Temperature dependence of the magnon spin diffusion length and magnon spin conductivity in the magnetic insulator yttrium iron garnet

We present a systematic study of the temperature dependence of diffusive magnon spin transport, using a non-local device geometry. In our measurements, we detect spin signals arising from electrical and thermal magnon generation, and we directly extract the magnon spin diffusion length $\lambda_m$ for temperatures from 2 to 293 K. Values of $\lambda_m$ obtained from electrical and thermal generation agree within the experimental error, with $\lambda_m=9.6\pm0.9$ $\mu$m at room temperature to a minimum of $\lambda_m=5.5\pm0.7$ $\mu$m at 30 K. Using a 2D finite element model to fit the data obtained for electrical magnon generation we extract the magnon spin conductivity $\sigma_m$ as a function of temperature, which is reduced from $\sigma_m=5.1\pm0.2\times10^5$ S/m at room temperature to $\sigma_m=0.7\pm0.4\times10^5$ S/m at 5 K. Finally, we observe an enhancement of the signal originating from thermally generated magnons for low temperatures, where a maximum is observed around $T=7$ K. An explanation for this low temperature enhancement is however still missing and requires additional investigations.