Large magnetic anisotropy predicted for rare-earth-free Fe16-xCoxN2 alloys.

Structures and magnetic properties of Fe16−xCoxN2 are studied using adaptive genetic algorithm and first-principles calculations. We show that substituting Fe with Co in Fe16N2 with a Co/Fe ratio ≤1 can greatly improve the magnetic anisotropy of the material. The magnetocrystalline anisotropy energy from first-principles calculations reaches 3.18MJ/m³ (245.6 μeV per metal atom) for Fe12Co4N2, much larger than that of Fe16N2, and is one of the largest among the reported rare-earth-free magnets. From our systematic crystal structure searches, we show that there is a structure transition from tetragonal Fe16N2 to cubic Co16N2 in Fe16−xCoxN2 as the Co concentration increases, which can be well explained by electron counting analysis. Different magnetic properties between the Fe-rich (x ≤ 8) and Co-rich (x>8) Fe16−xCoxN2 is closely related to the structural transition. [ABSTRACT FROM AUTHOR]