  The spin diffusion length for thermally excited magnon spins is measured by
utilizing a non-local spin-Seebeck effect measurement. In a bulk single crystal
of yttrium iron garnet (YIG) a focused laser thermally excites magnon spins.
The spins diffuse laterally and are sampled using a Pt inverse spin Hall effect
detector. Thermal transport modeling and temperature dependent measurements
demonstrate the absence of spurious temperature gradients beneath the Pt
detector and confirm the non-local nature of the experimental geometry.
Remarkably, we find that thermally excited magnon spins in YIG travel over 120
$\mu$m at 23 K, indicating that they are robust against inelastic scattering.
The spin diffusion length is found to be at least 47 $\mu$m and as high as 73
$\mu$m at 23 K in YIG, while at room temperature it drops to less than 10
$\mu$m. Based on this long spin diffusion length, we envision the development
of thermally powered spintronic devices based on electrically insulating, but
spin conducting materials.
