Charge-to-Spin Conversion and Spin Diffusion in Bi/Ag Bilayers Observed by Spin-Polarized Positron Beam

Charge-to-spin conversion induced by the Rashba-Edelstein effect was directly observed for the first time in samples with no magnetic layer. A spin-polarized positron beam was used to probe the spin polarization of the outermost surface electrons of $\mathrm{Bi}/\mathrm{Ag}/{\mathrm{Al}}_2{\mathrm{O}}_3$ and $\mathrm{Ag}/\mathrm{Bi}/{\mathrm{Al}}_2{\mathrm{O}}_3$ when charge currents were only associated with the Ag layers. An opposite surface spin polarization was found between $\mathrm{Bi}/\mathrm{Ag}/{\mathrm{Al}}_2{\mathrm{O}}_3$ and $\mathrm{Ag}/\mathrm{Bi}/{\mathrm{Al}}_2{\mathrm{O}}_3$ samples with the application of a charge current in the same direction. The surface spin polarizations of both systems decreased exponentially with the outermost layer thickness, suggesting the occurrence of spin diffusion from the Bi/Ag interface to the outermost surfaces. This work provides a new technique to measure spin diffusion length.

Figure 1

The positron beam and the samples. (a) Schematic of the spin-polarized positron beam. The sample layer stack of (b) $\mathrm{Bi}/\mathrm{Ag}(25)/{\mathrm{Al}}_2{\mathrm{O}}_3$ and (c) $\mathrm{Ag}/\mathrm{Bi}(8)/{\mathrm{Al}}_2{\mathrm{O}}_3$. The yellow blocks represent the Mo electrodes.

Figure 2

XRD patterns of (a) $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ substrate, (b) $\mathrm{Ag}(25)/{\mathrm{Al}}_2{\mathrm{O}}_3$, (c) $\mathrm{Bi}(8)/{\mathrm{Al}}_2{\mathrm{O}}_3$, and (d) $\mathrm{Ag}(25)/\mathrm{Bi}(8)/{\mathrm{Al}}_2{\mathrm{O}}_3$. The “filled diamond” marks represent the imperfections in the $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ substrates.

Figure 3

Variation of $\mathrm{\Delta }R$ as a function of input charge current of $+j_c$ and $-j_c$ for (a) $\mathrm{Ag}(25)/{\mathrm{Al}}_2{\mathrm{O}}_3$ at $j_c=89.3\mathrm{A}/\mathrm{m}$, (b) $\mathrm{Bi}(0.3)/\mathrm{Ag}(25)/{\mathrm{Al}}_2{\mathrm{O}}_3$ at $j_c=18.9\mathrm{A}/\mathrm{m}$, and (c) $\mathrm{Ag}(25)/\mathrm{Bi}(8)/{\mathrm{Al}}_2{\mathrm{O}}_3$ at $j_c=17.5\mathrm{A}/\mathrm{m}$.

Figure 4

The surface spin polarization of $\mathrm{Bi}/\mathrm{Ag}(25)/{\mathrm{Al}}_2{\mathrm{O}}_3$ samples as a function of Bi thickness. The six data points of $\mathrm{Bi}(\ge 0.3)/\mathrm{Ag}(25)/{\mathrm{Al}}_2{\mathrm{O}}_3$ were fitted to an exponential function of Eq. (4).

Figure 5

The surface spin polarization of $\mathrm{Ag}/\mathrm{Bi}(8)/{\mathrm{Al}}_2{\mathrm{O}}_3$ samples as a function of Ag thickness. The data were fitted to an exponential function of Eq. (5).