Against Other Iron Nitrides*

α″–Fe₁₆N₂ has been investigated as one of promising candidates for environment friendly magnets.[1] While giant saturation magnetization has previously been experimentally observed in α″–Fe₁₆N₂, its magnetic anisotropy and structural stability leave room for improvement. Recent theoretical studies have considered alloying Fe₁₆N₂with various elements to improve the magnetic properties and/or stability against decomposition.[2] However, estimates of stability in particular are typically restricted to simple ground-state-energy comparisons, i.e. effectively taken near 0 K.

For a more practical measure of stability, we therefore extend ground-state energies, obtained with the plane-wave density-functional theory (DFT) code Quantum ESPRESSO, with appropriate empirical and/or statistical corrections to obtain free energies at arbitrary temperature. We then compare the stability of Fe₁₆N₂ against the neighboring phases in the Fe-N binary system, to estimate the range of temperatures at which it is stable. We compare against experimental observations of the Fe–N phase diagram.[3]

[1] J.-P. Wang, J. Magn. and Magn. Mat. 497, 165962 (2020).

[2] P. Stoeckl, P. W. Swatek, & J.-P. Wang, AIP Advances 11(1), 015039 (2021).

[3] Cf. e.g. M. Hillert & M. Jarl, Metall. Transac. A 6A 553–9 (1975).