Magnetic anisotropy energy of disordered tetragonal Fe-Co systems from ab initio alloy theory

(Dated: November 26, 2012)We present results of systematic fully relativistic first-principles calculations of the uniaxial mag-netic anisotropy energy (MAE) of a disordered and partially ordered tetragonal Fe-Co alloy usingthe coherent potential approximation (CPA). This alloy has recently become a promising system forthin ferromagnetic films with a perpendicular magnetic anisotropy. We find that existing theoreticalapproaches to homogeneous random bulk Fe-Co alloys, based on a simple virtual crystal approxima-tion (VCA), overestimate the maximum MAE values obtained in the CPA by a factor of four. Thispronounced 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 Fermienergy and to the reduction of the MAE. The ordered Fe-Co alloys with a maximum L1