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

We predict that the spin-transfer, T-i,T-parallel to, and fieldlike, T-i,T-perpendicular to, components of the local spin torque are dramatically enhanced in double-barrier magnetic tunnel junctions. The spin-mixing enhancement is due to the energetic proximity of majority and minority quantum well states (QWSs) of different quantum numbers within the bias window. The local-spin-torque enhancement is not associated with a corresponding enhancement of the spin-polarized currents. T-i,T-parallel to exhibits a switch-on and switch-off steplike bias behavior when spin-polarized QWSs enter the bias window or exit the energy band, while T-i,T-perpendicular to changes sign between switch-on biases. The net T-perpendicular to exhibits an anomalous angular behavior due to the bias interplay of the bilinear and biquadratic effective exchange couplings