Quantitative Analysis of Coherent and Incoherent Tunneling Currents in MgO-Based Epitaxial Magnetic Tunnel Junctions

Abstract

We studied spin-dependent transport in fully epitaxial Fe/MgO/Cr/Fe(001) magnetic tunnel junctions (MTJs) with a Cr(001) layer grown above or below the MgO barrier layer to clarify the scattering process of tunneling electrons at the barrier/electrode interfaces. From the Cr-thickness dependence of tunneling resistance, it was clearly shown that the Cr layer having no Δ1 Bloch states at Fermi energy acts as a tunnel barrier for coherent-tunneling current (Icoh) carried by electrons with Δ1 symmetry and acts as a metallic layer for incoherent-tunneling current (Iinc) mediated by nonspecular scattering. We evaluated Icoh and Iinc in Fe/MgO/Fe MTJs from the Cr-thickness dependence. The Icoh/Iinc ratio for the upper MgO/Fe interface was estimated to be 16, which corresponded to tunneling spin polarization above 0.9. The lower Fe/MgO interface with higher-density dislocations had a smaller Icoh/Iinc ratio and spin polarization. Reducing structural defects at the lower interface is the key to achieving higher MR ratios.