  Spin-transfer torques occur in magnetic heterostructures because the
transverse component of a spin current that flows from a non-magnet into a
ferromagnet is absorbed at the interface. We demonstrate this fact explicitly
using free electron models and first principles electronic structure
calculations for real material interfaces. Three distinct processes contribute
to the absorption: (1) spin-dependent reflection and transmission; (2) rotation
of reflected and transmitted spins; and (3) spatial precession of spins in the
ferromagnet. When summed over all Fermi surface electrons, these processes
reduce the transverse component of the transmitted and reflected spin currents
to nearly zero for most systems of interest. Therefore, to a good
approximation, the torque on the magnetization is proportional to the
transverse piece of the incoming spin current.
