Competing ferromagnetic and anti-ferromagnetic interactions in iron nitride ζ-Fe2N

Highlights: • An extensive review of both experimental and computational reports on ζ-Fe₂N. • Presence of Competing ferromagnetic and antiferromagnetic interactions proposed. • Estimation of Curie temperature for ζ-Fe₂N using Heisenberg model. The paper discusses the magnetic state of zeta phase of iron nitride viz. ζ-Fe₂N on the basis of spin polarized first principles electronic structure calculations together with a review of already published data. Results of our first principles study suggest that the ground state of ζ-Fe₂N is ferromagnetic (FM) with a magnetic moment of $1.528{\mu }_{\text{B}}$ on the Fe site. The FM ground state is lower than the anti-ferromagnetic (AFM) state by 8.44 meV and non-magnetic (NM) state by 191 meV per formula unit. These results are important in view of reports which claim that ζ-Fe₂N undergoes an AFM transition below 10 K and others which do not observe any magnetic transition up to 4.2 K. We argue that the experimental results of AFM transition below 10 K are inconclusive and we propose the presence of competing FM and AFM superexchange interactions between Fe sites mediated by nitrogen atoms, which are consistent with Goodenough-Kanamori-Anderson rules. We find that the anti-ferromagnetically coupled Fe sites are outnumbered by ferromagnetically coupled Fe sites leading to a stable FM ground state. A Stoner analysis of the results also supports our claim of a FM ground state.