  We have recently reported that the spin transfer torque switching current
density is very sensitive to not only the junction sizes but also the exchange
stiffness constants of the free layer according to the micromagnetic
simulations. The results are very complicate and far from the macro-spin model
because of the non-coherent spin switching processes. The dependence of the
switching current density on the junction sizes and the exchange stiffness
constants becomes systematic when we employ the non-collinear polarizer layer.
It is found that the non-collinear polarizer layer enhances the coherency of
the spin dynamics by breaking symmetric spin configurations.
