Electronic structure and magnetic ordering of NiN and Ni$_2$N from first principles

The results of first-principles electronic structure calculations for the nitrogen-rich nickel nitrides ${\rm NiN}$ and ${\rm Ni_2N}$ are presented. The calculations are based on density functional theory and used the generalized gradient approximation (GGA) as well as the GGA$+U$ approach. The latter turned out to be crucial for a correct description of the crystal phase stability and magnetic instabilities of both compounds. While for ${\rm NiN}$ GGA calculations predict a non-magnetic ground state with the zincblende structure, GGA$+U$ calculations result in a half-metallic ferromagnet with the rocksalt structure in line with indications from the neighboring transition-metal nitrides making ${\rm NiN}$ a possible candidate for spin-filter devices. For ${\rm Ni_2N}$ GGA calculations likewise lead to a non-magnetic behavior, which is contrasted with a ferrimagnetic ordering obtained from the GGA$+U$ approach. This ground state results from complex three-dimensional exchange interaction via $\sigma$-type and $\pi$-type overlap of the Ni $3d$ orbitals with the N $2p$ orbitals and may explain the reported sensitivity of the magnetic ordering to details of the crystal structure. For both nitrides, experimental data are called for to confirm our predictions.