In some cases such as junctions with low magnetic thermal activation energy, the magnetization of the free layer in MgO-based magnetic tunnel junctions (MTJs) can back hop to its original direction after successful spin torque induced switching. The back-hopping is observed in both current directions corresponding to parallel-to-antiparallel and antiparallel-to-parallel switchings. For bias voltage pulses with increasing pulse width, the threshold voltage for back-hopping appears to decrease together with spin-torque switching and junction breakdown thresholds, but its rate of decrease is less. Increasing the anisotropy field H_k by increasing the MTJ aspect ratio can raise the threshold voltage of back-hopping significantly.
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