Gate-tunable charge-spin interconversion in graphene/heavy-metal heterostructures

Abstract

Spintronics has emerged as a promising field for next-generation devices, offering functionalities beyond complementary metal-oxide-semiconductor (CMOS). A critical challenge in spintronics is to develop systems that can efficiently generate spin currents and enable their long-distance transport. Here, we demonstrate a graphene (Gr)/heavy metal (HM) heterostructure system that combines strong charge-spin interconversion efficiency, induced by the spin Hall effect, with a long spin diffusion length. By employing an industry-friendly magnetron sputtering technique, we deposit HM layers onto few-layer Gr while minimizing structural damage. The proximity effect from the HM enhances the spin Hall angle of Gr while limiting the reduction in its spin diffusion length. Additionally, the spin Hall angle can be tuned via an applied gate voltage, offering high controllability of the system. Importantly, these properties are observed across heterostructures composed of different HMs, indicating the generality of this approach. Our findings establish Gr/HM heterostructures as a scalable and versatile platform for spin current generation, paving the way for advanced spintronic devices with high efficiency, long spin propagation, and straightforward fabrication processes.

Figures (25)

• Figure 1: (a) Raman spectra of Gr before (solid lines) and after (dashed lines) 1-nm-thick Ta sputtering deposition for monolayer (1L, red), bilayer (2L, green), trilayer (3L, cyan), and few-layer (blue) Gr. The spectra are normalized and vertically shifted for clarity. The characteristic peaks (D, G, and 2D) are labeled. (b) $V_{\text{G}}$-dependent sheet resistance ($R_{\text{sheet}}$) of pristine Gr (blue) and Gr/Ta (red) measured at 50 K. (c) Optical image of Gr/Ta heterostructure devices for transport measurements. (d) Schematic of the section of the device measured, corresponding to the outlined section in (c), showing labelled FM and NM contacts, the pristine Gr and the centre of the cross proximitized with the HM. The contacts corresponding to the data in (b) are shown as the red and blue voltage measurements, respectively.
• Figure 2: (a, b) Schematic of the device with non-local spin transport measurement geometry for pristine Gr (a) and Gr/Ta heterostructure (b). (c) Non-local resistance $R_\mathrm{NL}$ of the LSV measured as a function of $H_\mathrm{y}$. The red and blue lines correspond to results for pristine Gr and Gr/Ta heterostructure, respectively. (d, e) Hanle precession measurements of pristine Gr (d) and Gr/Ta heterostructure (e) measured as a function of $H_\mathrm{x}$. The red and blue curves represent measurements with the two FM electrodes of the LSV in parallel ($R_\mathrm{NL}^\mathrm{P}$) and antiparallel ($R_\mathrm{NL}^\mathrm{AP}$) configurations, respectively. (f) The difference in $R_\mathrm{NL}$ values ($\Delta R_{\text{NL}} = R_{\text{NL}}^\mathrm{P} - R_{\text{NL}}^\mathrm{AP}$) extracted from (d) (orange) and (e) (cyan). The black lines represent the fitting using a 3D FEM spin diffusion model. All data were obtained at 50 K without applied gate voltage.
• Figure 3: Temperature (a) and gate voltage (b) dependences of spin diffusion length ($\lambda_\mathrm{s}$) for pristine Gr (red) and the Gr/Ta heterostructure (blue). The values in (a) are extracted from measurements conducted without an applied gate voltage, while the values in (b) are determined at 50 K.
• Figure 4: (a) Schematic illustration of the CSI measurement geometry. (b) Non-local resistance $R_\mathrm{NL}$ for CSI measured as a function of $H_\mathrm{x}$ in the Gr/Ta heterostructure at 50 K without external gate voltage. The red ($R_{\text{NL}}^\uparrow$) and blue ($R_{\text{NL}}^\downarrow$) curves represent measurements with the initial magnetization of the FM electrodes along the $+y$ and $-y$ directions, respectively. (c) The CSI resistance $R_{\text{CSI}}$ (yellow) extracted from the difference of the red and blue curves in (b). The black curve represents the fitting results. (d, e) Gate voltage (d) and temperature (e) dependent spin Hall angle ($\theta_{\text{SH}}$) in the Gr/Ta heterostructure.
• Figure 5: (a, b) Non-local resistance $R_{\text{NL}}$ for CSI measured as a function of $H_\mathrm{x}$ in Gr/W (a), and Gr/Pt (b) heterostructures. (c, d) The CSI resistance $R_{\mathrm{CSI}}$ extracted from the difference of the red and blue curves in (a) and (b) for Gr/W (c) and Gr/Pt (d) heterostructures. All the data were measured at 50 K without external gate voltages.