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

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.

S pintronics aims to exploit the spin degree of freedom in solid state devices for data storage and information processing technologies 1,2 . The fundamental spintronic device concepts, such as creation, manipulation and detection of spin current has been demonstrated in semiconductors [3][4][5] and spin transistor structures [6][7][8][9] using both electrical and optical methods. However, an unsolved challenge in the field is the realization of all-electrical methods to control the spin current and spin lifetime at ambient temperature 2 . For this purpose, twodimensional (2D) crystals offer a unique platform for spintronics due to their remarkably broad range of spin-dependent properties [10][11][12] . Graphene has been demonstrated as an excellent material for spin transport and spin logic devices due to the low spin-orbit coupling (SOC) and high electron mobility [11][12][13][14][15][16][17][18][19][20] . However, this low SOC causes a big challenge for the electrical control of the spin polarization in graphene 11,12 . In contrast, transition metal di-chalcogenides (TMDCs) are semiconductors with a high SOC, several 10 meV in the conduction band and 100 meV in the valence band, which are orders of magnitude higher than in graphene 11,21 . Recently, such high-SOC TMDCs also showed fascinating spin dynamics and transport properties [22][23][24][25] .
To harvest these novel and contrasting spintronic properties of both 2D materials, hybrid devices, consisting of graphene and TMDC van der Waals heterostructures (vdWh) 26,27 , are proposed to be promising 28 . The proximity-induced strong SOC of graphene/TMDC vdWhs led to the observation of weak anti-localization 29,30 and spin Hall effect 31 . In addition to the advantage of spin transport in graphene, SOC in such vdWhs can be utilized to electrically control the spin polarization. Recently, a spin field-effect switch has been demonstrated by using non-local (NL) spin-valve measurements in such a vdWh up to 200 K (ref. 32). For comprehensive understanding and advancement of 2D materials vdWh-based spintronic devices, demonstration of electrical gate-tunable spin current and spin lifetime by both spin-valve and Hanle spin precession measurements at ambient temperatures is a crucial requirement.
In this article, we demonstrate the electronic gate control of the spin current and spin lifetime at room temperature by using both spin-valve and Hanle measurements on graphene/MoS 2 vdWh devices. Hanle spin precession measurements unambiguously prove the spin-polarized transport and demonstrate the electrical gate control of the spin lifetime and diffusion length in such 2D material heterostructures at room temperature. We further prove that the gate-voltage tuning of the Schottky barrier at the graphene-MoS 2 interface and MoS 2 channel conduction facilitate the spin in graphene to interact and dephase efficiently in high-SOC material MoS 2 . Extensive spintronic and electronic measurements in different devices at various gate voltages, injection bias currents and temperatures enable us to understand the gate control of spin relaxation mechanism in such vdWhs.

Results
Device and electrical gate control of spin current. A schematic representation of a gate-controlled graphene/MoS 2 vdWh spintronic device used for both spin-valve and Hanle measurements is shown in Fig. 1a. We fabricated the heterostructure devices with large-area chemical vapour deposited (CVD) graphene 13 and TMDC molybdenum disulfide (MoS 2 ) flakes 22 (Fig. 1b). The spin current is generated and detected in NL measurement geometry by using ferromagnetic (FM) tunnel contacts deposited on graphene either in parallel or antiparallel magnetization configuration (see 'Methods' section) 13 . Our devices satisfy the four main requirements for a transistor-like spin signal modulation at room temperature: 33 the electrical creation, diffusive transport, NL electrical detection and the crucial electrical control of a spin current by means of a gate voltage. Figure 1c shows the gate dependence of the measured spin signal modulation, demonstrating transistor-like ON/OFF states at room temperature. The measurements were performed by probing the NL voltage (DV NL ) while sweeping the gate voltage (V g ), for both parallel and antiparallel magnetization alignments of FM source and drain. To confirm such a gate-controlled spin device operations and to understand the spin relaxation mechanisms, we systematically investigated both the spin valve and Hanle spin precession measurements in graphene/MoS 2 heterostructures.
Electrical gate control of spin-valve signal. To investigate the spin transport behaviour in the graphene/MoS 2 heterostructure channel, spin-valve measurements were performed in NL geometry at room temperature. Sweeping an in-plane magnetic field, the magnetization configuration of the injector and detector FM contacts (Fig. 2a) can be aligned parallel or antiparallel, resulting in a magnetoresistance DR NL . We observed a clear spin-valve signal (Fig. 2b) at room temperature, demonstrating the possibility of spin transport in the graphene/MoS 2 heterostructures with a channel length of 2.6 mm and a width of 0.6 mm. The NL spin-valve signal DR NL % 0:04 O is smaller than usually measured in pristine CVD graphene channels of similar Next, we measured the spin-valve signal at different gate voltages (V g ) at room temperature (Fig. 2b,c). Below a threshold voltage V g o0 V, we observed spin-valve signals DR NL with almost constant amplitude. For the gate-voltage range of 0oV g o50 V, a drastic reduction in DR NL is observed, whereas for V g 450 V no spin signal could be detected. This behaviour reproduces the gate sweep NL resistance modulation behaviour we also measured in Fig. 1c. The strong modulation and vanishing spin current from ON to OFF state with application of gate voltage in our MoS 2 /graphene heterostructures indicates an electrical spin-control operation at room temperature. Spin-valve measurements with similar behaviour are recently reported at lower temperatures up to 200 K (ref. 32). The room temperature gate control in our spintronic devices can be attributed to better spin transport properties in graphene samples and less interfacial scattering. To further confirm the spin transport and to estimate the spin relaxation time, gate-dependent Hanle spin precession measurements in the graphene/MoS 2 channel are necessary.
Electrical gate control of spin lifetime. Consequently, we performed NL Hanle spin precession measurements in our graphene/MoS 2 vdWh devices (Fig. 3a). Figure 3b shows the Hanle data measured for different gate voltages at room temperature. In this geometry, the modulation of the signal stems from the spin precession about a perpendicular magnetic field 35 . The variation of this NL resistance DR NL due to precession and relaxation of the spins diffusing from the injector to the detector can be described by: With the channel length L, we can extract the spin lifetime t sf and diffusion constant D s % 0:006 À 0:025 m 2 s À 1 to calculate the spin diffusion length l sf ¼ ffiffiffiffiffiffiffiffi ffi Dt sf p (Fig. 3c). Similar to the spin-valve measurements, we observed an almost constant Hanle signal for V g o0 V (Fig. 3b). In this range, we obtain a spin lifetime t sf % 40 À 50 ps and diffusion length l sf % 0:8 À 1 mm at room temperature. The reduced spin lifetime in our vdWhs, compared to pristine graphene devices (200-300 ps) 13 , indicates a proximity-induced SOC in graphene through MoS 2 . Such a shortened spin lifetime has also been expected using the spinorbit relaxation time t so determined from spin Hall effect studies 31 , weak anti-localization measurements 29,30 and theoretical predictions 28,32 . The t sf % 40 ps in our graphene in proximity with MoS 2 corresponds to B1 meV of spin splitting at room temperature, which is gigantic compared to the few meV in pristine graphene 28   gate-controlled spin lifetime by an order of magnitude opens fascinating prospect for spin transport and spin relaxation in such graphene/TMDC hybrid systems.
The disappearance of the Hanle signal already at a gate voltage of about 30 V is due to the reduction of the amplitude by factor two compared to the spin-valve signal, which is consistent with the measurement principle (see Supplementary Fig. 1). Comparative measurements in a pristine graphene channel revealed only a weak modulation in DR NL and t sf due to a change in carrier density around the Dirac point 13 . The observed strong modulation and vanishing spin signal in our vdWhs can originate from the gate-controlled interaction of spin-polarized electrons in graphene with high-SOC MoS 2 . This electrical control of spin signal is reproducibly observed over several devices fabricated on different chips and measured at various bias voltages, gate-voltage sweeps and temperatures (see Supplementary Figs 2-4).

Discussion
To understand the gate-controlled spin modulation, we characterized the charge transport in both the MoS 2 channel and the graphene/MoS 2 vdWhs. The lateral MoS 2 field-effect transistor shows a typical n-type transfer characteristic with an enhancement of current up to 10 6 times with application of V g at room temperature (see Supplementary Fig. 5). Such a gate-tunable MoS 2 channel resistance in our MoS 2 /graphene vdWh would allow the tuning of a parallel spin transport channel next to graphene. If the spin current from graphene can enter MoS 2 in the ON state, spins would lose their coherence at a much faster rate, due to the high SOC of MoS 2 .
Next, we investigated the charge transport in vertical graphene/ MoS 2 devices, to understand the transport mechanisms at the interface (Fig. 4a). The transfer characteristic of such a device is typical for n-type field effect transistors with a current enhancement of more than 10 2 times (ON/OFF) at room temperature (Fig. 4b). The corresponding output characteristic shows an asymmetric line shape in the OFF state, which becomes symmetric in the ON state (Fig. 4c). This transistor behaviour can be attributed to the presence of a gate-tunable Schottky barrier (^) at the MoS 2 /graphene interface 36,37 . We determined this barrierf rom the temperature-dependent output characteristic using the thermionic emission model 22,36 (see Supplementary Fig. 6). Figure 4d shows the gate dependence of^at the graphene/MoS 2 interface, which changes from 300 to 50 meV when tuning the vertical transistor from OFF to ON state. This corresponds to a gate modulation of the graphene Fermi level with respect to the MoS 2 conduction band as depicted in the insets of Fig. 4d. Consequentially, in the spin-ON state, a high Schottky barrier at the graphene/MoS 2 interface and high-channel resistance of MoS 2 prevents spins in the graphene to enter the MoS 2 channel (Fig. 4e). In contrast, in the spin-OFF state, the Schottky barrier at the graphene/MoS 2 interface is significantly reduced yielding an almost Ohmic contact, in addition to a drastic reduction in MoS 2 channel resistance (Fig. 4f). Under these conditions, spins can easily enter MoS 2 , where they experience a much faster spin relaxation. This explains the reduction in spin lifetime in addition to the decrease in spin signal amplitude with gate voltage in our spin-modulation device. These findings demonstrate that the gate voltage can tune the band offset between materials having complementary spintronics properties, facilitating the spins in graphene to interact efficiently with MoS 2 .  In conclusion, the realization of the electrical control of spin current at room temperature using 2D material heterostructures is a significant step in the field of spintronics. Exploiting the complementary spintronic properties of graphene and MoS 2 creates a unique platform, where the spin current can be tuned in a controlled manner by gate voltage. Combining the gate dependence studies of spin valve, Hanle spin precession and Schottky barrier control in graphene/MoS 2 heterostructures, we experimentally demonstrate the remarkable control over the spin parameters. The all-electrical creation, transport and control of the spins in vdWhs also demonstrated an integration of novel gate-controlled spin device and non-volatile FM memory elements. The spin-ON state current can be increased by integrating FM source and drain contacts of Heusler alloys with high spin polarization 20 . The graphene/MoS 2 spintronic devices can be further improved by introducing insulating hexagonal boron nitride (h-BN) as tunnel barrier for efficient spin injection 38,39 , for gate-controlled spin polarization via magnetic proximity effects 40 , h-BN as substrate for improved graphene properties 41,42 , and as ultra-thin gate dielectric for efficient gate control of the spin parameters in the channel. Our findings also open a new platform for the interplay of spin, charge and orbital degrees of freedom for testing a plethora of exotic physical phenomena. Consequently, novel electronic materials with tunable SOC in electron-and hole-doped regime 43 and emerging materials with topological protection 44 can be used in heterostructures with graphene to create novel spin phenomena and provide opportunities for new discoveries.

Methods
Device fabrication. The vdWhs were prepared using CVD graphene (Graphenea) on highly doped Si (with a thermally grown 285-nm-thick SiO 2 layer) and cleaned by Ar/H 2 annealing at 450°C. The MoS 2 flakes (single crystals from SPI supplies) were transferred on top of graphene, which was patterned into individual channels either before or after the MoS 2 transfer. The graphene was patterned by photo-or electron beam lithography and oxygen plasma etching. Next, appropriate MoS 2 flakes of B20-35 nm located on graphene were identified by an optical microscope for device fabrication. The contacts were patterned on graphene (and MoS 2 flakes, in case of the vertical devices) by electron beam lithography. Finally, we used electron beam evaporation to deposit 8 Å Ti, followed by in situ oxidation in a pure oxygen atmosphere for 30 min, resulting in an about 1 nm thick TiO 2 layer. Without interrupting the vacuum, we deposited 80 nm Co and finalized the devices by lift-off in warm acetone at 60°C. In the final devices, the Co/TiO 2 contacts on graphene act as source and drain for spin-polarized electrons, the MoS 2 /graphene heterostructure region used as the channel and the Si/SiO 2 is used as a gate for manipulation of the spin polarization. A representative Dirac curve for the graphene vdWhs is shown in Supplementary Fig. 7.
Measurements. The measurements were performed in a cryostat with variable temperature and magnetic field facility. The current is applied using a Keithley 6221 current source and the NL voltage is detected by a Keithley 2182A nanovoltmeter; the gate voltage was applied using a Keithley 2612 source metre.
Data availability. The data that support the findings of this study are available from the corresponding authors on reasonable request.