  The spin-Seebeck effect (SSE) in ferromagnetic metals and insulators has been
investigated systematically by means of the inverse spin-Hall effect (ISHE) in
paramagnetic metals. The SSE generates a spin voltage as a result of a
temperature gradient in a ferromagnet, which injects a spin current into an
attached paramagnetic metal. In the paramagnet, this spin current is converted
into an electric field due to the ISHE, enabling the electric detection of the
SSE. The observation of the SSE is performed in longitudinal and transverse
configurations consisting of a ferromagnet/paramagnet hybrid structure, where
thermally generated spin currents flowing parallel and perpendicular to the
temperature gradient are detected, respectively. Our results explain the SSE in
terms of a two-step process: (1) the temperature gradient creates a
non-equilibrium state in the ferromagnet governed by both magnon and phonon
propagations and (2) the non-equilibrium between magnons in the ferromagnet and
electrons in the paramagnet at the contact interface leads to "thermal spin
pumping" and the ISHE signal. The non-equilibrium state of metallic magnets
(e.g. Ni81Fe19) under a temperature gradient is governed mainly by the phonons
in the sample and the substrate, while in insulating magnets (e.g. Y3Fe5O12)
both magnon and phonon propagations appear to be important. The phonon-mediated
non-equilibrium that drives the thermal spin pumping is confirmed also by
temperature-dependent measurements, giving rise to a giant enhancement of the
SSE signals at low temperatures.
