Effects of the atomic order on the half-metallic electronic structure in the Co2Fe(Ga0.5Ge0.5) Heusler alloy thin film

Abstract

The atomic order and its effect on the electronic structure is a key issue in the application of half-metallic Heusler alloys as spin-polarized electron sources. In this study, we investigated the atomic ordering and electronic structures of $\mathrm{C}{\mathrm{o}}_{2}\mathrm{Fe}(\mathrm{G}{\mathrm{e}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5})$ (CFGG) Heusler alloy thin films at various annealing temperatures using anomalous x-ray diffraction (AXRD) and hard x-ray photoelectron spectroscopy (HAXPES). AXRD measurements clearly showed that the Co-Fe disorder is large in the as-deposited state, which is reduced to almost zero by annealing at ${T}_{\mathrm{an}}=\phantom{\rule{0.16em}{0ex}}500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ for 30 min. The reduction of the Co-Fe disorder explains the observed increase in the spin polarization of currents estimated by ordinary and anisotropic magnetoresistance measurements. The photoelectron spectra of the valence band in CFGG thin films with different ${T}_{\mathrm{an}}$ agree well with the first-principles calculations that considers the atomic disorder. We also found that the characteristic peaks appearing in the valence band HAXPES spectra shifted to higher binding energies, compared to the calculated density of states for stoichiometric $\mathrm{L}{2}_{1}$-ordered CFGG. This indicates that the Fermi level is shifted and located in the vicinity of the conduction band edge in the present CFGG thin film due to electron doping by a mildly Co-rich chemical composition. Our first-principles calculations of partial density of state of $sp$ electron predicted that the high spin-polarization is obtainable in a wider energy region near Fermi level in the $\mathrm{L}{2}_{1}$-ordered structure than the B2 structure, which explains the enhancement of spin-polarization by promoting $\mathrm{L}{2}_{1}$-ordering in the present CFGG film.