Mechanism of paramagnetic spin Seebeck effect

We have theoretically investigated the spin Seebeck effect (SSE) in a normal metal (NM)/paramagnetic insulator (PI) bilayer system. Through a linear response approach, we calculated the thermal spin pumping from PI to NM and backflow spin current from NM to PI, where the spin-flip scattering via the interfacial exchange coupling between conduction-electron spin in NM and localized spin in PI is taken into account. We found a finite spin current appears at the interface under the difference in the effective temperatures between spins in NM and PI, and its intensity increases by increasing the density of the localized spin S. Our model well reproduces the magnetic-field-induced reduction of the paramagnetic SSE in Pt/Gd3Ga5O12 experimentally observed when the Zeeman energy is comparable to the thermal energy, which can be interpreted as the suppression of the interfacial spin-flip scattering. The present finding provides an insight into the mechanism of paramagnetic SSEs and the thermally induced spin-current generation in magnetic materials.

Automated summary

We have theoretically studied the spin Seebeck effect (SSE) in a bilayer system of normal metal (NM) and paramagnetic insulator (PI). By considering the spin-flip scattering through interfacial exchange coupling, we calculated the thermal spin pumping from PI to NM and backflow spin current from NM to PI using a linear response approach. Our results show the appearance of a finite spin current at the interface under a temperature difference between spins in NM and PI, which increases with the density of localized spin S. Additionally, our model successfully explains the experimentally observed reduction of paramagnetic SSE in Pt/Gd3Ga5O12 when the Zeeman energy is comparable to the thermal energy.