Structure and magnetic properties of the oxide layers formed on iron nanoparticles

α-Fe (bcc) and γ-Fe (fcc) nanoparticles have been prepared by laser-induced pyrolysis. The structures, morphologies and magnetic properties of the oxide layers covering the iron nanoparticles have been investigated using transmission electron microscopy, magnetic measurement, X-ray diffraction and annealing treatment. It is shown that the iron oxide layers consist of very fine crystallites of 2 - 5 nm in diameter, and the layers are non-ferromagnetic or superparamagnetic. The iron oxide layer formed on iron nanoparticles at room temperature is Fe₃O₄, rather than a mixture of Fe₃O₄ and γ-Fe₂O₃.