  Spin transport in magnetic tunnel junctions in the presence of spin diffusion
is considered theoretically. Combining ballistic tunneling across the barrier
and diffusive transport in the electrodes, we solve the spin dynamics equation
in the metallic layers. We show that spin diffusion mixes the transverse spin
current components and dramatically modifies the bias dependence of the
effective spin transfer torque. This leads to a significant linear bias
dependence of the out-of-plane torque, as well as a non-conventional thickness
dependence of both spin torque components.
