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We theoretically analyze the anisotropic magnetoresistance (AMR) effects of bcc Fe (þ), fcc Co (þ), fcc Ni (þ), Fe 4 N (À), and a half-metallic ferromagnet (À). The sign in each parenthesis represents the sign of the AMR ratio observed experimentally. We here use the two-current model for a system consisting of a spin-polarized conduction state and localized d states with spin-orbit interaction. From the model, we first derive a general expression of the AMR ratio. The expression consists of a resistivity of the conduction state of the spin ( ¼ " or #), s , and resistivities due to s-d scattering processes from the conduction state to the localized d states. On the basis of this expression, we next find a relation between the sign of the AMR ratio and the s-d scattering process. In addition, we obtain expressions of the AMR ratios appropriate to the respective materials. Using the expressions, we evaluate their AMR ratios, where the expressions take into account the values of s# = s" of the respective materials. The evaluated AMR ratios correspond well to the experimental results.

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The negative anisotropic magnetoresistance (AMR) effect is observed in pseudo-single-crystal γ'-Fe4N films from 4 to 300 K. Below 50 K, the changes in the AMR ratio depend on the crystal direction along which the sensing current flows. A large stepwise change of the AMR ratio is observed along the [100] direction. The anomalous cos (4θ) component appears on the AMR curves below 30 K. A first-principles calculation of γ'-Fe4N indicates that the electron occupation of the 3d orbitals is modified as the magnetic moment direction changes with respect to the crystal axes. The anomalous behavior of the AMR effect might be due to the change of the partial density of states of 3d-orbitals at the Fermi level.