Thermal spin pumping and magnon-phonon-mediated spin-Seebeck effect

TL;DR

This paper systematically investigates the spin-Seebeck effect in ferromagnetic metals and insulators, revealing the roles of magnons and phonons in thermal spin pumping and demonstrating a significant enhancement of SSE signals at low temperatures.

Contribution

It introduces a comprehensive explanation of the SSE involving magnon-phonon interactions and thermal spin pumping, supported by temperature-dependent measurements.

Findings

Non-equilibrium states are governed by magnons and phonons.

Thermal spin pumping is driven mainly by phonons in metallic magnets.

Giant SSE signal enhancement observed at low temperatures.

Abstract

The spin-Seebeck effect (SSE) in ferromagnetic metals and insulators has been investigated systematically by means of the inverse spin-Hall effect (ISHE) in paramagnetic metals. The SSE generates a spin voltage as a result of a temperature gradient in a ferromagnet, which injects a spin current into an attached paramagnetic metal. In the paramagnet, this spin current is converted into an electric field due to the ISHE, enabling the electric detection of the SSE. The observation of the SSE is performed in longitudinal and transverse configurations consisting of a ferromagnet/paramagnet hybrid structure, where thermally generated spin currents flowing parallel and perpendicular to the temperature gradient are detected, respectively. Our results explain the SSE in terms of a two-step process: (1) the temperature gradient creates a non-equilibrium state in the ferromagnet governed by both magnon and phonon propagations and (2) the non-equilibrium between magnons in the ferromagnet and electrons in the paramagnet at the contact interface leads to "thermal spin pumping" and the ISHE signal. The non-equilibrium state of metallic magnets (e.g. Ni81Fe19) under a temperature gradient is governed mainly by the phonons in the sample and the substrate, while in insulating magnets (e.g. Y3Fe5O12) both magnon and phonon propagations appear to be important. The phonon-mediated non-equilibrium that drives the thermal spin pumping is confirmed also by temperature-dependent measurements, giving rise to a giant enhancement of the SSE signals at low temperatures.