Effect of disorder on spin-transfer torque in magnetic tunnel junctions

We have generalized the nonequilibrium Green’s functions Keldysh formalism to study the effect of interfacial disorder on the average spin transfer torque, $〈T∥〉$⁠, in magnetic tunnel junctions (MTJs). We find a sinusoidal angular behavior of the average $〈T∥〉$ as in ideal MTJs. We demonstrate for the first time that the general expression of the bias behavior of the average $〈T∥〉$ in terms of the interplay of average spin current densities in collinear configurations is valid even in the presence of disorder. This explains the strong enhancement and sign reversal of $〈T∥〉$ in the positive bias region, due to the disorder-induced resonance states at interface which selectively assist the transmission of right-coming electrons.