Theoretical study of thermal stability of α″–Fe16N2 against other iron nitrides

Abstract

α″–Fe16N2 has been investigated as one of promising candidates for environment-friendly magnets. 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 Fe16N2 with various elements to improve the magnetic properties and/or stability against decomposition. However, estimates of stability in particular are typically restricted to simple ground-state-energy comparisons, i.e. effectively taken at 0 K. For a more practical measure of stability, we therefore extend ground-state energies, obtained with the plane-wave density-functional theory 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.