We have generalized the nonequilibrium Green's functions Keldysh formalism to study the effect of interfacial disorder on the average spin transfer torque, < T(parallel to)>, in magnetic tunnel junctions (MTJs). We find a sinusoidal angular behavior of the average < T(parallel to)> as in ideal MTJs. We demonstrate for the first time that the general expression of the bias behavior of the average < T(parallel to)> 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(parallel to)> in the positive bias region, due to the disorder-induced resonance states at interface which selectively assist the transmission of right-coming electrons. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3565396]
