Temperature-dependence of current-perpendicular-to-the-plane giant magnetoresistance spin-valves using Co₂(Mn_1−xFe_x)Ge Heusler alloys

The properties of Co₂(Mn_1−xFe_x)Ge (CMFG) (x = 0–0.4) Heusler alloy magnetic layers within polycrystalline current-perpendicular-to-the plane giant magnetoresistance (CPP-GMR) spin-valves are investigated. CMFG films annealed at 220–320 °C exhibit partly ordered B2 structure with an order parameter S_B2 = 0.3–0.4, and a lower S_B2 was found for a higher Fe content. Nevertheless, CPP-GMR spin-valve devices exhibit a relatively high magnetoresistance ratio of ∼13% and a magnetoresistance-area product (ΔRA) of ∼6 mΩ μm² at room temperature, which is almost independent of the Fe content in the CMFG films. By contrast, at low temperatures, ΔRA clearly increases with higher Fe content, despite the lower B2 ordering for increasing the Fe content. Indeed, first-principles calculations reveal that the CMFG alloy with a partially disordered B2 structure has a greater density of d-state at the Fermi level in the minority band compared to the Fe-free (Co₂MnGe) alloy. This could explain the larger ΔRA measured on CMFG at low temperatures by assuming that s-d scattering mainly determines the spin asymmetry of resistivity as described in Mott's theory.