正方晶Fe-Co合金の磁気異方性に関する理論的検証

概要

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We studied magnetic anisotropy in tetragonal distorted Fe-Co alloys, which are promising as novel hard magnetic materials that are free from of rare-elements, by using first-principles electronic structure calculations. Coherent potential approximation (CPA) and virtual crystal approximation (VCA) were employed to calculate the electronic structure in disordered Fe-Co alloys; CPA and VCA are the most reasonable and simplest approximations to deal with the random distribution of Fe and Co atoms. In comparing the uniaxial anisotropy constant calculated with these approximations, the values obtained from CPA were smaller than those from VCA even though the same compositions were considered. We analyzed Bloch spectral functions that reflected the electronic structure calculated with CPA and VCA to verify this inconsistency. Broadening of the spectral function due to electron scattering in disordered alloys drastically influences its magnetocrystalline anisotropy in Fe0.5Co0.5 with an axial ratio of c/a = 1.25 where a giant magnetocrystalline anisotropy is expected. We also discuss the effect of the finite lifetime of electrons due to scattering via the analysis of energy variations within second order perturbation theory in terms of spin-orbit interactions.