PHYSICAL REVIEW B 103, L140405 (2021) Letter Editors’ Suggestion Elucidation of the strong effect of an interfacial monolayer on magnetoresistance in giant magnetoresistive devices with current perpendicular to the plane Björn Büker, 1, 2 JinWon Jung , 1 Taisuke Sasaki, 1 Yuya Sakuraba , 1 , * Yoshio Miura , 1 Tomoya Nakatani, 1 Andreas Hütten , 2 and Kazuhiro Hono 1 1 Research Center for Magnetic and Spintronic Materials, NIMS, 1-2-1 Sengen, Tsukuba, 305-0047 Ibaraki, Japan 2 Department of Physics, Bielefeld University, Universitätsstraße 25, 33615 Bielefeld, Germany (Received 28 January 2021; revised 1 March 2021; accepted 17 March 2021; published 12 April 2021) 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 magnetore- sistance (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 Co 2 FeGa 0.5 Ge 0.5 (CFGG)/ 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. DOI: 10.1103/PhysRevB.103.L140405 I. INTRODUCTION The current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) effect, which was originally observed in Ag/Co multilayers by Pratt et al. in 1991 [1], is one of the most thoroughly investigated phenomena in spintronics. Currently, owing to its promising applications in read heads [2–6] and spin-torque oscillators [7–10] for next-generation ultrahigh density hard disk drives and magnetic sensors [11], there is significant interest in CPP-GMR. Compared to the current-in- plane GMR effect, CPP-GMR has attracted much more inter- est in terms of fundamental spin-dependent transport [12,13]. This is because it is possible to interpret the spin-dependent transport more accurately. Valet and Fert have proposed a Boltzmann equation-based macroscopic expression for the CPP-GMR, considering the spin-diffusion length [14]. The Valet-Fert model treats spin-dependent scatterings using bulk and interfacial spin asymmetry coefficients, β and γ . This model predicts that higher magnetoresistance (MR) ratio is obtained by selecting a ferromagnetic (FM) material with a larger β and a suitable nonmagnetic (NM) spacer for larger γ . Recent studies have demonstrated large MR ratios of over 30% at room temperature (RT) in the CPP-GMR devices using half-metallic Co-based Heusler alloy layers, such as Co 2 FeGa 0.5 Ge 0.5 (CFGG) and an Ag spacer [15–19]. These * Corresponding author: SAKURABA.Yuya@nims.go.jp large MR values are because of the large β and γ values originating from the half-metallicity of the Heusler alloys and good electronic band matching at the Heusler/Ag interfaces [17–19]. Recently, Jung et al. demonstrated a significant en- hancement of the MR ratio to approximately 80% at RT in CFGG/Ag/CFGG structures with very thin 0.21-nm-thick (1.5 monolayers) NiAl insertion layers at the CFGG/Ag interfaces [20]. The reason for the enhancement of the MR ratio by NiAl insertion is unclear because the transmission electron microscopy (TEM) images taken for these samples do not give an element-resolved termination structure at the interface. However, it is presumed to be because of the enhancement of γ caused by the improvement of interfacial band matching by the presence of a NiAl layer at/near the interface. The concept of the interfacial band matching in CPP-GMR has not been mentioned in the initial studies [1,14], where only spin-dependent scattering by interfacial defects and roughness has been considered. Schep et al. have calculated spin-dependent ballistic conductance in Co/Cu multilayers based on the local-spin-density approximation and predicted the importance of interfacial intrinsic band matching [21]. Subsequently, theoretical and experimental studies have con- firmed that realistic electronic band matching is a crucial factor for CPP transport that determines the magnitude and sign of γ [22–25]. However, these studies did not investi- gate the effect of the atomic-level interfacial microstructure on the electronic band matching neither experimentally nor theoretically. Therefore, no one has ever experimentally 2469-9950/2021/103(14)/L140405(8) L140405-1 ©2021 American Physical Society