Physical Nature of Magnon Spin Seebeck Effect in Ferrimagnetic Insulators

TL;DR

This paper develops a quantitative transport theory for the magnon spin Seebeck effect in ferrimagnetic insulators, revealing phonon-dominated scattering, temperature-dependent relaxation, and ballistic transport at low temperatures, aligning well with experiments.

Contribution

It introduces a comprehensive magnon transport theory combining Boltzmann and quantum scattering approaches, explaining experimental SSE results in ferrimagnetic insulators.

Findings

Magnon scattering is dominated by phonons rather than magnons.

Magnon relaxation time is inversely proportional to the cube of temperature.

At very low temperatures, magnons exhibit ballistic transport.

Abstract

The spin Seebeck effect (SSE) in ferrimagnetic insulators (FMI) provides a simple method of using heat to manipulate magnons, which could be used as carriers of information and energy conversion. However, a theory that can quantitively interpret experimental results is still lacking. In this paper, we develop a transport theory of magnons in FMI at low temperatures by combining the macroscopic Boltzmann equation with microscopic quantum scattering theory. It is found that the scattering of magnons is dominated by phonons rather than magnons, and the relaxation time of magnon is inversely proportional to the cube of temperature. At extremely low temperature region, the magnon enters the ballistic transport process. In addition, we also derive the linear spatial distribution of the transverse SSE signal with sample position. All the theoretical results are in excellent agreement with the experimental data.