetic properties of the formed iron oxide layers and ultrafine oxide particles.
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