Long-range chemical order effects upon the magnetic anisotropy of FePt alloys from an ab initio electronic structure theory

The magnetocrystalline anisotropy (MCA) of bulk and thick films of FePt is calculated from a 'first-principles' theory. The starting point is a description from electronic density functional theory for systems of interacting electrons moving in lattices of ions. Relativistic effects such as spin-orbit coupling are included. FePt readily transforms into a CuAu-type (L1(0)) ordered phase and this coincides with the material's high anisotropy. Here we describe how to calculate the MCA of a partially ordered alloy and to extract its dependence on the long range chemical order parameter eta. We present calculations of the MCA of FePt as a function of eta and find excellent agreement with the experimental data of Okamoto et al (2002 Phys. Rev. B 66 024413) and others with respect to the magnetic easy axis, the magnitude of the MCA and its trend with decreasing eta. We also study the paramagnetic phase of the ordered alloy using the 'disordered local moment' picture of metallic magnetism at finite temperatures. We calculate a Curie temperature of 935 K in reasonable agreement with experiment (710 K) and find the easy axis for the onset of ferromagnetic order to coincide with the magnetic easy axis found at low temperatures