Enhanced magnetic properties and thermal stability of highly ordered ε-Fe3N1+x (−0.12 ≤ x ≤ −0.01) nanoparticles

Abstract

ε-Iron nitrides with the general formula ε-Fe₃N_1+x (−0.40 < x < 0.48) have been widely studied due to their interesting magnetism. However, the phase diagram of the Fe–N binary system indicates the absence of monophasic ε-Fe₃N_1+x (x < 0) compounds that are stable below their synthetic temperatures. Here, ε-Fe₃N_1+x (−0.12 ≤ x ≤ −0.01) nanoparticles with excellent thermal stability and magnetic properties were synthesized by a simple chemical solution method. The ε-Fe₃N_1+x nanoparticles with space group P6₃22 have excellent oxidation resistance due to a carbon shell with a thickness of 2–3 nm. NPD refinements suggest that the ε-Fe₃N_1+x nanoparticles possess a highly ordered arrangement of N atoms and their magnetic moments align parallel to the c axis. The Curie temperature (T_C) and room temperature saturation magnetization (M_S) increase with decreasing N content, which results in record-high T_C (632 K) and M_S (169.2 emu g⁻¹) at x = −0.12, much higher than the magnetic properties of the corresponding bulk materials. The significant enhancements in the intrinsic magnetic properties and thermal stability of ε-Fe₃N_1+x are ascribed to chemically engineering the stoichiometry and N occupancy from the disordered to the ordered site.