MnMn/O Interface Termination at the Co 2 Mn α Si/MgO Interface in Magnetic Tunnel Junctions Investigated by Scanning Transmission Electron Microscopy Fengyuan Shi, 1 Hongxi Liu, 2 Masafumi Yamamoto 2 and Paul M. Voyles 1 1. Materials Science and Engineering Department, University of Wisconsin-Madison, Madison, WI, 53706, US 2. Division of Electronics for Informatics, Hokkaido University, Sapporo 060-0814, Japan We have discovered MnMn/O and CoCo/O interface terminations at the MgO/Co 2 MnSi (CMS) interfaces in CoFe/MgO/Co 2 Mn α Si (α = 0.80, 1.0 and 1.29) magnetic tunnel junctions (MTJs) from high-resolution STEM images. The existence of MnMn/O and CoCo/O interface termination explains the high tunnel magnetoresistence (TMR) obtained in similar CoFe/MgO/Co 2 Mn 1.29 Si MTJs [1] because simulation shows that only the metastable MnMn/O interface termination gives 100% spin polarization across the CMS/MgO interface [2]. Steps in the MgO barrier induce switching of the interface termination between MnMn/O and CoCo/O terminations. We also found that increasing Mn concentration in the CMS electrodes increased the fraction of MnMn/O termination. Other interface terminations like SiSi/Mg were not observed in the MTJs. The mixed terminations we found at the CMS/MgO interface provide a key to understand the high TMR and strong temperature dependence of these MTJs [1]. Figure 1(a) is an HRSTEM image of the MTJ multilayer structure consisting of (from the lower side) CoFe/MgO/Co 2 Mn 1.29 Si. Steps at the CMS/MgO are visible due to the very flat MgO tunnel barrier. Figure 1(b) shows CoCo/O and MnMn/O terminations on the left and right side of image, respectively. The changes of the interface terminations are associated with the steps of the MgO tunnel barrier labeled by the orange arrows in Figure 1(b). Away from the interface, the L2 1 structure with the Mn and Si positions interchanged in subsequent MnSi planes is visible in the line profiles in Figure 1(c). At the interface, all the atomic columns have the same intensity. The distances in Figure 2(a) and (b) between the last CMS and first MgO layers are 0.20±0.01 nm and 0.25±0.01 nm for the CoCo/O and MnMn/O terminations, respectively, which are in good agreement with calculations by Hülsen et al. [2]. They disagree with simulated images based on a disordered MnSi/O interface [3]. The intensity of the atom columns at the interface is suppressed by interfacial strain [4], as shown by Figure 2(b)-(d). We increased the HAADF detector angles to suppress the interfacial strain contrast in the STEM images at CMS/MgO interface. The difference between the intensity of the Mn column in the CMS electrode and that of Mn at the interface decreases with increasing the detector angles. Figure 2(e) and (f) shows the simultaneously acquired HAADF and ABF STEM images at the CMS/MgO interface. SiSi/Mg interface termination can be excluded due to the metal-oxygen bonding shown in Figure 2(f). Our results indicate that the metastable MnMn/O termination is favored by nonequilibrium sputter deposition with excess Mn in the CMS. The Mn concentration in the CMS also affects the interface terminations at CMS/MgO interface. Saito et al. reported MnSi/O interface termination at CMS/MgO interface with a Mn deficient CMS electrode [5], although they did not consider MnMn/O in detail. Figure 3 shows the fraction of the MnMn/O termination increases with the increasing Mn concentration. We also found MnSi/O interface termination in the MTJ with Co 2 Mn 0.80 Si electrode, but CoCo/O and MnMn/O terminations are still the dominant species in our MTJs [6]. 336 doi:10.1017/S143192761300367X Microsc. Microanal. 19 (Suppl 2), 2013 © Microscopy Society of America 2013 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S143192761300367X Downloaded from https://www.cambridge.org/core. IP address: 168.151.137.97, on 27 Sep 2017 at 22:29:44, subject to the Cambridge Core terms of use, available at