Nonlinear bias dependence of spin-transfer torque from atomic first principles

We report first-principles analysis on the bias dependence of spin-transfer torque (STT) in Fe/MgO/Fe magnetic tunnel junctions. The in-plane STT changes from linear to nonlinear dependence as the bias voltage is increased from zero. The angle dependence of STT is symmetric at low bias but asymmetric at high bias. The nonlinear behavior is marked by a threshold point in the STT versus bias curve. The high-bias nonlinear STT is found to be controlled by a resonant transmission channel in the anti-parallel configuration of the magnetic moments. Disorder scattering due to oxygen vacancies in MgO significantly changes the STT threshold bias.