Ab initio comparison of spin-transport properties in MgO-spaced ferrimagnetic tunnel junctions based on Mn3Ga and Mn3Al

Abstract

We report on first-principles spin-polarised quantum transport calculations (from NEGF + DFT) in MgO-spaced magnetic tunnel junctions (MTJs) based on two different Mn-based Heusler ferrimagnetic metals, namely Mn3Al and Mn3Ga in their tetragonal DO22 phase. The former is a fully compensated half-metallic ferrimagnet, while the latter is a low-moment high-spin-polarisation ferrimagnet, both with a small lattice mismatch from MgO. In identical symmetric and asymmetric interface reconstructions across a 3-monolayer thick MgO barrier for both ferrimagets, the linear response (low-voltage) spin-transfer torque (STT) and tunneling magneto-resistance (TMR) effects are evaluated. A larger staggered in-plane STT is found in the Mn3Ga case, while the STT in Mn3Al vanishes quickly away from the interface (similarly to STT in ferromagnetic MTJs). The roles are reversed for the TMR, which is practically 100% in the half-metallic Mn3Al-based MTJs (using the conservative definition) as opposed to 60% in the Mn3Ga case. The weak dependence on the exact interface reconstruction would suggest Mn3Ga-Mn3Al solid solutions as a possible route towards optimal trade-off of STT and TMR in the low-bias, low-temperature transport regime.