  We investigate the dependence of the switching current density on the
exchange stiffness constant in the spin transfer torque magnetic tunneling
junction structure with micromagnetic simulations. Since the widely accepted
analytic expression of the switching current density is based on the macro-spin
model, there is no dependence of the exchange stiffness constant. When the
switching is occurred, however, the spin configuration forms C-, S-type, or
complicated domain structures. Since the spin configuration is determined by
the shape anisotropy and the exchange stiffness constant, the switching current
density is very sensitive on their variations. It implies that there are more
rooms for the optimization of the switching current density with by controlling
the exchange stiffness constant, which is determined by composition and the
detail fabrication processes.
