Ab initiotheoretical description of the dependence of magnetocrystalline anisotropy on both compositional order and lattice distortion in transition metal alloys

Recently, we outlined a scheme to investigate the effects of both short-ranged and long-ranged compositional order on the magnetocrystalline anisotropy of alloys from a first-principles electronic structure point of view [Phys. Rev. Lett. 82, 5369 (1999)] and showed that in the Co0.5Pt0.5 alloy compositional order enhances the magnitude of magnetocrystalline anisotropy energy (MAE) by some two orders of magnitude. Here we describe our scheme in detail and study some more transition metal alloys. In the Co0.25Pt0.75 alloy we find the perfect L1₂ structure to be magnetically soft whereas imposition of directional order greatly enhances its MAE. We also present the effect of lattice distortion (tetragonalization) on MAE on the same footing and find that in the Co0.5Pt0.5 alloy it accounts for only about 20% of the observed enhancement, thus confirming that compositional order is the major player in this effect. Tetragonalization of the lattice has also a modest effect on the MAE of the Fe0.5Co0.5 alloy. We also examine the electronic effects which underpin the directional chemical order that is produced by magnetic annealing of permalloy which we study within the same framework.