Structure, Thermal Stability, and Magnetism of Ni4N Thin Films

Abstract

Herein, the synthesis, structure, thermal stability, and magnetic properties of Ni4N thin films are studied. Ni4N is difficult to synthesize in the correct chemical order and stoichiometry due to unfavorable thermodynamics. During the synthesis of Ni–N thin films, it is found that the substrate temperature (Ts) is a critical parameter affecting the growth of a Ni4N phase. The Ni4N phase transforms into an amalgamation of [Ni+Ni3N] phases even if the Ts rises just above 300 K. These results elucidate a correlation between atomic diffusion and Ts. N self‐diffusion measurements carried out using secondary‐ion mass spectroscopy indicate that substantial N self‐diffusion is occurring at low Ts. A comparison of N self‐diffusion coefficients in Fe–N, Co–N, and Ni–N indicates that N self‐diffusion in Ni–N lies between that of Fe–N and Co–N and exhibits a greater correlation with the enthalpy of formation. A transformation from the cubic–to‐tetragonal deformation in the Ni4N crystal lattice as a function of increasing N concentration is evident from the X‐ray diffraction and X‐ray absorption near‐edge spectroscopy measurements. Magnetization measurements confirm a nonferromagnetic state of Ni4N at 300 K, which transforms into a ferromagnetic state at 15 K.