Non-local thermal spin injection: Mapping the magnon spin diffusion length in Yttrium Iron Garnet (YIG)*

The non-local spin detection geometry was developed to sample a pure electron spin current in the absence of an electric field, thereby removing parasitic transport effects [1]. Here we demonstrate the non-local detection of magnon spins that are thermally injected via the spin Seebeck effect in single crystal YIG. A laser is used to thermally generate a spin current under an electrically isolated Pt absorbing pad. The spin current is detected on a remote Pt strip via the inverse spin Hall effect ($V_{ISHE}^{non-local})$. Spatial maps of the spin current are acquired by measuring $V_{ISHE}^{non-local} $ while scanning the laser to different absorbing pads. Temperature modeling shows the laser-induced temperature gradient contained within 50$\mu $m of the Pt absorbing pad [2]. Thus, the spin detector is isolated from thermal effects unrelated to the spin current. Although the thermal magnon diffusion length at 21K is $\sim$ 1 $\mu$m [3], $V_{ISHE}^{non-local} $ is detected at displacements of more than 150um with an exponential decay constant of 40 $\mu $m at 25K.\\[4pt] [1] Jedema, \textit{et al}. Nature 416, 713 (2002).\\[0pt] [2] Z. Yang et al. 2015 APS March meeting abstract.\\[0pt] [3] Boona \textit{et al}. Phys. Rev. B 90, 064421 (2014).