The dependence of TMR on the barrier thickness, bias voltage and asymmetry in Fe/ZnO/Fe MTJs: A DFT study

Density functional theory (DFT) and non-equilibrium Green's function (NEGF) methods are used to simulate spin-polarized transport properties in Fe(001)/ZnO(001)/Fe(001) magnetic tunnel junctions (MTJs) with an even number of ZnO monolayers (ML). The barrier thickness conductance, tunneling magneto-resistance (TMR) ratio, and tunneling current are investigated for parallel and antiparallel alignment of the magnetization of electrodes. The results indicate that in zero and non-zero bias voltages the slower decay of conductance in the parallel alignment configuration, compared to antiparallel alignment, leads to increased TMR ratio for thicker barriers. Additionally, it is shown that, across all thicknesses of the ZnO barrier, the TMR ratio is highly sensitive to the applied bias voltage and decreases with increasing voltage. Moreover, analysis of the 4-ML MTJ reveals that, for the same density of states (DOS) in the symmetric structure, increasing the DOS at the Fermi level of antiparallel alignment configuration in the asymmetric case results in a negative TMR ratio. The results suggest that a ZnO rock-salt type potential barrier is as suitable as the traditional MgO barrier for spintronic purposes in MTJs.