  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.
