Linear-response theory of the longitudinal spin Seebeck effect

Adachi, Hiroto; Maekawa, Sadamichi

Spin Seebeck effect refers to a thermal spin injection from a ferromagnet into an attached nonmagnetic metal that occurs over macroscopic scale of several millimeters. The spin Seebeck effect is now established as a universal aspect of ferromagnets since it is observed in a variety of materials ranging from a metallic ferromagnet (NiFe) and semiconducting ferromagnet (GaMnAs) to an insulating magnet (YIG). Here we theoretically discuss the phonon-drag contribution to the spin Seebeck effect. The spin Seebeck effect is driven by nonequilibrium phonons that drag the low-lying spin excitations. This scenario explains simultaneously the local nature of the spin Seebeck effect and the signal enhancement at low temperatures.