Abstract
We report on a first principles study of the spin transfer torque (STT) induced by electric voltages across and temperature gradients through ${\mathrm{Fe}}_{0.5}{\mathrm{Co}}_{0.5}/\mathrm{MgO}/{\mathrm{Fe}}_{0.5}{\mathrm{Co}}_{0.5}$ (001) magnetic tunnel junctions (MTJs). A small amount of interfacial oxygen vacancies (OVs) has demonstrated considerable effects on the bias-dependent STT, enabling the ``threshold voltage'' for in-plane STT to be optimized to 0.2 V, and one sample with an asymmetric OV distribution reproduced the experimental measurements. The spin torque efficiency increases with the MgO barrier thickness until it reaches a constant value, while interfacial OVs tend to reduce the efficiency when compared with that of the perfect junction. The thermal spin torque (TST) is also affected greatly by imperfections at the interface, leading to enhancements in FeCo/MgO/FeCo junctions, whereas a weakening effect is found in Fe/MgO/Fe junctions. The TST magnitude in FeCo/MgO/FeCo MTJs is typically smaller than that of a similar MTJ with Fe electrodes.
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