Spin diffusion in Pt as probed by optically generated spin currents

Abstract

We investigate the dependence of the photoinduced inverse spin-Hall effect (ISHE) signal in a set of Pt/Ge(001) junctions as a function of the Pt thickness ${t}_{\mathrm{Pt}}$ in the 1--20-nm range at room temperature. The spin-polarized electrons, photoexcited through optical orientation at the direct gap of bulk Ge, diffuse towards the Pt layer, where spin-dependent scattering yields a transverse electromotive field ${\mathbf{E}}_{\mathrm{ISHE}}$. We show that the experimental data can be properly interpreted in the frame of the spin diffusion model, provided that we consider a variation of the spin diffusion length ${L}_{\mathrm{s}}$ in platinum when the Pt film thickness is increased. Indeed, for ${t}_{\mathrm{Pt}}g10$ nm we estimate ${L}_{\mathrm{s}}=8.2\ifmmode\pm\else\textpm\fi{}3.5$ nm, whereas for ${t}_{\mathrm{Pt}}l10$ nm the spin diffusion length decreases to ${L}_{\mathrm{s}}=3.5\ifmmode\pm\else\textpm\fi{}2.1$ nm. Moreover, the experimental results clearly indicate that for ${t}_{\mathrm{Pt}}l10$ nm the spin Hall angle $\ensuremath{\gamma}$ is three times larger than the value corresponding to thick Pt layers, suggesting a drastic change of the spin-dependent scattering mechanism for very thin Pt films.