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Current perpendicular-to-plane (CPP) giant magnetoresistance (GMR) effects in devices including Co2Fe0.4Mn0.6Si (CFMS)/Ag100−x Mg x /CFMS structures were investigated theoretically and experimentally. First-principles transport calculation revealed that the Fermi surface matching between CFMS and L12 Ag3Mg is better than that between CFMS and fcc-Ag. In the experiments the Mg composition, x was changed from 0 to 26 at.%, in which both face centered cubic phase and L12 phase of Ag–Mg alloys are included depending on the Mg composition. It was confirmed by a cross-sectional high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) image that the Ag–Mg spacer layer with L12 ordered phase was successfully fabricated for x = 22 at.%. The maximum CPP–GMR ratio and the change of the areal resistance () were 56% and 20 mm2, respectively, for x = 22 at.% at room temperature, which is much higher than that of the conventionally used pure Ag spacer devices. It was suggested from the HAADF-STEM images and the results of the temperature dependence of CPP–GMR effects that the diffusion of Mn atoms occurred less at the CFMS/Ag–Mg interfaces for the L12 ordered Ag–Mg spacer devices than the Ag spacer devices, which might be a key factor for the enhancement of the value. The newly developed L12 Ag–Mg spacer is a promising material for realizing large of the CPP–GMR devices.

The current perpendicular to film plane type giant magnetoresistance (CPP-GMR) effect in Co2Fe0.4Mn0.6Si/Ag83Mg17/Co2Fe0.4Mn0.6Si junctions was investigated. An epitaxially grown 5-nm-thick Ag83Mg17 film having partially ordered L12 structure was fabricated on the Co2Fe0.4Mn0.6Si layer by magnetron sputtering. CPP-GMR effects were observed in the submicrometer-sized junctions, and the maximum value of the observed (intrinsic) MR ratio was 40% (48%) at room temperature. The average change in the resistance–area product was 23 mΩ·µm2 for the Ag–Mg junctions, which was higher than those of conventional CPP-GMR junctions using a Ag spacer layer.