Sub-micron sized ε-FexN (2 ≤q x ≤q 3) particles were synthesized by heating 180nm Fe3O4 particles in an atmosphere of NH3, both with and without H2, and their structural and magnetic properties were studied. The nitride samples (first step nitridation) prepared by NH3 and annealed at 623 K (ε-Fe2N) had an expanded unit cell and were paramagnetic at room temperature. The purpose of H2 annealing, which was successively applied on the first step nitride sample, was to dissolve excess nitrogen atoms and reduce the lattice constant. The nitrogen concentrations were estimated from X-ray diffraction patterns. Saturation magnetization was found to increase with the H2 annealing time. The nitrogen diffusion constant in the submicron hexagonal close-packed particles was found to be 6.33 x 10-20m2/s, which was about three orders of magnitude smaller than that of the bulk hexagonal close-packed iron nitrides. This might result from the coating materials, such as SiO2 and others, covering the inner core nitrides, where nitrogen atoms passed through during diffusion.

Synthesis of ε-FexN (2 ≤q x ≤q 3) Submicron Particles and the Diffusion Mechanism of Nitrogen Atoms. M.Minagawa, H.Yanagihara, M.Kishimoto, E.Kita: Materials Transactions, 2010, 51[12], 2173-6