The effect of the magnon dispersion on the longitudinal spin Seebeck effect in yttrium iron garnets (YIG)

TL;DR

This study investigates how magnon dispersion influences the temperature-dependent longitudinal spin Seebeck effect in YIG/Pt systems, revealing the role of subthermal magnons and energy gaps in magnon contributions.

Contribution

It introduces a new empirical model linking magnon flux to the LSSE temperature dependence, highlighting the impact of magnon dispersion and energy gaps.

Findings

LSSE peaks at a temperature dependent on YIG thickness

High magnetic fields cause LSSE freeze-out due to magnon energy gap

Subthermal magnons significantly contribute to LSSE at room temperature

Abstract

We study the temperature dependence of the longitudinal spin-Seebeck effect (LSSE) in a yttrium iron garnet Y3Fe5O12 (YIG) / Pt system for samples of different thicknesses. In this system, the thermal spin torque is magnon-driven. The LSSE signal peaks at a specific temperature that depends on the YIG sample thickness. We also observe freeze-out of the LSSE signal at high magnetic fields, which we attribute to the opening of an energy gap in the magnon dispersion. We observe partial freeze-out of the LSSE signal even at room temperature, where kBT is much larger than the gap. This suggests that a subset of the magnon population with an energy below kB x TC (TC about 40 K) contribute disproportionately to the LSSE; at temperatures below TC, we label these magnons subthermal magnons. The T-dependence of the LSSE at temperatures below the maximum is interpreted in terms of a new empirical model that ascribes most of the temperature dependence to that of the thermally driven magnon flux.