Surface-State-Dominated Spin-Charge Current Conversion in Topological-Insulator–Ferromagnetic-Insulator Heterostructures

We report the observation of ferromagnetic resonance-driven spin pumping signals at room temperature in three-dimensional topological insulator thin films—${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$—deposited by molecular beam epitaxy on ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ thin films. By systematically varying the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ film thickness, we show that the spin-charge conversion efficiency, characterized by the inverse Rashba-Edelstein effect length (${\lambda }_{\mathrm{IREE}}$), increases dramatically as the film thickness is increased from two quintuple layers, saturating above six quintuple layers. This suggests a dominant role of surface states in spin and charge interconversion in topological-insulator–ferromagnet heterostructures. Our conclusion is further corroborated by studying a series of ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}/{(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ heterostructures. Finally, we use the ferromagnetic resonance linewidth broadening and the inverse Rashba-Edelstein signals to determine the effective interfacial spin mixing conductance and ${\lambda }_{\mathrm{IREE}}$.

Figure 1

(a) Cross-sectional high-angle annular dark-field scanning TEM image of the $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ interface. (b) Atomic force microscopy image of the $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ (6 QL) sample with a surface roughness of 0.71 nm. (c) Semilog $\theta -2\theta$ XRD scan of a $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ (40 QL) sample which exhibits clear x-ray scattering peaks from the (003) to (0021) planes of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. (d) A representative room-temperature FMR derivative spectrum of a 30-nm YIG film with an in-plane field, which gives a peak-to-peak line width of 9.2 Oe at $f=3\mathrm{GHz}$.

Figure 2

(a) Schematic of the experimental setup for FMR spin pumping measurements. (b) Resistivity of 6 QL ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ thin films grown on YIG as a function of temperature. (c) $V_{\mathrm{S}\mathrm{P}}$ vs $H$ spectra of $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ (6 QL) at $f=2$, 3, and 4 GHz using 100 mW microwave power. (d) $V_{\mathrm{S}\mathrm{P}}$ vs $H$ spectra of the $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ (6 QL) sample for the microwave power of 18, 32, 56, and 100 mW (blue, black, green, red curves, respectively) at $f=3\mathrm{GHz}$. Inset: rf power dependence of the corresponding $V_{\mathrm{S}\mathrm{P}}$ at ${\theta }_H=90°$.

Figure 3

(a) $V_{\mathrm{SP}}$ vs $H$ spectra of $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ (4 QL), $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ (6 QL), $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ (24 QL), and $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ (40 QL) at $f=3\mathrm{GHz}$ using 100 mW microwave power. The $x$ axis is shifted by the resonance field ($H_R$) for clarity. (b) Dependence of $V_{\mathrm{SP}}$ (blue points) and the spin-to-charge conversion efficiency $J_c/J_s$ determined by ${\lambda }_{\mathrm{IREE}}$ (red points) on the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thickness. (c) $V_{\mathrm{SP}}$ vs $H$ spectra of control sample B: $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3(1\mathrm{QL})/{(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3(6\mathrm{QL})$ (blue curve); sample C: $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3(6\mathrm{QL})/{(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3(6\mathrm{QL})$ (green curve); sample D: $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3(1\mathrm{QL})/{\mathrm{Cr}}_{0.2}({\mathrm{Bi}}_{0.5}{\mathrm{Sb}}_{0.5}{)}_{1.8}{\mathrm{Te}}_3(6\mathrm{QL})$ (red curve), and sample E: $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3(6\mathrm{QL})/{\mathrm{Cr}}_{0.2}({\mathrm{Bi}}_{0.5}{\mathrm{Sb}}_{0.5}{)}_{1.8}{\mathrm{Te}}_3(6\mathrm{QL})$ (black curve) at 3 GHz and 100 mW. (d) Comparison of $J_c/J_s$ for the control samples with the corresponding values for $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$.

Figure 4

(a) $V_{\mathrm{ISHE}}$ vs $H$ spectra of $\mathrm{YIG}/\mathrm{Pt}$ (5 nm) bilayer at radio-frequency of 3 GHz and 100 mW microwave power. (b) Dependence of the $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ interfacial spin mixing conductance $g_{\uparrow \downarrow }$ on ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thickness. The blue dashed line indicates the value of $g_{\uparrow \downarrow }$ at the $\mathrm{YIG}/\mathrm{Pt}$ interface.