Origin of the difference in spin-dependent transport properties between half-metallic Co2FeGa0.5Ge0.5 and Co2Mn0.5Fe0.5Si with the Ag spacer

Current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) devices are promising for next-generation magnetic read heads in hard disk drives due to their low resistance area product compared to tunnel magnetoresistive devices. While Co2FeGa0.5Ge0.5 (CFGG) and Co2Mn0.5Fe0.5Si (CMFS) are representative half-metallic Co-based Heusler alloys with experimentally observed high magnetoresistance (MR) in CPP-GMR with Ag spacer, there is no understanding of the origin of the different band matching with the Ag spacer for evaluation of ferromagnetic electrodes. In this study, first-principles calculations of the ballistic conductance in (001)-CFGG/Ag/CFGG and CMFS/Ag/CMFS were performed to clarify an origin of the difference the spin-dependent conductance. We found that the majority-spin conductance of the CFGG/Ag(001) at maximum was about 45% higher than that of the CMFS/Ag(001) over all interfacial terminations, indicating the advantageous potential of CFGG for achieving a larger MR ratio in CPP-GMR. The Fermi surface analysis revealed that a hybridization between the d states of the CFGG electrode and the p states of the Ag spacer around the Brillouin zone edge plays an important role in the Fermi surface matching at the interface. In addition, calculated exchange stiffness constant revealed that CFGG/Ag(001) interfaces exhibit higher stability to thermal fluctuations of the spin-moment than CMFS/Ag(001) interfaces, indicating suppression of the decrease of MR ratios at finite temperatures. All these findings provide important insights for the selection of materials high-performance spintronic devices.