Structure, Thermal Stability, and Magnetism of Ni₄N Thin Films

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 (T_s) 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 T_s rises just above 300 K. These results elucidate a correlation between atomic diffusion and T_s. N self‐diffusion measurements carried out using secondary‐ion mass spectroscopy indicate that substantial N self‐diffusion is occurring at low T_s. 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.