Two-Dimensional Van Der Waals Materials for Spin-Orbit Torque Applications

Spin-orbit torque (SOT) provides an efficient approach to control the magnetic state and dynamics in different classes of materials. Recent years, the crossover between two-dimensional van der Waals (2D vdW) materials and SOT opens a new prospect to push SOT devices to the 2D limit. In this mini-review, we summarize the latest progress in 2D vdW materials for SOT applications, highlighting the comparison of the performance between devices with various structures. It is prospected that the large family of 2D vdW materials and numerous combinations of heterostructures will widely extend the material choices and bring new opportunities to SOT devices in the future.

vdW materials. Recent years, emerging studies were carried out on 2D SOT devices. Liu et al. have summarized the recent progress of SOT studies based on transition-metal dichalcogenides (TMDs) and show how these results are in line with the symmetry arguments (Liu and Shao, 2020). Husain et al. reviewed the current progress in research on 2D TMDs for generating SOTs in spin-logic devices (Husain et al., 2020b). Both of them focused on SOT devices based on 2D TMDs. However, the large family of 2D vdW materials and numerous combinations of heterostructures has widely extended the material choices of SOT devices, and there is a lack of overviews of studies on 2D vdW materials beside 2D TMDs for SOT applications.
This review gives an overview of the recent progress on 2D vdW materials for SOT applications. The remainder of this review is structured as follows. We start by introducing the basic principles underlying the physical behaviour of SOT. Then, we outline several magnetic heterostructures based on 2D vdW materials focusing on their pivotal use in SOT devices, including 2D vdW material/FM heterostructures, 2D vdW magnet/HM heterostructures, and all-vdW heterostructures. Additionally, performance modulation of conventional SOT devices by 2D vdW materials is also reviewed. At last, we provide a conclusion and discuss the future perspectives briefly in this field.

ORIGIN OF SOT
SOT phenomenon describes the effect of spin-polarized electrons acting on magnetic moments. The mechanism of SOT can be illustrated from two aspects. Firstly, spinpolarized electrons are generated by an electric current due to SOC effects (Manchon and Zhang, 2009). The main SOC effects attributed to generating spin accumulation are spin Hall effect (SHE) and interface Rashba-Edelstein effect, which will be introduced later. Secondly, the spin polarized electrons exert torques on adjacent magnetic layers, leading to the dissipation, precession or switching of the magnetic moments. Here, we introduce the basic principles underlying the physical behavior of SOTs, including the origin of spin-polarized electrons and the effect of various torques.

Spin Hall Effect
Spin Hall effect (SHE) is one of the most general ways to generate pure spin current in source materials with large SOC using electrical methods. The SHE was theoretically predicted by Dyakonov and Perel (1971) and then be revisited by Hirsch Hirsch (1999) and Zhang Zhang (2000). Although theoretically predicted a long time ago, evidence of SHE had not been experimentally confirmed until 21st century. In 2004, using magneto-optical Kerr microscope, Kato et al. (2004) observed the spin accumulation at the material boundary induced by the SHE in semiconductor GaAs for the first time. Figure 1A illustrates the mechanism of SHE in a conventional magnetic heterostructure. When an unpolarized charge current J C is injected into a source material with SOC, electrons with the spin up and down deflect in the opposite direction due to the spin-dependent asymmetric scattering, leading to a transverse spin current J S perpendicular to the direction of both charge current and the polarization σ of the accumulated spins. Accordingly, the SHE can be represented by Where Z is the reduced Planck constant divided by 2π, e is the elementary charge, and θ SH is the Spin Hall angle (SHA) (Niimi and Otani, 2015). The value of θ SH determines the charge-spin conversion efficiency of the source material, and the sign of θ SH denotes the polarized direction of spin accumulation at the heterostructure interface. The spin-dependent asymmetric scattering originates from the skew scattering (Niimi et al., 2011), side-jump (Levy et al., 2013), and the intrinsic mechanisms (Zhang, 2000), which are caused by coherent band-mixing effects induced by the external electric field and the disorder potential in the presence of SOC. Therefore, to improve the efficiency of charge-spin conversion, materials with large SOC can be selected as the source of the spinpolarized electrons.

Rashba Effect
Another effective method to generate spin-polarized electrons is the interface current-induced spin accumulation known as the Rashba-Edelstein effect (Dresselhaus, 1955). It was initially Frontiers in Nanotechnology | www.frontiersin.org August 2021 | Volume 3 | Article 732916 proposed in the context of wurtzite semiconductors and 2D electron gases with broken inversion symmetry (Meijer et al., 2005). Over the last decade, the Rashba-Edelstein effect has been extended to HM/FM structures with broken inversion symmetry (Grytsyuk et al., 2016). Its application for conversion between charge and spin currents has been theoretically predicted and experimentally demonstrated in recent years, as have been reviewed by Koo et al. Koo et al. (2020). As is illustrated in Figure 1B, in structures with broken inversion symmetry, an internal electric field E is generated at the interface along the direction of symmetry breaking due to the potential drop (Manchon, 2020). When the conduction electrons with momentum p move near the interface, an effective magnetic field is induced parallel to E×p. Therefore, the interfacial Rashba effect can be expressed by the following Hamiltonian (Manchon, 2020), Where α R is the Rashba parameter governed by the potential drop at the interface. The Rashba effect results in spin split 2D dispersion surfaces and the spin-momentum locking, which have been investigated across various surfaces and interfaces (Edelstein, 1990). Interface alloying of heavy elements with intermediate-weight metals can enhance the in-plane potential gradient via hybridization, leading to more pronounced Rashba effects, as on the Bi/Ag (111) alloyed interface (Sánchez et al., 2013).

Varieties of Torques
The SOTs induced by spin-polarized electrons act on the precessional magnetization, and the dynamics of the magnetization subject to SOT is governed by the Landau-Lifshitz-Gilbert (LLG) equation, Where τ SOT is the sum of several spin orbit torques (Brataas et al., 2012). Traditionally in materials with mirror symmetries, two kinds of SOTs are proposed, including an out-of-plane field-like torque, . ξ FL ∝ m × y, and an in-plane damping-like (or Slonczewski) torque, (Miron et al., 2011;Liu et al., 2012a;Avci et al., 2014). In both cases, we define the applied current as always being in the y direction, and m present the direction of the magnetization. Breaking of the mirror symmetry generates an additional out-of-plane damping-like torque, et al., 2017a;MacNeill et al., 2017b;Stiehl et al., 2019a). Recent experiments have demonstrated that . ξ DL, Out−of −plane can be generated using 2D vdW materials like WTe 2 (MacNeill et al., 2017a;MacNeill et al., 2017b) and MoTe 2 (Stiehl et al., 2019a) with low crystal symmetry as the spin source material (Stiehl et al., 2019a). This novel SOT holds an excellent promise for field-free switching of the perpendicularly magnetized system. Apart from these torques, an in-plane field-like torque, ξ T . ∝ m × z is observed in some 2D vdW spin source materials such as NbSe 2 (Guimarães et al., 2018). Possible mechanisms contributing to this kind of torque may be the strain-induced symmetry breaking during the fabrication procedure, but further studies are still needed. Recently, another current-induced torque ξ C . ∝ m × x with Dresselhaus symmetry was observed in TaTe 2 (Stiehl et al., 2019b). However, the dominant mechanism behind ξ C . is not the SOT effect but rather the Oersted field due to the resistance anisotropy in 2D materials. Since the Dresselhaus component exists in a wide variety of heterostructures incorporating spin source materials with low crystal symmetries, special attention should be paid to the measurement of SOTs.
It is noted that in systems dominated by the damping-like torque, the value of ξ DL .
can be equivalent to the SHA. The ratio between SHA and the resistivity of the source material is known as spin Hall conductivity, which is proportional to the power consumption of a SOT device.

CONSTRUCTIONS OF 2D SOT DEVICES
SOT devices based on 2D vdW Materials has been rapidly developed in the last few years. By assorting with material construction, there are mainly three kinds of 2D vdW SOT devices which are based on 2D vdW material/FM heterostructures, 2D vdW magnet/HM heterostructures, and all vdW heterostructures. In this section, we first introduce the preparation methods of 2D vdW materials for SOT devices and then give an overview of 2D vdW SOT devices with different constructions.

Preparing Methods of 2D vdW Materials for SOT Devices
Substantial efforts have been devoted to preparing atomically thin 2D materials with controllable dimensions and thicknesses. Preparing methods of 2D vdW materials for SOT applications mainly includes mechanical exfoliations, chemical vapour deposition (CVD) and magnetron sputtering.
At present, mechanical exfoliations remain the workhorse to prepare 2D vdW materials for SOT studies. Materials prepared by exfoliation techniques are characterized by excellent crystal structures and provide conveniences for controlling the stacking order of 2D materials. In recent years, although various approaches have been reported to enhance the lateral size of exfoliated flakes (Desai et al., 2016;Velický et al., 2018;Huang Y. et al., 2020), the critical limitations of uniform coverage and controlled flake shape restrict its practical applications. On the other hand, by mechanical exfoliation, multiple combinations of van der Waals heterostructures can be stacked, which is beneficial to develop novel SOT devices. However, potential problems such as organic residues, hydrolysis, and oxidation at the interface during exfoliation may become key factors restricting the performance of SOT devices .
CVD prevails as the most industry-relevant technique so far. To data, TMDs such as WS 2 , WSe 2 and MoS 2 have been prepared by CVD for SOT applications (Zhang W. et al., 2016;Shao et al., 2016;Lv et al., 2018). 2D magnetic materials with simple components, such as CrTe (Wang M. et al., 2020), FeTe 2  and CrSe 2 , have also been successfully prepared by CVD, which provide a good platform for studying the engineering of SOT devices at the 2D scale. The bottlenecks of this method are mainly concentrated in two aspects. The first challenge is the synthesis of complex alloys, like ternary magnets, e.g. Fe 3 GeTe 2 (FGT) (Huang et al., 2017;Fei et al., 2018), Cr 2 Ge 2 Te 6 (CGT) (Gong et al., 2017), and CrSiTe 3 (Ito et al., 2019), which are mainstream SOT devices based on 2D magnets. The second challenge concerns the construction of magnetic heterostructures with sharp and ordered interfaces. The preparation of heterostructures by CVD usually requires refuelling or connecting with other equipment, thus the quality of the interface is difficult to guarantee. Another 2D vdW material preparation method is magnetron sputtering, which is compatible with the current semiconductor industry. The preparation of 2D materials by magnetron sputtering is usually performed in multiple steps while controlling strict growth and annealing conditions. To date, 2D materials for SOT devices prepared by sputtering include PtTe 2  and TaS 2 (Husain et al., 2020a). Waferscale PtTe 2 films are prepared through a two-step process, which has been previously used for fabricating PtSe 2 (Wang et al., 2015;Yim et al., 2016) and WTe 2 (Zhou et al., 2017). During preparation, Pt thin films are first deposited on Si/SiO 2 wafers by a magnetron sputtering system. They are then transformed into uniform and homogenous PtTe 2 thin films by annealing them in tellurium vapour. Similarly, for TaS 2 (Husain et al., 2020a), as-deposited Ta ultra-thin films are transferred to a high vacuum chamber for the plasma-assisted sulfurization process. It is noted that the spin Hall conductivity of PtTe 2 and TaS 2 prepared by sputtering are superior to values reported for many other 2D vdW materials, denoting the high-quality of 2D vdW materials prepared by the sputtering method.
In addition to the above three preparation methods, molecular beam epitaxy (MBE) has also attracted considerable attention (Walsh and Hinkle, 2017). MBE offers promising solutions to improve the interface quality of magnetic heterostructures thanks to its high purity solid source materials and ultra-high vacuum growth environment. Recently, heterostructures such as CGT/(Bi,Sb) 2 Te 3 (Mogi et al., 2018) and FGT/Bi 2 Te 3  with high-quality interfaces have been successfully prepared. Although it has not been widely used for SOT applications yet, it is believed that MBE has its irreplaceable role for 2D SOT devices in the near future.

SOT Devices Based on 2D vdW Material/FM Heterostructures
Extensive 2D Materials including semiconductors, semimetals, metals and insulators have been used as the spin source layer, among which TMDs draw the most attention for high SOT efficiency and the out-of-plane damping-like torque induced by the broken symmetry. Following, we review the main results on 2D vdW material/FM heterostructures for SOT applications, as is listed in Table 1.

2D Semiconductor/FM
Current induced SOT was first observed in monolayer 2D vdW semiconductor MoS 2 /Py by Zhang et al. using spin-torque ferromagnetic resonance (ST-FMR) (Zhang W. et al., 2016). Both out-of-plane field-like torque ζ FL and in-plane dampinglike torque ζ DL are observed, and a torque ratio ζ FL /ζ DL 0.19 ± 0.01 was quantified by the lineshape analysis, indicating the potential of 2D TMDs for the use of interfacial spin-orbitronics applications. Later on, Shao et al. examined the SOT in monolayer MoS 2 and WSe 2 coupled with CoFeB by another measurement technique, second-harmonic Hall (SHH) (Shao et al., 2016). In contrast to the results by Zhang and co-workers, Shao et al. detected only the out-of-plane field-like torque ζ FL , and there is no evidence of damping-like torques in both MoS 2 / CoFeB and WSe 2 /CoFeB bilayers. The torques are temperaturedependent, and the corresponding spin Hall conductivity σ FL is equal to 2.88 × 10 3 (ħ/2e) (Ωm) -1 and 5.52 × 10 3 (ħ/2e) (Ωm) -1 at room temperature for MoS 2 and WSe 2 respectively. They attributed the SOTs to the Rashba-Edelstein effect. Furthermore, Lv et al. demonstrated that the SOT ratio between the field-like and damping-like torques can be controlled by applying the back-gate voltage in WS 2 /Py heterostructures (Lv et al., 2018), as shown in Figure 2A,B The effective modulation of SOTs by electrical voltage opens a new door for developing and applying SOT devices in data storage and processing. It is noted that although the spin-charge conservation efficiency in 2D vdW semiconductors is comparable to conventional HMs, their resistivity is several orders of magnitude higher than that of HMs, leading to most of the current flowing in the FM layer and hence much power consumption. In order to overcome this limitation and improve the energy efficiency of SOT devices, more and more studies are focused on 2D semimetals with large conductivity, as will be introduced as follow.

2D Semimetal/FM
Pioneering works on 2D semimetals for SOT applications are performed by MacNeill et al. on WTe 2 (MacNeill et al., 2017a), a Weyl semimetal that features an open Fermi arc with strong SOC and spin-momentum locking effect. Notably, the surface crystal structure of WTe 2 has only one mirror plane. In their study, the in-plane damping-like torque (ζ DL, in-plane 0.029) and the outof-plane field-like torque (ζ FL 0.033) are both detected by ST-FMR, and the corresponding spin-torque conductivities are 8 × 10 3 (ħ/2e) (Ωm) -1 and 9 × 10 3 (ħ/2e) (Ωm) -1 respectively. Particularly, when an electric current is applied along the lowsymmetry axis of WTe 2 , an unconventional out-of-plane damping-like torque (ζ DL, out-of-plane 0.013, σ DL, out-of-plane 3.6 × 10 3 (ħ/2e) (Ωm) -1 ) is applied on the adjacent Py film, as is shown in Figure 2C,D The torques were further confirmed by the SHH technique (MacNeill et al., 2017b) and remain large down to WTe 2 monolayer, providing a new strategy for optimizing the manipulation of magnetisation in SOT devices with perpendicular magnetic anisotropy (PMA). Recently, a step forward has been made using SOTs in WTe 2 /Py heterostructures to switch magnetization (Shi et al., 2019), where the damping-like SOT was as large as 0.51, and the switching current density is in the order of 10 5 A/cm 2 . The power consumption required to switch the magnetization in WTe 2 /Py is much lower than average values in Bi 2 Se 3 /Py or Pt/Py. Similar studies were also performed by Stiehl et al. on another 2D semimetal 1T'-MoTe 2 (Stiehl et al., 2019a). Unlike WTe 2 , 1T'-MoTe 2 retains a bulk inversion symmetry while the surface symmetry is limited to just one mirror plane. By ST-FMR, the Frontiers in Nanotechnology | www.frontiersin.org August 2021 | Volume 3 | Article 732916 average in-plane damping-like torque conductivity is extracted as 5.8 × 10 3 (ħ/2e) (Ωm) -1 , and the field-like torque conductivity is 0.8 × 10 3 (ħ/2e) (Ωm) -1 . When the current is perpendicular to the single mirror plane, the spin conductivity of the out-of-plane damping-like torque induced by interfacial inversion symmetry breaking of MoTe 2 is 1.0 × 10 3 (ħ/2e) (Ωm) -1 , which is about 1/3 of that of WTe 2 (MacNeill et al., 2017a). Recently, a large spin Hall angle of 0.32 accompanied by a long spin-diffusion length of 2.2 μm in MoTe 2 was reported by Song et al. (2020), identifying MoTe 2 as an excellent candidate for simultaneous spin generation, transport and detection. More recently, magnetization switching was realized by Liang and coauthors in MoTe 2 /Py (Liang et al., 2020), and the critical current density is in the order of 10 5 A/cm 2 , which is similar to that of WTe 2 /Py. To explore materials with more significant spin-torque conductivity, Xu et al. examined the type-II Dirac semimetal, PtTe 2 , whose resistivity is lower than WTe 2 and the nontrivial topological invariant gives rise to topological surface states (TSSs) with spin-momentum locking. They found that the SOT conductivity of PtTe 2 is as large as 160 × 10 3 (ħ/2e) (Ωm) -1 , which is comparable to that of the bestperforming heavy metals and topological insulators. Additionally, Xu and coauthors realized switching of perpendicular magnetization in PtTe 2 /Au/CoTb devices with the critical current density in the order of 10 6 ∼10 7 A/m 2 . It is noted that although the SOT conductivity of PtTe 2 is larger than WTe 2 and MoTe 2 , the critical current density for magnetization switching in PtTe 2 shows an opposite trend. Such a discrepancy between the SOT conductivity and the critical switching current density have also been reported by , and the physics at work might depend on the saturation magnetization, the Dzyaloshinskii-Moriya interaction, and the magnetic damping, which require future efforts to understand. Recently, another topological semimetal, Ta 3 As, was predicted to has a large SOT conductivity around 150 × 10 3 (ħ/2e) (Ωm) -1 (Hou et al., 2021). This prediction promotes further research on SOT devices based on 2D semimetals with topological and superconductivity properties.

2D Metal/FM
Compared with 2D semiconductors and semimetals, there are fewer kinds of 2D metallic materials, and only a tiny part of them are used for SOTapplications. Although by far there are only a few of studies reported, 2D metals hold the most potential for SOT application, due to their extremely low resistivities. The first research of SOTs generated by a metallic 2D vdW material is performed on NbSe 2 (Guimarães et al., 2018), where an in-plane damping-like torque with torque conductivity σ DL ≈ 3 × 10 3 (ħ/ 2e) (Ωm) -1 is detected by ST-FMR. Besides, an in-plane field-like torque is observed and varies from sample to sample, consisting of the symmetry breaking by a uniaxial strain that might result from device fabrication. Another metallic 2D vdW material used for SOT applications is TaS 2 (Husain et al., 2020a), which exhibits to date the lowest room-temperature resistivity (≈16.9 μΩ cm) among metallic TMDs ( Figure 2E). The SOTs were measured by using both ST-FMR and SHH, and the damping-like SOT conductivity is found to be 1,490 × 10 3 (ħ/2e) (Ωm) -1 , which is superior to values reported on other 2D vdW materials.

2D Insulator/FM
During the development of SOT devices, 2D insulators have received the least attention than other material counterparts. To date, only one experimental study was reported on antiferromagnetic insulator NiPS3/Py bilayers (Hidding and Guimarães, 2020), where an ample in-plane damping-like torque was induced at the interface. Figure 2F shows the result detected from the angular dependent SHH measurement, the most immense value of the damping-like SOT conductivity they detected at room temperature is 220 × 10 3 (ħ/2e) (Ωm) -1 , which is larger than most of the values reported in other 2D vdW materials. Moreover, temperature-dependent measurements revealed an increased SOT with a decreasing temperature below the Néel temperature of NiPS 3 (T N ≈ 170 K), suggesting a possible effect of the magnetic ordering on SOTs.

SOT Devices Based on 2D vdW Magnet/HM Heterostructures
In the past 4 years, notable examples of magnetic order have been observed in 2D vdW materials, which bring new prospects for spintronic devices. Manipulation of the magnetization in 2D magnets via SOTs is a new branch of spin-orbitronics, a feasible scheme of which is combining the 2D magnet with heavy metal film.
FGT is currently one of the most attractive vdW layered ferromagnets due to its stable PMA and high Curie temperature. SOTs in FGT/Pt heterostructures have been studied by Wang et al. (2019) and Alghamdi et al. (2019). Similar results show that the lower-bound damping-like torque in FGT/Pt bilayer is about 0.1, and the critical current density for magnetization switching is in the order of 10 7 A/cm 2 , as shown in Figure 3A-C Most recently, by further improving the quality of the interface, the minimum of SOT efficiency in FGT/Pt is determined to be 0.18 , which is higher than that in previous studies, and comparable to the values for the Pt/Co and Pt/CoFeB bilayers (Pai et al., 2015;Nguyen et al., 2016), indicating the importance of the high-quality interface for SOT devices.
Apart from the metallic 2D magnet FGT, investigations on SOTs in semiconducting 2D ferromagnets were also reperted recently. Ostwal et al. (2020) and Gupta et al. (2020) separately studied the current induced switching of a ferromagnetic semiconductor CGT. It was found that a current with a density as low as 10 5 A cm -2 is sufficient to switch the out-ofplane magnetization of CGT in CGT/Ta heterostructure, as is shown in Figure 3D-F This current density is about two orders of magnitude lower than those required for switching metallic ferromagnets such as CoFeB (Zhang C. et al., 2016), Py (Liu et al., 2011) and FGT (Alghamdi et al., 2019;Wang et al., 2019), benefiting from the negligible shunting effect of the electrical current in such structures. Recently, Su et al. demenstrated the current dependent magnetoresistance (MR) of CrI 3 by combining with Pt (Su et al., 2020), indicating the potential effect of SOTs on CrI 3 induced by SHE in Pt.
Evidence of SOTs on 2D ferromagnets are also observed in a 2D ferromagnetic insulator Co-Doped MoS 2 (Huang M. et al., 2020) by measuring the spin Hall magnetoresistance (SMR) in Co-Doped MoS 2 /Ta bilayer. We believe that the emerging family of vdW magnets remarkably extend the material choices and construct new building blocks for spintronic applications.

SOT Devices Based on All-vdW Heterostructures
Because of the vdW nature of 2D materials, an atomically sharp interface can be achieved in an all-vdW heterostructure, which is crucially important for a high SOT efficiency. SOT devices based on all-vdW heterostructures was first put forward by Dolui et al. (2020). They theoretically predicted that the field-free magnetization switching can be induced by SOTs in a bilayer-CrI 3 /monolayer-TaSe 2 vdW heterostructure, which has attracted great attentions in this field. Another theoretical work focused on CGT/ graphene/WS 2 vdW heterostructure was reported by Zollner et al. (2020b). Using first principles combined with quantum transport calculations, they found that a dampinglike SOT is able to be generated by skew scattering, and the ratio of field-like and damiping-like components of SOTs can be tuned by two orders of magnitude via gate voltage. In a single device consisting of a bilayer graphene sandwiched by a CGT and a monolayer WS 2 , it is able not only to generate, but also to swap the two spin interactions of exchange and spin-orbit coupling (Zollner et al., 2020a). These pioneering theoretical researches provide bases for further experiments and applications of all-van der Waals SOT devices.
Experimental studies on SOTs in all-vdW heterostructures were preliminarily explored very recently. A significant SOT efficiency of 4.6 along with a dramatically high SOT conductivity of 1,035 × 10 3 (ħ/2e) (Ωm) -1 was reported in WTe 2 /FGT bilayers (Shin et al., 2021). The switching current density is about 3.9 × 10 6 A/cm 2 . Such an extraordinary value of SOT conductivity may benefit from the clean interface between WTe 2 and FGT in their devices. Utilizing the out-of-plane damping-like SOT induced in WTe 2 with low-crystal symmetry, Kao et al. (2020) achieved the field-free deterministic magnetic switching of FGT, highlighting the superiority of 2D vdW heterostructures in energy-efficient magnetization control in SOT-based spintronics.

PERFORMANCE MODULATION OF CONVENTIONAL SOT DEVICES BY 2D VDW MATERIALS
In addition to the main effect in the above structures, 2D vdW materials are also used to assist in improving the SOT efficiency in conventional HM/FM heterostructures.
Recently, the hybrid between 2D vdW materials with HM/FM films has proved to be a good candidate for energy-efficient SOT devices (Xie et al., 2019;Debashis et al., 2020;Lv et al., 2021). It was experimentally demonstrated that the SOT efficiency in Pt/ (Co/Ni) 2 was 17 times enhanced by MoS 2 underlayer (Xie et al., Frontiers in Nanotechnology | www.frontiersin.org August 2021 | Volume 3 | Article 732916 2019), but the underlying mechanisms need to be further investigated. A similar phenomenon was also observed in Ta/ CoFeB with a monolayer WSe 2 inserted as an underlayer (Debashis et al., 2020). In that work, the SOT efficiency is enhanced by more than 25 times due to the potential spin absorption at the Ta/WSe 2 interface. Besides, as an inset layer between FM and HM, it is expected that the 2D vdW materials may work for interfacial engineering. Evidence can be observed in several relevant studies reported in the recent past (Dastgeer et al., 2019;Lee et al., 2020;Lee et al., 2021a;Lee et al., 2021b;Novakov et al., 2021). Novakov and coauthors observed enhanced spin torques from the Rashba spin current in heterostructures of Py and WSe 2 , indicating that the insertion of low layer WSe 2 can be used as an interlayer "scattering promoter" in heterostructure interfaces without quenching the original polarization (Novakov et al., 2021). Lee et al. investigated the longitudinal spin Seebeck effect in magnetic heterostructures with vdW interface, and found that the spin fluctuation in inserted WSe 2 (Lee et al., 2021a) or MoS 2 (Lee et al., 2020;Lee et al., 2021b) can amplify the spin current transmission between HM and FM films. Further works in this area are worthy to be carried on.

CONCLUSION AND FUTURE PERSPECTIVE
Over the last decade, 2D vdW materials have brought fresh stimuli to SOT devices. The large family of vdW and numerous combinations of vdW heterostructures will widely extend the material choices and bring new opportunities in SOT devices in the future. Numerous advantages make vdW materials promising candidates for emerging applications in spin-orbitronics. Based on this review, we suggest the following three important research directions in this field. The first direction is the preparing and interface engineering of novel 2D SOT devices. The quality of the material and the interface condition of the heterostructure play decisive roles in the performance of SOT devices. At present, there are still difficulties in preparing large-area 2D vdW materials with high quality and controllable thickness, thus interfacial engineering remains a challenge for most of the 2D SOT devices. Innovative exfoliation techniques such as the vacuum exfoliation approach  need to be developed. CVD methods could be further optimized, especially in the synthesis of large-scaled complex alloys. Notably, we look forward to the employment of an in-situ growth strategy for 2D vdW magnetic heterostructures for SOT applications, which holds unique advantages to the quality of the material and the interface.
The second direction is the performance modulation of SOT devices. Pioneer works in this aspect were performed by Lv et al. (2018), and affective modulation of SOTs by electrical voltage is successfully realised, suggesting the feasibility of the idea of modulated SOT performance. In recent years, it has been reported that the rich magneto-electric properties of 2D vdW materials can be regulated by electric field (Deng et al., 2018;Huang et al., 2018;Jiang et al., 2018;Park et al., 2020;Zhuo et al., 2021), light , and strain Hu et al., 2020;Zhuo et al., 2021). Therefore, it is expected that the performance of 2D SOT devices can be manipulated by controlling multi-field conditions. The third direction is the field-free magnetization switching using 2D vdW materials with low crystal symmetries. The crystal symmetry induced out-of-plane damping-like SOT in some 2D vdW materials suggests a strategy for field-free switching of perpendicularly magnetize system . To date, various approaches for field-free SOT-induced magnetization switching of PMA systems have been proposed and demonstrated. Examples include using a wedgy oxide capping layer (Yu et al., 2014), a polarized ferroelectric substrate (Cai et al., 2017), lateral SOT (Cao et al., 2020a), introducing an interlayer coupling (Fukami et al., 2016;Lau et al., 2016). Compared with these previous methods, 2D vdW materials broaden the scope of material engineering for conventional SOT devices. Moreover, SOT driven magnetization switching in 2D vdW ferromagnets fuels the low-dimensional spintronic applications, but further work is still needed to push the 2D vdW ferromagnet down to the monolayer limit.

AUTHOR CONTRIBUTIONS
YZ and QC conceived and initiated this project. MT and with the help received from QC and MJ wrote the paper. YZ, WJ and JL were instrumental in making the table and figures. ZH and ZZ helped in drafting the content and correction of the review. All authors contributed to the article and approved the submitted version.