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

TL;DR

This paper uses relativistic first-principles calculations to study the magnetic anisotropy energy of disordered tetragonal Fe-Co alloys, revealing the importance of disorder effects neglected in simpler models.

Contribution

It provides a detailed CPA-based analysis of MAE in disordered Fe-Co alloys, highlighting the limitations of the virtual crystal approximation.

Findings

CPA predicts MAE values four times lower than VCA.

Disorder in the minority spin channel broadens eigenstates and reduces MAE.

Perfect L1_0 order yields high MAE, but small disorder significantly decreases it.

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 four. 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.