Phonon-drag contribution to the spin Seebeck effect

Adachi, Hiroto; Oe, Junichiro*; Takahashi, Saburo; Maekawa, Sadamichi

The spin Seebeck effect, i.e., the generation of the spin voltage by a temperature gradient, is now established as an universal aspect of ferromagnets as it is observed in several ferromagnetic materials ranging from metallic ferromagnet NiFe and semiconducting ferromagnet GaMnAs to an insulating magnet LaYFeO. Recent theoretical and experimental efforts have clarified that the phonon degrees of freedom is of crucial importance in the spin Seebeck effect. Here we theoretically discuss the phonon-drag contribution to the spin Seebeck effect; the spin Seebeck effect is enormously enhanced by nonequilibrium phonons that drag the low-lying spin excitations. This scenario predicts that the spin Seebeck signal at low temperature tracks the phonon-dominated thermal conductivity of the system, and the predicted behaviour has been confirmed by the recent experiments for yttrium iron garnet and for GaMnAs.