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

epsilon-Iron nitrides with the general formula epsilon-Fe3N1+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 epsilon-Fe3N1+x (x < 0) compounds that are stable below their synthetic temperatures. Here, epsilon-Fe3N1+x (-0.12 <= x <= -0.01) nanoparticles with excellent thermal stability and magnetic properties were synthesized by a simple chemical solution method. The epsilon-Fe3N1+x nanoparticles with space group P6(3)22 have excellent oxidation resistance due to a carbon shell with a thickness of 2-3 nm. NPD refinements suggest that the epsilon-Fe3N1+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 epsilon-Fe3N1+x are ascribed to chemically engineering the stoichiometry and N occupancy from the disordered to the ordered site.