Influence of asymmetry on bias behavior of spin torque

We report calculations, based on the tight-binding model and the nonequilibrium Keldysh formalism, of the effect of band-filling (BF) asymmetry between the ferromagnetic leads on the bias behavior of the spin torque and the tunneling magnetoresistance (TMR) in magnetic tunnel junctions. The underlying mechanism for the asymmetry-induced change in the bias dependence of TMR and the spin-transfer component, Tk, is the interplay of charge and spin currents in the ferromagnetic (FM) and antiferromagnetic (AF) configurations. The BF asymmetry has a dramatic effect on the low-bias behavior of the fieldlike component, T_, which can vary from linear to quadratic bias dependence with positive or negative curvature, thus reconciling the apparently contradictory experimental results. A general expression is derived relating T_ with four independent nonequilibrium interlayer exchange couplings (NEIECs), JFM, associated with the majority- and minority-spin channels, o,o'=^ ,', of the two leads solely in the FM configuration. The bias behavior of the NEIEC components can be selectively tuned with the BF of the free and pinned FM layers, thus opening a new avenue for controlling experimentally T_.