Highly Spin-Polarized Tunneling in Fully Epitaxial Magnetic Tunnel Junctions with a Co-Based Full-Heusler Alloy Thin Film and a MgO Barrier

Abstract

Co-based full-Heusler alloy (Co2YZ) thin films are highly preferable ferromagnetic materials in spintronic devices because of the half-metallic ferromagnetic nature at room temperature (RT) theoretically predicted for some of these alloys. We developed fully epitaxial magnetic tunnel junctions (MTJs) that have a Co2 YZ thin film of Co2Cr0.6Fe0.4Al (CCFA), Co2MnSi (CMS), or Co2MnGe (CMG) as a lower electrode, and a MgO tunnel barrier, and have demonstrated a relatively high tunnel magnetoresistance (TMR) ratio of 109% at RT (317% at 4.2K) for CCFA/MgO/Co50Fe50 MTJs and a TMR ratio of 90% at RT (192% at 4.2K) for CMS/MgO/Co50Fe50 MTJs. A high tunneling spin polarization of 0.88 at 4.2K was estimated for epitaxial CCFA films with the B2 structure. The demonstrated high TMR ratios confirmed that fully epitaxial MTJs with a MgO tunnel barrier are promising as a key device structure for fully utilizing the high spin polarization of Co-based full-Heusler alloy thin films.