Synthesis of high moment and high coercivity iron nitride particles

Fe2N particles were prepared by nitriding acicular γ-Fe2O3 particles (precoated with sodium silicate to prevent the particles from sintering) at about 450 °C for 3–6 h under a mixed stream of H2-NH3 (1:4 molar ratio). X-ray diffraction showed mostly Fe4N and small amounts of α-Fe and Fe2O3, but no Fe8N(α″-Fe16N2). The saturation magnetic moment density σS increased to 140–160 emu/g and the coercivity Hc increased to about 1000 Oe. Although the ultimate goal of preparing Fe8N particles was not attained, an impressive combination of magnetization and coercivity was achieved. To obtain Fe8N by chemical doping, the nitrogen content of Fe4N was decreased by coating with metal hydroxides of Ti, Nd, and Sm, and reducing under H2 at 300 °C: the magnetization dropped precipitously, and no Fe8N was formed.