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

TLDR

The spin‑Seebeck effect generates a spin voltage from a temperature gradient in a ferromagnet, injecting a spin current into an attached paramagnet that is converted into an electric field by the inverse spin‑Hall effect. The effect is measured in longitudinal and transverse geometries of a ferromagnet/paramagnet hybrid, detecting spin currents flowing parallel and perpendicular to the temperature gradient. The study shows that a temperature gradient induces a magnon–phonon non‑equilibrium in the ferromagnet, and the resulting mismatch with electrons in the paramagnet drives thermal spin pumping that produces the ISHE signal, with phonon‑mediated effects yielding a giant SSE enhancement 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.