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

Abstract

We present a systematic study of the temperature dependence of diffusive magnon spin transport using nonlocal devices fabricated on a 210-nm yttrium iron garnet film on a gadolinium gallium garnet substrate. In our measurements, we detect spin signals arising from electrical and thermal magnon generation, and we directly extract the magnon spin diffusion length ${\ensuremath{\lambda}}_{m}$ for temperatures from 2 to 293 K. Values of ${\ensuremath{\lambda}}_{m}$ obtained from electrical and thermal generation agree within the experimental error with ${\ensuremath{\lambda}}_{m}=9.6\ifmmode\pm\else\textpm\fi{}0.9\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ at room temperature to a minimum of ${\ensuremath{\lambda}}_{m}=5.5\ifmmode\pm\else\textpm\fi{}0.7\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ at 30 K. Using a two-dimensional finite element model to fit the data obtained for electrical magnon generation we extract the magnon spin conductivity ${\ensuremath{\sigma}}_{m}$ as a function of temperature, which is reduced from ${\ensuremath{\sigma}}_{m}=3.7\ifmmode\pm\else\textpm\fi{}0.3\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.28em}{0ex}}\mathrm{S}/\mathrm{m}$ at room temperature to ${\ensuremath{\sigma}}_{m}=0.9\ifmmode\pm\else\textpm\fi{}0.6\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.28em}{0ex}}\mathrm{S}/\mathrm{m}$ 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 $T=7\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. An explanation for this low-temperature enhancement is however still missing and requires additional investigation.