Electrical gate control of spin current in van der Waals heterostructures at room temperature

Abstract

Two-dimensional (2D) crystals offer a unique platform due to their remarkable and contrasting spintronic properties, such as weak spin-orbit coupling (SOC) in graphene and strong SOC in molybdenum disulfide (MoS₂). Here we combine graphene and MoS₂ in a van der Waals heterostructure (vdWh) to demonstrate the electric gate control of the spin current and spin lifetime at room temperature. By performing non-local spin valve and Hanle measurements, we unambiguously prove the gate tunability of the spin current and spin lifetime in graphene/MoS₂ vdWhs at 300 K. This unprecedented control over the spin parameters by orders of magnitude stems from the gate tuning of the Schottky barrier at the MoS₂/graphene interface and MoS₂ channel conductivity leading to spin dephasing in high-SOC material. Our findings demonstrate an all-electrical spintronic device at room temperature with the creation, transport and control of the spin in 2D materials heterostructures, which can be key building blocks in future device architectures.

Figure 1. MoS₂/graphene vdWh.

(a) Schematics of graphene/MoS₂ heterostructure channel with FM source (S) and drain (D) contacts. This structure allows spin injection into graphene from the source (S), diffusive spin transport in the graphene/MoS₂ channel, spin manipulation by a gate voltage and detection of spin signal by the drain (D). (b) Coloured scanning electron microscope image of a fabricated device with a CVD graphene/MoS₂ heterostructure channel and multiple FM tunnel contacts of TiO₂(1 nm)/Co(80 nm) (scale bar, 1 μm). The devices are fabricated on Si/SiO₂ substrate, which is used as a gate electrode for control of the spin polarization in the channel. (c) Gate dependence of the measured NL resistance normalized to the maximum value, showing transistor-like ON/OFF spin signal modulation at room temperature, for parallel and antiparallel magnetization alignments of source and drain.

Figure 2. Gate-controlled spin valve signal at room temperature.

(a) Schematic of the NL spin-valve measurement geometry, where the spin current injector circuit (I) and the voltage detector circuit (V) are placed in a NL geometry. The spin current diffusing in the heterostructure channel is detected as a voltage signal by the FM detector. The magnetization of the injector/detector FM contact and also the sign of spin accumulation are controlled by an in-plane magnetic field B_||. (b) NL spin-valve magnetoresistance measurements at 300 K by application of different gate voltages V_g. Measurements are performed at a constant current source of I=30 μA. A NL linear background (few μV) due to stray charge current is subtracted from the signal. (c) Modulation of spin-valve signal magnitude ΔR_NL with gate voltage V_g, showing ON/OFF states at 300 K. The error bar is derived from the root mean square of the noise in the measured signal.

Figure 3. Gate-controlled Hanle spin precession at room temperature.

(a) NL Hanle geometry, where source and drain magnetization are aligned parallel while sweeping a perpendicular magnetic field . (b) NL Hanle spin signal measured at 300 K at different gate voltages. Measurements are performed at a constant current source of I=30 μA. The raw data points are fitted with equation (1) (red line) to extract spin lifetime τ_sf and diffusion length λ_sf. (c) The gate voltage V_g dependence of spin lifetime τ_sf (black) and diffusion length λ_sf (orange) at 300 K showing modulation of spin parameters from ON to OFF state. The error is derived from the error of the Hanle fit.

Figure 4. Tuning the Schottky barrier at the MoS₂/graphene interface by gate voltage.

(a) Schematics of the graphene/MoS₂ vertical heterostructure. (b) Transfer characteristic (drain–source current I_ds versus gate voltage V_g for different drain–source voltages V_ds) in a vertical device with MoS₂ thickness of 35 nm. (c) Output characteristics (I_ds versus V_ds) at . (d) Schottky barrier height Ф obtained for different V_g. Inset: Band structures at the MoS₂/graphene interface for V_g<0 V and V_g>30 V. (e) Representative circuit diagram of the graphene and MoS₂ parallel transport channels connected by Schottky barrier resistors in the heterostructure. At gate voltages V_g<0 V, the large Schottky barrier and high MoS₂ resistance prevents spins from interacting with the high SOC MoS₂ channel, resulting in a finite spin transport in graphene and corresponds to the spin-ON state. (f) At high gate voltages V_g>30 V, the reduced Schottky barrier and MoS₂ channel resistance allows spins to tunnel into MoS₂ and hence dephasing in the high-SOC material, resulting in the spin-OFF state.