Tunnel magnetoresistance in ultrathin L1₀ MnGa/MgO perpendicular magnetic tunnel junctions

L1₀ MnGa is one of the interesting magnetic alloys for spin-transfer-torque based applications because such alloys have high perpendicular magnetic anisotropy, small magnetization, and low Gilbert damping. Magnetic tunnel junctions (MTJs) with ultrathin MnGa electrodes have recently been demonstrated using the room temperature growth technique of MnGa on paramagnetic B2-ordered CoGa templates, which exhibited a small TMR ratio of ∼3%. To obtain a higher TMR ratio, we systematically investigated the annealing dependence of the TMR ratio with MTJs with 1-5 nm thick MnGa electrodes in this study. The TMR ratios were 2%-3% without annealing, which were the same as those reported previously, and the TMR ratios reached their maximum values of 6%-8% at an annealing temperature of approximately 250 °C for the MTJs with 2-5 nm MnGa electrodes. The TMR ratio increased to approximately 25% at 10 K for those MTJs. These TMR ratios were slightly higher than those reported in MTJs with 30 nm-thick MnGa electrodes. The annealing temperature at which TMR showed the maximum value tended to decrease with decreasing MnGa thickness, and this low annealing endurance may be attributed to the atomic mixing between MnGa and barrier/buffer layers. The TMR ratio was discussed in terms of both coherent tunneling based on first principles calculations with different element terminations at the interface and incoherent tunneling.