Magnetic properties of nanocrystalline γ-Fe-Ni-N nitride systems

Nanocrystalline gamma-Fe0.75Ni0.25-N, gamma-Fe0.5Ni0.5-N, and gamma-Fe0.25Ni0.75-N nitrides have been synthesized by borohydride reduction followed by nitridation at 773 K using NH3(g). gamma-Fe0.75Ni0.25-N, gamma-Fe0.5Ni0.5-N and gamma-Fe0.25Ni0.75-N nitrides crystallize in fcc cubic structures having unit cell parameters: a = 3.588, 3.556 and 3.534 Angstrom, respectively. X-ray diffraction and electron microscopic studies reveal the ultrafine nature of the particles. Chemisorption of atmospheric oxygen results in the formation of nonmagnetic oxide and/or a complex oxynitride layer at the surface of the ultrafine particles. The reduction of room temperature saturation magnetization is attributed to fine particle size effects and the formation of nonmagnetic oxide and/or an oxynitride layer at the surface of the ultrafine particles. The change in Curie temperature T-c in gamma-Fe-Ni-N nitride systems compared to their corresponding bulk Fe-Ni alloys is related to the presence of concentration inhomogeneities in the nitride materials. Mossbauer study of gamma-Fe0.75Ni0.25-N, gamma-Fe0.5Ni0.5-N and gamma-Fe0.25Ni0.75-N nitride particles exhibit the coexistence of ferromagnetic and superparamagnetic fractions and corroborate the observed magnetic properties. (C) 1999 American Institute of Physics. [S0021-8979(99)08217-1].