Pure spin current transport in Alq3 by spin pumping

The use of organic semiconductors (OSCs) in spintronics has aroused considerable interests, owing to their much longer spin-relaxation times of OSCs than those of inorganic counterparts. The most studied example is the organic spin valve (OSV), in which magnetoresistance (MR) effect is frequently reported. However, studies on pure spin current injection and transport in OSCs are scarce. Recently, the pioneering work by Watanabe et al. demonstrated that pure spin current can be pumped into and propagates in semiconducting polymers [1]. In the present work we extend the study to small molecule OSCs, and demonstrate that pure spin current can be injected into Alq$_{3}$ from the adjacent magnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$ (YIG) by spin pumping. The pure spin current is detected by inverse spin Hall effect (ISHE) in Pd after propagation through Alq$_{3}$. From the ISHE voltage $V_{ISHE}$ as a function of the Alq$_{3}$ thickness, the spin diffusion length is determined to be $\sim$ 50 nm and does not depend on temperature. This result indicates the MR decrease as increasing temperature in OSVs is not due to the reduced spin diffusion length. \\[4pt] [1] S. Watanabe, et.al, Nat. Phys. \textbf{10}, 308 (2014).