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 firstprinciples 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 3.18 MJ/m(3) (245.6 mu eV permetal 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. Differentmagnetic properties between the Fe-rich (x <= 8) and Co-rich (x > 8) Fe16-xCoxN2 is closely related to the structural transition.