Enhancing the spin-transfer torque through proximity of quantum well states

We predict that the spin-transfer, $T_{i,||}$, and field-like, $T_{i,\bot}$, components of the {\it local} spin torque are dramatically enhanced in double-barrier magnetic tunnel junctions. The {\it spin-mixing} enhancement is due to the energetic proximity of majority and minority quantum well states (QWS) of different quantum numbers within the bias window. $T_{i,||}$ exhibits a switch-on and switch-off step-like bias behavior when spin polarized QWS enter the bias window or exit the energy band, while $T_{i,\bot}$, changes sign between switch-on biases. The {\it net} $T_{\bot}$ exhibits an anomalous angular behavior due to the bias interplay of the bilinear and biquadratic effective exchange couplings.