Current-induced spin polarization in transition metals and Bi/Ag bilayers observed by spin-polarized positron beam

Current-induced spin polarization (CISP) on the outermost surfaces of Au, Cu, Pt, Pd, Ta, and W films were studied by spin-polarized positron beam (SPPB). The Au and Cu surfaces showed no significant CISP. In contrast, the Pt, Pd, Ta, and W films exhibited large CISP (3 $\sim$ 15\% per charge current of $10^{5}$ A/cm$^{2}$) and the CISP of Ta and W were opposite to those of Pt and Pd. The sign of the CISP obeys the same rule in spin Hall effect suggesting that the spin-orbit coupling is mainly responsible for the CISP.\footnote{H. J. Zhang \emph{et al}., Scientific Reports {\bf4}, 4844 (2014).} The outermost spin poalrization of Bi/Ag/Al$_{2}$O$_{3}$ and Ag/Bi/Al$_{2}$O$_{3}$ (charge currents directly connected to Ag layers) were probed by SPPB. The opposite outermost spin polarization of Bi/Ag/Al$_{2}$O$_{3}$ and Ag/Bi/Al$_{2}$O$_{3}$ clarified the charge-to-spin conversion in Bi/Ag bilayers. Nevertheless, the magnitudes of the outermost spin polarization of Bi(0.3$\sim$5)/Ag(25)/Al$_{2}$O$_{3}$ (numbers in parentheses denote thickness in nm) and Ag(25$\sim$500)/Bi(8)/Al$_{2}$O$_{3}$ decrease exponentially with increasing Bi thickness and Ag thickness, respectively. This provides probably the first direct evidence for spin diffusion mechanism.