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 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 themagnon 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 +/- 0.9 mu m at room temperature to a minimum of lambda(m) = 5.5 +/- 0.7 mu 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 sigma(m) as a function of temperature, which is reduced from sm = 3.7 +/- 0.3 x 10(5) S/m at room temperature to sigma(m) = 0.9 +/- 0.6 x 10(4) S/m at 5 K. Finally, we observe an enhancement of the signal originating from thermally generated magnons for low temperatures where amaximum is observed around T = 7 K. An explanation for this low-temperature enhancement is however still missing and requires additional investigation.