Effect of bias-induced spin flips on spin torque: Enhancement of current and spin transfer in magnetic tunnel junctions

Recent data on the bias dependence of the spin transfer effect in magnetic tunnel junctions have shown that torque remains intact at bias voltages for which the tunneling magnetoresistance has been strongly reduced. We determine the effect of magnons emitted by hot electrons on charge and spin transport in these junctions by using Caroli's formalism. The transport is described in terms of the direct, spin-dependent, and spin-flip probabilities for transmission across the tunnel junction. By adopting values for these adjustable parameters that were found by fitting the bias dependence of the conductance and magnetoresistance, we find that the excitations due to hot electrons, while reducing the magnetoresistance, enhance both the charge current and spin transfer in magnetic tunnel junctions in such a manner that the ratio of the torque to the charge current does not significantly change.