Effect of the magnon dispersion on the longitudinal spin Seebeck effect in yttrium iron garnets

Abstract

We study the temperature dependence of the longitudinal spin Seebeck effect (LSSE) in an 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 k(B)T is much larger than the gap. This suggests that a subset of the magnon population with an energy below k(B)T(C) (T-C similar to 40 K) contributes disproportionately to the LSSE; at temperatures above T-C, we label these magnons subthermal magnons. The T dependence of the LSSE at temperatures below the maximum is interpreted in terms of an empirical model that ascribes most of the temperature dependence to that of the thermally driven magnon flux, which is related to the details of the magnon dispersion.