  The extra heat generation in spin transport is usually interpreted in terms
of the spin relaxation. By reformulating the heat generation rate, we found
alternative current-force pairs without cross effects, which enable us to
interpret the product of each pair as a distinct mechanism of heat generation.
The results show that the spin-dependent part of the heat generation includes
two terms. One of them is proportional to the square of the spin accumulation
and arises from the spin relaxation. However, the other is proportional to the
square of the spin-accumulation gradient and should be attributed to another
mechanism, the spin diffusion. We illustrated the characteristics of the two
mechanisms in a typical spin valve with a finite nonmagnetic spacer layer.
