  Using a macroscopic approach, we studied theoretically the heat generation
due to spin transport in a typical spin valve with nonmagnetic spacer layer of
finite thickness. Our analysis shows that the spin-dependent heat generation
can also be caused by another mechanism, the spin-conserving scattering in the
presence of spin accumulation gradient, in addition to the well-known spin-flip
scattering. The two mechanisms have equal contributions in semi-infinite
layers, such as the ferromagnetic layers of the spin valve. However, in the
nonmagnetic layer of a thickness much smaller than its spin-diffusion length,
the spin-dependent heat generation is dominated by the spin-flip scattering in
the antiparallel configuration, and by the spin-conserving scattering in the
parallel configuration. We also proved that the spin-dependent heat generation
cannot be interpreted as the Joule heating of the spin-coupled interface
resistance in each individual layer. An effective resistance is proposed as an
alternative so that the heat generation can still be described simply by
applying Joule's law to an equivalent circuit.
