Spin-to-Charge Conversion in $\mathrm{Ag}/\mathrm{Bi}$ Bilayer Revisited

The spin-to-charge conversion of the $\mathrm{Ag}/\mathrm{Bi}$ interface is studied in a device in which a spin current can be injected from either side selectively. The charge voltages generated by the two counterpropagating spin currents show opposite signs, that is consistent with the inverse spin Hall effect rather than the well-accepted inverse Rashba-Eldestein effect in the $\mathrm{Ag}/\mathrm{Bi}$ bilayer. Femtosecond laser is further employed to generate the spin-current-induced terahertz signal in a $\mathrm{Ag}/\mathrm{Bi}$ bilayer, which shows no evidence for the inverse Rashba-Eldestein effect, either. This work provides a clear-cut method to identify the spin-to-charge mechanism in a Rashba electronic state and delivers new understanding for the relevant spin-transport phenomena.

Figure 1

(a),(b) Schematic of a spin current injected into an interface between two normal metals with a Rashba spin splitting. The spin current from either the top side or the bottom side is converted into a charge current of the same direction. (c),(d) Schematic of spin current injected into normal metal. A spin current from the top (bottom) side is converted into a charge current to the left (right) side.

Figure 2

(a) Schematic of spin-pumping signal generated by the inverse Rashba-Edelstein effect in a device where spin current can be selectively injected to the Rashba interface from both sides under different magnetic field. The polarity of the generated charge current does not depend on the injection direction of the spin current. (b) Schematic of spin-pumping signal generated by the inverse spin Hall effect with the spin current injected from opposite directions, in which the generated charge current changes its sign with the injection direction of the spin current. (c) Schematic of the multilayer structure device $\mathrm{Fe}/\mathrm{Ag}/\mathrm{Bi}/\mathrm{Py}$ used in this study measured by spin pumping. Fe and Py are two ferromagnetic materials from which a spin current can be injected into $\mathrm{Ag}/\mathrm{Bi}$ from either top or bottom side at different ferromagnetic resonance fields. (d) Spin-pumping signal measured from sample $\mathrm{Fe}(10\mathrm{nm})/\mathrm{Ag}(5\mathrm{nm})/\mathrm{Bi}(6\mathrm{nm})/\mathrm{Py}(10\mathrm{nm})$, indicating the dominant contribution of the ISHE in the $\mathrm{Ag}/\mathrm{Bi}$ bilayer. (e) Spin-pumping signal measured from sample $\mathrm{Fe}(10\mathrm{nm})/\mathrm{Bi}(6\mathrm{nm})/\mathrm{Ag}(5\mathrm{nm})/\mathrm{Py}(10\mathrm{nm})$.

Figure 3

(a),(b) RHEED patterns for $\mathrm{Ag}/\mathrm{Fe}/\mathrm{MgO}$ and $\mathrm{Bi}/\mathrm{Ag}/\mathrm{Fe}/\mathrm{MgO}$. (c) Schematic of terahertz emission from sample $\mathrm{Bi}/\mathrm{Ag}/\mathrm{Fe}$ by the excitation of an ultrafast laser pulse. (d),(e) RHEED patterns for $\mathrm{Bi}/\mathrm{Fe}/\mathrm{MgO}$ and $\mathrm{Ag}/\mathrm{Bi}/\mathrm{Fe}/\mathrm{MgO}$. (f) Schematic of terahertz emission from sample $\mathrm{Ag}/\mathrm{Bi}/\mathrm{Fe}$. (g) Time-domain terahertz signals in $\mathrm{Pt}(3\mathrm{nm})/\mathrm{Fe}(2\mathrm{nm})/\mathrm{MgO}(001)$, $\mathrm{Bi}(4\mathrm{nm})/\mathrm{Ag}(3\mathrm{nm})/\mathrm{Fe}(2\mathrm{nm})/\mathrm{MgO}(001)$ and $\mathrm{Ag}(3\mathrm{nm})/\mathrm{Bi}(4\mathrm{nm})/\mathrm{Fe}(2\mathrm{nm})/\mathrm{MgO}(001)$ films. (h) Time-domain terahertz signals obtained from the Ag-Bi alloy of different atomic ratios. The spin Hall angle changes its sign with varying components. The orange line serves as a guide for the eye.