Anisotropy of Magnetic Moments and Energy in Tetragonal Fe-Co Alloys from First Principles

Magnetic anisotropies of random body-centered tetragonal Fe-Co alloys are studied by means of relativistic ab initio electronic structure calculations. Special attention is paid to the values and trends of magnetic moments and magnetic anisotropy energies as well as to their sensitivity to current approximations employed. It is found that the high uniaxial anisotropy energy K-u, predicted for alloys with 60 % of Co by several authors, is reduced significantly in the coherent potential approximation describing the chemical disorder in real alloys. On the other hand, the four-component Dirac formalism leads to enhanced values of the K-u as compared to a simple perturbative treatment of the spin-orbit interaction. Similar behavior is found for the orbital magnetic moments, the values of which are systematically smaller than the measured ones irrespective of the theoretical approach used.