Prediction of half-metallic gap formation and Fermi level position in Co-based Heusler alloy epitaxial thin films through anisotropic magnetoresistance effect

We have investigated the temperature (T) dependence of the anisotropic magnetoresistance (AMR) effect of Co2FeGa0.5Ge0.5 (CFGG) epitaxial thin films having different compositions and atomic orders to examine the relation between AMR and the half-metallic electronic structure based on a developed theoretical model. The T dependence of the resistance change (Delta rho) of the AMR normalized at 10 K is minimal in the CFGG films having a standard composition and high atomic order. In contrast, the films having a large atomic disorder due to the Co-rich composition exhibit a large reduction of Delta rho with T. Our theoretical model of AMR well explains this behavior; namely, the half-metallic ferromagnets having the Fermi level (E-F) around the center of the half-metallic gap are predicted to show a small T dependence of Delta rho because of no/few localized d down arrow states at E-F. On the contrary, a large change of Delta rho with T is expected for the half-metallic materials having E-F close to gap edges or large in-gap states because of the contribution of thermally excited s-d scattering involving the d down arrow states. Moreover, we have also investigated the variation of Delta rho with T for the thin films of various half-metallic Co-based Heusler alloys and found the behavior that agrees with our theoretical prediction. The present study proves that the formation of a half-metallic gap and the position of E-F in a half-metallic material are readily predictable from the T dependence of Delta rho of the AMR effect, which can be a facile way for efficient screening of half-metallic materials.