  The spin Seebeck effect is a spin-motive force generated by a temperature
gradient in a ferromagnet that can be detected via normal metal contacts
through the inverse spin Hall effect [K. Uchida {\it et al.}, Nature {\bf 455},
778-781 (2008)]. We explain this effect by spin pumping at the contact that is
proportional to the spin-mixing conductance of the interface, the inverse of a
temperature-dependent magnetic coherence volume, and the difference between the
magnon temperature in the ferromagnet and the electron temperature in the
normal metal [D. J. Sanders and D. Walton, Phys. Rev. B {\bf 15}, 1489 (1977)].
