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Effects of alloying and strain on the magnetic properties of Fe$_{16}$N$_2$

The electronic structure and magnetic properties of pure and doped {Fe$_{16}$N$_2$} systems have been studied in the local-density (LDA) and quasiparticle self-consistent {\emph{GW}} approximations. The {\emph{GW}} magnetic moment of pure {Fe$_{16}$N$_2$} is somewhat larger compared to LDA but not anomalously large. The effects of doping on magnetic moment and exchange coupling were analyzed using the coherent potential approximation. The theoretical Curie temperature in pure {Fe$_{16}$N$_2$} is significantly higher than the measured value, which is attributed to the quality of available samples and the interpretation of experimental results. We found that different Fe sites contribute very differently to the magnetocrystalline anisotropy energy (MAE), which offers a way to increase MAE by small additions of Co or Ti. MAE also increases under tetragonal strain.