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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 Hk by increasing the MTJ aspect ratio can raise the threshold voltage of back-hopping significantly.