Specific heat and thermal conductivity of ferromagnetic magnons in Yttrium Iron Garnet

TL;DR

This study measures the specific heat and thermal conductivity of Yttrium Iron Garnet at millikelvin temperatures, revealing the effects of magnetic interactions on ferromagnetic magnons' thermodynamic and transport properties.

Contribution

It provides new experimental data on magnon behavior at very low temperatures, highlighting the influence of magnetic dipole-dipole interactions and defects.

Findings

Magnon specific heat follows a T^{1.5} dependence down to 0.77 K.

Deviation from T^{1.5} indicates magnetic dipole-dipole interactions.

Thermal conductivity data fitted considering defect scattering and dipole interactions.

Abstract

The specific heat and thermal conductivity of the insulating ferrimagnet Y$_3$Fe$_5$O$_{12}$ (Yttrium Iron Garnet, YIG) single crystal were measured down to 50 mK. The ferromagnetic magnon specific heat $C$$_m$ shows a characteristic $T^{1.5}$ dependence down to 0.77 K. Below 0.77 K, a downward deviation is observed, which is attributed to the magnetic dipole-dipole interaction with typical magnitude of 10$^{-4}$ eV. The ferromagnetic magnon thermal conductivity $\kappa_m$ does not show the characteristic $T^2$ dependence below 0.8 K. To fit the $\kappa_m$ data, both magnetic defect scattering effect and dipole-dipole interaction are taken into account. These results complete our understanding of the thermodynamic and thermal transport properties of the low-lying ferromagnetic magnons.