Spin-to-Charge Conversion in Bi Films and $\mathrm{Bi}/\mathrm{Ag}$ Bilayers

Thermally injected pure spin current phenomena have been investigated in $\mathrm{Bi}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ and $\mathrm{Bi}/\mathrm{Ag}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ structures at room temperature. We show that although pure spin current has been injected into the Bi layer and the $\mathrm{Bi}/\mathrm{Ag}$ bilayer, there is little detectable signal of spin-to-charge conversion, except the distinctive Nernst signal from the Bi layer, in sharp contrast to the inverse Rashba-Edelstein effect claimed in these systems.

Figure 1

(a) Schematic diagram for measuring LSSE in metallic thin film/YIG samples. The device dimension is $2\mathrm{mm}\times 6\mathrm{mm}\times 0.5\mathrm{mm}$. (b) XRD results of $\mathrm{Bi}(100\mathrm{nm})/\mathrm{Ag}(100\mathrm{nm})/\mathrm{YIG}/\mathrm{GGG}$ grown by low-temperature sputtering. (c) XRR results of low-temperature sputtered $\mathrm{Bi}(30\mathrm{nm})/\mathrm{YIG}$ slab (scaled by a factor of $10^3$) and $\mathrm{Ag}(30\mathrm{nm})/{\mathrm{SiO}}_x/\mathrm{Si}$. The red curves are fits to XRR results, indicating that the thickness of Bi(Ag) is 31.6 nm(30.5 nm) and the surface roughness of Bi(Ag) is 0.4 nm(0.5 nm). (d) XRR of the superlattice $[\mathrm{Bi}(8\mathrm{nm})/\mathrm{Ag}(1.5\mathrm{nm}){]}_5/{\mathrm{SiO}}_x/\mathrm{Si}$ grown by low-temperature sputtering. (e) RHEED patterns of $\mathrm{YIG}/\mathrm{GGG}$ substrates and $\mathrm{Bi}(30\mathrm{nm})/\mathrm{YIG}/\mathrm{GGG}$ samples grown by MBE. (f) XRD results of $\mathrm{Bi}(60\mathrm{nm})/\mathrm{YIG}/\mathrm{GGG}$ grown by MBE.

Figure 2

(a)–(e) Transverse thermovoltages as the function of the in-plane applied magnetic field measured in (a) $\mathrm{Pt}(3\mathrm{nm})/\mathrm{YIG}$ slab and $\mathrm{Pt}(3\mathrm{nm})/\mathrm{MgO}(3\mathrm{nm})/\mathrm{YIG}$ slab, (b) $\mathrm{Bi}(15\mathrm{nm})/\mathrm{YIG}$ slab and $\mathrm{Bi}(15\mathrm{nm})/\mathrm{MgO}(3\mathrm{nm})/\mathrm{YIG}$ slab, (c) $\mathrm{Pt}(3\mathrm{nm})/\mathrm{YIG}/\mathrm{GGG}$ and $\mathrm{Bi}(8\mathrm{nm})/\mathrm{YIG}/\mathrm{GGG}$, (d) $\mathrm{Bi}(15\mathrm{nm})/\mathrm{YIG}/\mathrm{GGG}$ and $\mathrm{Bi}(60\mathrm{nm})/\mathrm{YIG}/\mathrm{GGG}$, (e) $\mathrm{Pt}(3\mathrm{nm})/\mathrm{YIG}$ slab, $\mathrm{Pt}(3\mathrm{nm})/\mathrm{Bi}(3\mathrm{nm})/\mathrm{YIG}$ slab, $\mathrm{Pt}(3\mathrm{nm})/\mathrm{Bi}(6\mathrm{nm})/\mathrm{YIG}$ slab, and $\mathrm{W}(3\mathrm{nm})/\mathrm{Bi}(3\mathrm{nm})/\mathrm{YIG}$ slab (scaled by a factor of 5). Samples in (a)–(c) and (e) are grown by sputtering, whereas those in (d) by MBE. The inset to (b) shows the thermovoltages in $\mathrm{Bi}(15\mathrm{nm})/\mathrm{YIG}$ slab after subtracting a straight line from the raw data. The inset to (c) shows an enlarged plot of the thermovoltages in $\mathrm{Bi}(8\mathrm{nm})/\mathrm{YIG}/\mathrm{GGG}$. (f) The amplitudes of ISHE voltages $|\mathrm{\Delta }V|$ in $\mathrm{Pt}(3\mathrm{nm})/\mathrm{Bi}(t_{\mathrm{Bi}})/\mathrm{YIG}$ slab (red squares) and $\mathrm{W}(3\mathrm{nm})/\mathrm{Bi}(t_{\mathrm{Bi}})/\mathrm{YIG}$ slab (blue circles) as the function of $t_{\mathrm{Bi}}$. The lines are exponential-decay-function fits to the data ($t_{\mathrm{Bi}}\ge 2\mathrm{nm}$) with the effective decay length $\lambda$.

Figure 3

Transverse thermovoltages as the function of the in-plane external magnetic field measured in (a) $\mathrm{Pt}(3\mathrm{nm})/\mathrm{Ag}(2\mathrm{nm})/\mathrm{YIG}$ slab, $\mathrm{Pt}(3\mathrm{nm})/\mathrm{Ag}(2\mathrm{nm})/\mathrm{MgO}(3\mathrm{nm})/\mathrm{YIG}$ slab, and $\mathrm{Ag}(2\mathrm{nm})/\mathrm{YIG}$ slab in the inset, (b) $\mathrm{Bi}(15\mathrm{nm})/\mathrm{Ag}(2\mathrm{nm})/\mathrm{YIG}$ slab and $\mathrm{Bi}(15\mathrm{nm})/\mathrm{Ag}(2\mathrm{nm})/\mathrm{MgO}(3\mathrm{nm})/\mathrm{YIG}$ slab, (c) $\mathrm{Pt}(3\mathrm{nm})/\mathrm{Ag}(2\mathrm{nm})/\mathrm{YIG}/\mathrm{GGG}$ and $\mathrm{Bi}(8\mathrm{nm})/\mathrm{Ag}(2\mathrm{nm})/\mathrm{YIG}/\mathrm{GGG}$. The inset to (c) shows the zoom-in plot of the thermovoltages in $\mathrm{Bi}(8\mathrm{nm})/\mathrm{Ag}(2\mathrm{nm})/\mathrm{YIG}/\mathrm{GGG}$.