Interplay of Wave Localization and Turbulence in Spin Seebeck Effect

TL;DR

This paper introduces a wave-based theory for the spin Seebeck effect, emphasizing the roles of wave localization and turbulence, and offers explanations for experimental discrepancies and a new perspective beyond local equilibrium assumptions.

Contribution

It develops a wave theory that explains the spin Seebeck effect by considering wave localization and turbulence, challenging the local equilibrium approximation used in prior models.

Findings

Wave localization suppresses the transverse spin Seebeck effect.

Wave turbulence can revive the effect by delocalizing magnons.

The theory suggests experimental tests to resolve current debates.

Abstract

One of the most important discoveries in spintronics is the spin Seebeck effect (SSE) recently observed in both insulating and (semi-)conducting magnets. However, the very existence of the effect in transverse configuration is still a subject of current debates, due to conflicting results reported in different experiments. Present understanding of the SSE is mainly based on a particle-like picture with the local equilibrium approximation (LEA), i.e., spatially resolved temperature-field assumed to describe the system. In this work, we abandon the LEA to some extent and develop a wave theory to explain the SSE, by highlighting the interplay between wave localization and turbulence. We show that the emerging SSE with a sign change in the high/low-temperature regions is closely related to the extendedness of the spin wave that senses an average temperature of the system. On the one hand, ubiquitous disorders (or magnetic field gradients) can strongly suppress the transverse spin Seebeck effect (TSSE) due to Anderson (or Wannier-Zeeman) spin-wave localization. On the other hand, the competing wave turbulence of interacting magnons tends to delocalize the wave, and thus remarkably revives the TSSE before the magnon self-trapping. Our theory provides a promising route to resolve the heated debate on TSSE with a clear experiment scheme to test it in future spin caloritronic devices.