Bulk vs interface effects on the inverse spin-Hall effect in C₇₀ thin films

Ferromagnetic resonance induced spin pumping allows for the generation of pure spin currents by ferromagnetic materials into non-magnetic materials [1,2]. Spin pumping can be observed using spin to charge conversion as provided by the inverse Spin-Hall effect (ISHE), which, for carbon based semiconductors with generally weak SOC, is very subtle, yet detectable when pumped by very powerful ferromagnetic resonant pulses (FMR) [1]. We have studied the ISHE in fullerene C₇₀ thin films. Repeated ISHE measurements were conducted on films with varying thickness between 10 nm and 60 nm showing that, while the ISHE current decreases with an increasing ratio of film thickness and spin diffusion length, the spin Hall angle obtained from these measurements reproduces within the error margins, suggesting that there is no thickness dependence of the mechanism behind the ISHE. This confirms our hypothesis that the measured ISHE is a bulk effect and not an interface effect. [1] D. Sun et al., Nat. Mater. 15 863-869 (2016). [2] S. Watanaba et al., Nature Phys. 10 308-313 (2014).