  How magnetism affects the Seebeck effect is an important issue widely
concerned in the thermoelectric community yet remaining elusive. Based on a
thermodynamic analysis of spin degrees of freedom on varied $d$-electron based
ferro- and anti-ferromagnets, we demonstrate that in itinerant or partially
itinerant magnetic compounds there exists a generic spin contribution to the
Seebeck effect over an extended temperature range from slightly below to well
above the magnetic transition temperature. This contribution is interpreted as
resulting from transport spin entropy of (partially) delocalized conducting $d$
electrons with strong thermal spin fluctuations, even semiquantitatively in a
single-band case, in addition to the conventional diffusion part arising from
their kinetic degrees of freedom. As a highly generic effect, the
spin-dependent Seebeck effect might pave a feasible way to efficient "magnetic
thermoelectrics".
