Abstract
We present results of systematic fully relativistic first-principles calculations of the uniaxial magnetic anisotropy energy (MAE) of a disordered and partially ordered tetragonal Fe-Co alloy using the coherent potential approximation (CPA). This alloy has recently become a promising system for thin ferromagnetic films with a perpendicular magnetic anisotropy.
We find that existing theoretical approaches to homogeneous random bulk Fe-Co alloys, based on a simple virtual crystal approximation (VCA), overestimate the maximum MAE values obtained in the CPA by a factor of 4. This pronounced difference is ascribed to the strong disorder in the minority spin channel of real alloys, which is neglected in the VCA and which leads to a broadening of the d-like eigenstates at the Fermi energy and to the reduction of the MAE.
The ordered Fe-Co alloys with a maximum L1(0)-like atomic long-range order can exhibit high values of the MAE, which, however, get dramatically reduced by small perturbations of the perfect order.