Anisotropic magnetoresistance effect of a strong ferromagnet: Magnetization direction dependence in a model with crystal field

We theoretically study magnetization direction dependence of the anisotropic magnetoresistance (AMR) effect of a strong ferromagnet. We here use the two-current model which takes into account the s -d scattering, where s (d) is the conduction state (the localized d states). The d states have the spin-orbit interaction, exchange field, and crystal field of tetragonal symmetry. From the model, we derive analytic expressions of twofold and fourfold symmetric terms of the AMR ratio. The dominant component in the twofold symmetric term is proportional to the difference of partial density-of-states (DOS) at the Fermi energy (E_F) between dγ and dε orbitals. The fourfold symmetric term is proportional to the difference of partial DOS at E_F among dε orbitals. In addition, using the expressions, we analyze an experimental result of the AMR ratio of Fe₄N.