  We study spin accumulation and spin transfer torque in a domain wall by
solving the Boltzmann equation with a diffusion approximation. We obtained the
analytical expressions of spin accumulation and spin transfer torque. Both the
adiabatic and the non-adiabatic components of the spin transfer torque
oscillate with the thickness of the domain wall. We show that the oscillation
plays a dominant role in the non-adiabatic torque when the domain wall
thickness is less than the spin-flip length, which is defined by the product of
the Fermi velocity and the spin-flip scattering time.
