Electronic band matching at the interface between ferromagnetic (FM) and nonmagnetic (NM) metals has been considered a key factor that affects the spin-dependent transport properties such as giant magnetoresistance (GMR) effect. However, to date, there has not been an experimental explanation on the effect of a few monolayer atomic structures at the FM/NM interface on the band matching with a direct observation of the atomic- and element-resolved interfacial microstructure. In this study, we fabricated fully epitaxial current-perpendicular-to-plane GMR pseudo-spin-valve (PSV) films of half-metallic ${\mathrm{Co}}_2\mathrm{Fe}{\mathrm{Ga}}_{0.5}{\mathrm{Ge}}_{0.5}\left(\mathrm{CFGG}\right)/$ Ag spacer/CFGG structure with very thin (0 to 1 nm thick) Ni insertion layers at the CFGG/Ag interfaces. The MR ratio was significantly enhanced (from 23.1% for the PSV without Ni to 32.5% for that) with 0.21 nm-thick Ni insertion. Through an aberration-corrected scanning transmission electron microscopy (STEM), the state-of-the-art atomic-scale microstructure analysis revealed that the Co atoms in a second termination layer from the Ag interface are replaced with Ni monolayer via insertion of 0.21-nm-thick Ni. Our first-principles calculations of ballistic transmittance for the stacking structures modeled by the STEM images indicated that substituting the Co termination layer with Ni improved electronic band matching of majority spin electrons. This study proves that even a monolayer near the interface critically affects the interfacial band matching and MR properties.

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• Magnetoresistance
• Microstructure
• Spin accumulation
• Spin diffusion
• Spin polarization
• Spintronics
• Heusler alloy