Theory for the nuclear spin Seebeck effect

The spin Seebeck effect (SSE) involves transfer of spin angular momentum between a magnet and a metal from internal thermal fluctuations. SSE is usually dominated by electronic, rather than nuclear, spins, since interfacial exchange is much stronger than interfacial hyperfine coupling. At low temperatures, however, electronic magnons freeze out while nuclear spins remain thermally active. Along these lines, we introduce a theory for a new type of SSE: the nuclear SSE, and compare our results to novel ultralow-temperature experiments where this physics is believed to have been observed for the first time. The dominant mechanism for nuclear spin relaxation in a metal is into the Fermi sea, called Korringa relaxation, which drives the nuclear spin current in our theory. The nuclear SSE is then determined by competing rates: thermalization with phonons via hyperfine coupling to electrons in the magnet, and thermalization with metallic electrons via an interfacial Korringa-like interaction.