Enhanced Magnetoresistance under Bias Voltage in Fe / MgO / MgAl2O4 / MgO / Fe Trilayer Tunneling Barrier Junction

Abstract

Experiments on magnetic tunnel junctions (MTJs) show that the use of a (001)-oriented spinel ${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ barrier improves the robustness of the tunneling magnetoresistance (TMR) ratio against bias voltage [Sukegawa et al., Appl. Phys. Lett. 96, 212505 (2010)]; however, the maximum TMR ratio is very small compared with that of the $\mathrm{Mg}\mathrm{O}$-based MTJ. To overcome this problem, we propose a MTJ with a trilayered tunnel-barrier junction, $\mathrm{Fe}$/$\mathrm{Mg}\mathrm{O}$/${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$/$\mathrm{Mg}\mathrm{O}$/$\mathrm{Fe}$, from first-principles calculations. The presence of the $\mathrm{Mg}\mathrm{O}$ interlayer between $\mathrm{Fe}$ and ${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ has the effect of enhancing the TMR ratio to more than $1000\mathrm{%}$ at zero bias. The large TMR is maintained under a bias voltage. The results indicate the potential of a hybrid-type tunnel barrier that combines the advantages of MTJs containing a single $\mathrm{Mg}\mathrm{O}$ barrier (high TMR) and a single ${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ barrier (robustness to bias voltages). The $\mathrm{Mg}\mathrm{O}$ interlayer is found to play a key role in suppressing the transmittance of the minority-spin channel, and thus, the tunneling conductance of antiparallel magnetization is significantly reduced.