Mechanism of paramagnetic spin Seebeck effect

TL;DR

This paper presents a theoretical model explaining the paramagnetic spin Seebeck effect, showing how temperature differences induce spin currents at interfaces, with results matching experimental observations in specific materials.

Contribution

The study introduces a linear response theoretical framework for the paramagnetic SSE, highlighting the role of localized spins and interfacial spin-flip scattering in spin-current generation.

Findings

Finite spin current arises from temperature differences between spins in NM and PI.

Increasing localized spin density enhances the spin current.

Model reproduces experimental magnetic-field effects on paramagnetic SSE.

Abstract

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/Gd$_3$Ga$_5$O$_{12}$ 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.