Voltage Asymmetry of Spin-Transfer Torques

Experimentally, it is seen that the free magnetic layer of a spin torque transfer (STT) device experiences a larger in-plane torque when a negative (rather than positive) voltage is applied to the fixed layer. This is surprising because magnets do not have any intrinsic asymmetry. In this paper, we 1) provide a simple physical explanation, based on the polarization of fixed layer in the energy range of transport; 2) extend it to explain the asymmetric bias dependence of out-of-plane torque as observed in some of the experiments; and 3) propose an asymmetric STT structure that can lead to a significant difference in the in-plane torques exerted on two contacts, even if they are identical. This effect 3 has not been observed to our knowledge and if demonstrated can find important applications.