Control of spin–charge conversion in van der Waals heterostructures Cite as: APL Mater. 9, 100901 (2021); https://doi.org/10.1063/5.0054865 Submitted: 22 April 2021 • Accepted: 06 July 2021 • Published Online: 12 October 2021 Regina Galceran, Bo Tian, Junzhu Li, et al. COLLECTIONS Paper published as part of the special topic on Emerging Materials for Spin-Charge Interconversion ARTICLES YOU MAY BE INTERESTED IN Emerging materials for spin–charge interconversion APL Materials 9, 120401 (2021); https://doi.org/10.1063/5.0076924 Topological insulators for efficient spin–orbit torques APL Materials 9, 060901 (2021); https://doi.org/10.1063/5.0048619 Spin–charge conversion in transition metal oxides APL Materials 9, 060908 (2021); https://doi.org/10.1063/5.0052304 APL Mater. 9, 100901 (2021); https://doi.org/10.1063/5.0054865 9, 100901 © 2021 Author(s). APL Materials PERSPECTIVE scitation.org/journal/apm Control of spin–charge conversion in van der Waals heterostructures Cite as: APL Mater. 9, 100901 (2021); doi: 10.1063/5.0054865 Submitted: 22 April 2021 • Accepted: 6 July 2021 • Published Online: 12 October 2021 Regina Galceran,1 Bo Tian,2 Junzhu Li,2 Frédéric Bonell,3 Matthieu Jamet,3 3 3 1 Céline Vergnaud, Alain Marty, Jose H. García, Juan F. Sierra,1 Marius V. Costache,1 Stephan Roche,1,4 Sergio O. Valenzuela,1,4 Aurélien Manchon,5 Xixiang Zhang,2 2,a) and Udo Schwingenschlögl AFFILIATIONS 1 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain 2 King Abdullah University of Science and Technology (KAUST), Physical Science and Engineering (PSE), Thuwal 23955-6900, Saudi Arabia 3 University of Grenoble Alpes, CNRS, CEA, Spintec, 38000 Grenoble, France 4 ICREA–Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain 5 Aix-Marseille Université, CNRS, CINaM, Marseille, France Note: This paper is part of the Special Topic on Emerging Materials for Spin-Charge Interconversion. a) Author to whom correspondence should be addressed: udo.schwingenschlogl@kaust.edu.sa ABSTRACT The interconversion between spin and charge degrees of freedom offers incredible potential for spintronic devices, opening routes for spin injection, detection, and manipulation alternative to the use of ferromagnets. The understanding and control of such interconversion mech- anisms, which rely on spin–orbit coupling, is therefore an exciting prospect. The emergence of van der Waals materials possessing large spin–orbit coupling (such as transition metal dichalcogenides or topological insulators) and/or recently discovered van der Waals layered ferromagnets further extends the possibility of spin-to-charge interconversion to ultrathin spintronic devices. Additionally, they offer abun- dant room for progress in discovering and analyzing novel spin–charge interconversion phenomena. Modifying the properties of van der Waals materials through proximity effects is an added degree of tunability also under exploration. This Perspective discusses the recent advances toward spin-to-charge interconversion in van der Waals materials. It highlights scientific developments which include techniques for large-scale growth, device physics, and theoretical aspects. © 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0054865 I. INTRODUCTION interaction is mediated by 1000 times weaker van der Waals (vdW) forces, which enables an easy delamination of these layered materi- In the search for continued miniaturization of microelectronic als. The interfaces obtained are defect-free and extremely sensitive devices, two-dimensional (2D) materials, such as graphene and to their environment.4 This offers a unique playground of study, as its siblings, offer opportunities to radically change the technol- the properties of one material can be transferred to its neighbor by ogy landscape by enabling the discovery of revolutionary device proximity effects when stacking different materials in complex vdW paradigms.1,2 Although the path to technological implementation heterostructures. While the device performance is yet to challenge is long and hard, the extraordinary burst of 2D material synthe- the existing traditional technology,5 their ability for unconventional sis during the last decade allowed for the fabrication of a wide operation nurtures the search for innovative concepts, such as the variety of 2D materials,1,3 opening thrilling avenues for the design inclusion of the spin, pseudospin, and valley degrees of freedom.6,7 of innovative atomically flat devices. 2D materials possess strong To address the issues related to the energy loss in devices covalent bonds between atoms in the plane, but their out-of-plane (e.g., gate leakages, interconnects, and refreshing non-volatile APL Mater. 9, 100901 (2021); doi: 10.1063/5.0054865 9, 100901-1 © Author(s) 2021 APL Materials PERSPECTIVE scitation.org/journal/apm memories), spin angular momentum can be used to store, trans- possess a large bandgap that promotes optoelectronic operation53,54 mit, and manipulate information with high efficiency in terms of and the realization of light-driven spin–valley coupling.55–60 Beyond energy consumption.2,8–11 In fact, highly efficient spin transport12–14 the most commonly found semiconducting phase, the TMD fam- and long spin diffusion15–21 can be achieved in graphene and transi- ily also exhibits a wide variety of electronic behaviors ranging from tion metal dichalcogenides (TMDs),22,23 enabling the fabrication of a Weyl semi-metallic state61–63 to superconductivity64,65 as well spin-field effect transistors24–26 and novel spin filtering devices.27,28 as magnetism.66 These features are particularly appealing for the However, traditional spin-devices require spin injection from mag- realization of vdW heterostructures50,51 that could combine all of netic elements, usually made of transition metal materials such as them.25,26 NiFe or CoFe, and the application of magnetic fields to define their Another important class of vdW materials concern bismuth- magnetization orientation. This is a challenge because good interface antimony chalcogenides that display topological properties. TIs quality between the ferromagnet (FM) and the 2D vdW substrate have an insulating bulk and spin–orbit coupled surface states.67,68 is of critical importance and difficult to achieve.12,13,29 Furthermore, The large spin-momentum locking of their surface states makes the use of magnetic fields to switch the magnetization limits the den- them ideal candidates for SCI. Recent progress includes the real- sity of magnetic elements. These limitations severely hindered the ization of current-driven SOT in systems using Bi-chalcogenide TIs performance of flat spintronic devices despite the apparent appeal of (Bi2 Se3 /NiFe)69,70 and the realization of current-driven switching at 2D systems. However, they may be circumvented and new opportu- room temperature71–73 with record low currents required. Note that nities may arise by adopting different strategies. These range from SOT also was realized using TMD monolayers74,75 but with a much the use of recently discovered 2D magnetic materials (CrSe2 , VSe2 , weaker efficiency as compared to TIs. etc.) to the exploitation of the spin–charge interconversion (SCI) In this Perspective, we present the current state of the art that takes place in materials with large spin–orbit coupling (SOC), and perspectives on the control of SCI in vdW materials toward among which we find TMD semimetals and topological insulators the realization of ultra-compact spintronic devices. The complexity (TIs). The SCI enables electrical generation of spin currents or spin of the studied phenomena and the continued surge of new mate- densities, which in turn can be used to generate a torque on an rials make synergy between experiment and theory particularly adjacent FM, thus allowing for all-electrical manipulation of the useful in developing improved large-scale growth methods of such magnetization direction in the FM. vdW materials to facilitate their implementation in upcoming SOC is the responsible interaction for the locking between the technologies. spin and the orbital angular momenta and results in a large number of physical phenomena, such as magnetic anisotropy and damp- II. GROWTH AND MANIPULATION OF 2D MATERIALS ing,30,31 spin relaxation,32 and the spin Hall effect (SHE).33,34 The AND VDW HETEROSTRUCTURES latter is of particular importance for spintronic technologies because it offers a route for the electrical generation of spin currents, one of A. Chemical vapor deposition (CVD) the most crucial limiting factors in current devices. This is why in Currently, most 2D materials for fundamental research are pro- recent years much attention is drawn toward materials presenting duced by mechanical exfoliation, which ensures high crystal quality large SOC and inversion symmetry breaking, both needed for most but with limited yield and size.76 Since the report on the synthe- of the spin–orbit phenomena of interest. In addition, a particular sis of graphene using chemical vapor deposition (CVD),77 in which type of SOC that enables the direct coupling between the spin of volatile precursors react or decompose, this method developed into the electron and its linear momentum can be used for the electrical a popular and reliable technique for the synthesis of different 2D generation of non-equilibrium spin density, offering a different path materials, such as h-BN, MoS2 , WSe2 , CrI3 , CrTe, MoSSe, and 2D for the electrical manipulation of magnets through the spin. Such an vdW heterostructures50,78 (Fig. 1). effect is referred to as the inverse spin galvanic effect (iSGE) or the For wafer-scale synthesis of 2D materials, a number of Rashba–Edelstein effect. approaches were developed, including precise control of the growth Recently, researchers proposed to use the iSGE35,36 and SHE37,38 process,79–81 substrate engineering,82–84 chemical doping,85–87 to generate current-driven spin–orbit torque (SOT), enabling the transfer,88–90 surface morphology,91–93 and twisted bilayers.94–96 electrical manipulation of ultrathin magnets.39–41 The recent obser- Interestingly, adlayer-free large-area single-crystal graphene was vation of current-driven magnetization switching in inversion sym- realized on single-crystal Cu foil,97 which was obtained from metry broken transition metal systems (Pt/Co,42 Ta/NiFe,43 etc.) commercial Cu foil by the contact-free annealing method.98 The initiated a massive amount of effort in this direction. Ultrafast CVD growth of h-BN, a popular 2D insulator resembling the switching was demonstrated,44,45 and SOT is currently considered hexagonal crystal structure of graphene, and 2D heterostructures a promising method for the next generation of magnetic random- of h-BN with graphene and TMDs attracted great interest.99–101 access memories, as explicitly stated in the international roadmap For example, large-area high-quality h-BN was synthesized on for devices and systems and currently being pursued by major single-crystal Cu.102 industries.45 Many 2D materials with excellent semiconductor character- Among the vast catalog of novel 2D materials, TMDs stand istics and high current on/off ratios, such as MoS2 , MoSe2 , WS2 , out as a versatile platform for the advancement of disruptive and WSe2 , were fabricated using CVD.78,103,104 In particular, stable flat microelectronics and spintronics.5,6,11,46–48 Indeed, in contrast and controllable growth of large-scale MoS2 films was achieved by to graphene, TMDs have a large SOC49–51 that was successfully a number of research groups.105,106 A growth and transfer method exploited for gate-controlled spin manipulation in graphene/TMD of large-area high-quality single-crystal MoS2 was also reported.107 interfaces.25,26,52 In addition, some TMDs, such as WS2 and MoS2 , Using S and MoO3 as the growth source and Ar and Ar/O2 as APL Mater. 9, 100901 (2021); doi: 10.1063/5.0054865 9, 100901-2 © Author(s) 2021 APL Materials PERSPECTIVE scitation.org/journal/apm after decomposition with WO3 powder, eventually leading to a 4-in. wafer-scale single-layer WS2 film.111 Going beyond individ- ual TMDs, synthesis and precise control of 2D vdW heterostructure arrays was also achieved.112 For applications in the field of spintronics, potential magnetic 2D materials include FeX2 , CoX2 , NiX2 , VX2 , CrX2 , and MnX2 (X = I, Br, and Cl).113 For the synthesis of large-scale 2D transition metal tellurides by CVD, the use of a mixture of Cr, CrCl3 , and Te was reported to lead to a CrTe film.114 B. Molecular beam epitaxy (MBE) An alternative method to grow large-scale 2D materials is molecular beam epitaxy (MBE), a physical deposition technique based on co-evaporation of individual elements under ultra-high vacuum on a crystal surface held at a controlled temperature (Fig. 2). Directional molecular beams are produced by thermal evapora- tion in effusion cells or by electron-beam evaporation. MBE yields materials with high crystalline quality and centimeter-scale areas. The ultra-high vacuum environment ensures minimal contamina- tion and offers the possibility to use surface analysis techniques to characterize in situ the properties of the films. The film compo- sition, doping, and thickness can be accurately controlled, thanks to the low fluxes (typically 1–10 Å/min). Heterostructures consist- FIG. 1. 2D materials synthesized by the CVD method: graphene, BN, TMDs, mag- ing of a stack of vdW materials can be formed with abrupt inter- netic 2D materials, 2D Janus materials, and 2D van der Waals heterostructures. faces and an arbitrary number of layers. The growth of layered materials by MBE is based on the method of vdW epitaxy, consist- ing in film growth either on a passivated surface with a very low density of dangling bonds [e.g., F-terminated BaF2 (111)] or on a the carrier gas, the growth was successfully achieved at 930 ○ C on layered vdW substrate (e.g., SiC/graphene or mica).116 The weak the sapphire substrate. In addition to MoS2 , there were attempts interaction between the epilayer and the substrate largely releases to grow MoSe2 by CVD.108,109 A promoter-assisted liquid-phase the constraint of lattice matching and leads to the formation of fully CVD method was reported to synthesize high-quality large-area relaxed layers. The advantage of vdW epitaxy is the potential to arbi- monolayer MoSe2 .110 A method to grow wafer-scale monolayer WS2 trarily combine vdW materials with atomically sharp interfaces, free used H2 S gas instead of sulfur powder, as the sulfur source reacts from contaminants. FIG. 2. (a) Schematics of the MBE deposition technique. (b) Reflection high-energy electron diffraction (RHEED) intensity oscillations recorded during the epitaxy of various vdW materials, showing layer-by-layer growth. (c)–(f) Characterization of a WSe2 monolayer grown by MBE on mica. (c) RHEED patterns in two azimuths showing the single crystallinity of the film. (d)–(f) Plane-view transmission electron microscopy near the boundary between in-plane rotated twin domains (marked A and B). (g) and (h) Cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of a multilayer WSe2 film grown on SiC/graphene. (i) Cross-sectional HAADF-STEM image of a Cr2 Ge2 Te6 /(Bi,Sb)2 Te3 /Cr2 Ge2 Te6 epitaxial heterostructure [reproduced with permission from Mogi et al., Phys. Rev. Lett. 123, 016804 (2019). Copyright 2019, American Physical Society].115 APL Mater. 9, 100901 (2021); doi: 10.1063/5.0054865 9, 100901-3 © Author(s) 2021 APL Materials PERSPECTIVE scitation.org/journal/apm The epitaxial growth of layered TIs by MBE was reported on configurations being metastable, high-temperature growth and/or various substrates [e.g., graphene, Si(111), Al2 O3 (0001), GaAs(111), post-deposition annealing might improve the in-plane crystal qual- Ge(111), BaF2 (111), InP(111), and SrTiO3 (111)].117–123 Crystals of ity. Another strategy consists in orienting the grains by nucleation high quality are commonly produced, but suppressing defects such at step edges using vicinal surfaces. In addition, quasi-vdW epitaxy as chalcogen vacancies or twin boundaries requires fine optimization on non-vdW surfaces may promote a better crystal alignment at the of the growth parameters.118,120,123 It is possible by MBE to adjust expense of a stronger substrate interaction146 and a possible loss of in (Bi2−x Sbx )(Te3−y Sey ) alloys the Dirac point energy through the the intrinsic TMD properties. Recently, substrate-induced strain was Te/Se ratio124 and the chemical potential through the Bi/Sb rela- used to stabilize the metastable semimetallic 1T′ phase in WSe2 150 tive content,119,120 yielding bulk-insulating materials with designed and WTe2 .151 topological Fermion dynamics. Magnetic doping of (Bi2−x Sbx )Te3 Reports on the MBE of 2D-FMs include diluted magnetic with Cr and V125–128 as well as epitaxy of the antiferromagnets TMDs, such as V-doped WSe2 152,153 and Mn-doped MoSe2 ,154,155 MnBi2 Se4 129,130 and MnBi2 Te4 131,132 were reported, enabling the intercalated vdW compounds, such as V5 Se8 156 and Cr2 Te3 ,157 experimental realization of the quantum anomalous Hall effect.133 and intrinsic FMs, such as Cr2 Ge2 Te6 ,158 Fe3 GeTe2 ,159–161 CrBr3 ,162 For the large-area growth of TIs, other physical deposition tech- CrCl3 ,163 and CrI3 .164 Intrinsic ferromagnetism was also claimed for niques were employed recently, such as sputtering and pulsed laser MBE-grown monolayer MnSex 165 and VSe2 ,66 although in the latter deposition.73,134 case, a charge density wave tends to suppress ferromagnetism.166,167 Efforts in the MBE of TMDs are more recent. Epitaxy was TI/2D-FM multilayers with abrupt epitaxial interfaces were achieved reported for a variety of TMDs and TMD heterostructures, including by MBE only recently.158,161 MoSe2 , WSe2 , HfSe2 , PtSe2 , VSe2 , ZrTe2 , MoTe2 , and WTe2 .66,135–149 Inert vdW surfaces, such as graphene, mica, or hBN, make it possible to stabilize very flat monolayers of high structural quality, as shown III. FABRICATION OF DEVICES for mica/WSe2 .149 However, the weak interfacial interaction often SCI can be probed and exploited in different types of spin- results in an in-plane misorientation of crystal grains, which proba- tronic devices. The two most prominent examples are the non-local bly remains the main difficulty in the MBE of TMD monolayers. The Hall cross spin devices and the SOT devices. The non-local Hall misorientation is due to atomic position coincidences between the cross devices [sketched in Fig. 3(a)] combine different materials TMD monolayer and the substrate, favoring the formation of grains with specific purposes. In the case of vdW heterostructures, they with an orientation away from the exact epitaxial relationship. These consist of a spin channel made of a low-SOC material (typically FIG. 3. Schematic illustration of the (a) non-local lateral device, (b) second-harmonic Hall device, and (c) SOT-FMR device. (d) Representative room-temperature non- local resistance (Rxy = Vxy /I) vs magnetic field (Bx ) measurements in the iSHE configuration in a graphene-WS2 device. Open and closed symbols correspond to the magnetization of the FM aligned in the −y and +y directions, respectively. (e) Representative second-harmonic Hall voltage for a TMD/NiFe bilayer as a function of the angle between the in-plane magnetic field and the direction of the current for different magnitudes of the magnetic field. (f) Typical SOT-FMR resonances in a TMD/NiFe bilayer for different frequencies with the in-plane magnetic field applied at 35○ relative to the direction of the current. The characteristic resonance frequencies fmin and fmax are in the 1–20 GHz range. (d) Adapted with permission from Benítez et al., Nat. Mater. 19, 170–175 (2020). Copyright 2020, Springer Nature Limited. (e) Adapted with permission from Gupta et al., Nano Lett. 20, 7482–7488 (2020). Copyright 2020, American Chemical Society. APL Mater. 9, 100901 (2021); doi: 10.1063/5.0054865 9, 100901-4 © Author(s) 2021 APL Materials PERSPECTIVE scitation.org/journal/apm graphene) where the spins can propagate over long distances (tens of some cases, preceded by a short etching with Ar plasma to clean the μm). Spins are injected and detected either through a FM or by tak- interface175,176 ). ing advantage of SCI in materials with large SOC. Due to the atomi- SOT devices (with sizes of up to 20 × 100 μm2 ) are commonly cally thin nature of graphene, its very weak SOC can be enhanced by patterned either by electron beam or by optical lithography tech- proximity effects when stacking the material with other vdW mate- niques, followed by ion milling and the deposition of metal contacts. rials, including TMDs, semimetals, and TIs, which are characterized However, when investigating magnetization switching mechanisms, by large SOC. it is important to take into account the dimensions of the device; for As the in situ growth of vdW heterostructures is still at an large devices, switching occurs by domain nucleation and motion, early stage, most of the studied systems to date are obtained by which results in low switching currents. To date, fewer studies stacking mechanically exfoliated crystals. Owing to the nature of the were realized for all-vdW SOT devices. These include in situ MBE- studied effects and stability issues of some of the utilized materi- grown (Bi0.5 Sb0.5 )2 Te3 /(Cr0.08 Bi0.54 Sb0.38 )2 Te3 bilayers with a cap- als at ambient conditions, the heterostructures are fabricated under ping, which were patterned by photolithography and dry etching.177 controlled atmosphere with a low concentration of H2 O and O2 More recently, switching of Fe3 GeTe2 on WTe2 was reported.178 (around 0.1 ppm), thus providing at the same time ultra clean inter- The samples were prepared similarly to the above-mentioned lat- faces. The stacks are obtained by transferring the 2D materials using eral structures by dry transfer of mechanically exfoliated flakes inside a dry transfer method, in which viscoelastic stamps are used to an inert Ar atmosphere glove box, further protected by electron- pick-up and release flakes in a controlled manner. Precise align- beam evaporated Al capping. Progress in the in situ growth of ment between the flakes is achieved by using a micro-positioner vdW heterostructures may greatly simplify the fabrication steps stage and an optical microscope. To avoid gas bubbles and blis- required to preserve the interfaces from degradation due to air ters and to ensure better attachment between the layers, the fin- exposure.161 ished stacks are usually annealed in an Ar/H2 atmosphere or high vacuum. IV. SCI: EXPERIMENTS The stacking process is followed by electron-beam lithography and reactive etching steps to define electrical contacts and to pattern A. Non-local Hall cross devices the heterostructure with specific shapes and dimensions. Devices for Pioneering SCI experiments were conducted in metallic sys- SCI experiments consist of graphene etched in a Hall bar geome- tems using non-local detection techniques. These devices use a FM try. The graphene channel, typically 1 μm wide and tens of μm long, contact for injecting spins into a non-magnet, typically Cu and Al, includes a graphene Hall arm capped with a large-SOC material (see acting as the spin transport channel. Spins diffuse and are absorbed Sec. IV for further details). The device includes a combination of by a heavy-metal, e.g., Pt and Ta, in which a transversal voltage, metal and FM contacts. Metal contacts, typically Ti/Au or Ti/Pd, are generated by the iSHE, is measured in a Hall cross geometry.33 attached to the graphene Hall cross and can be used either for apply- Later, the same geometry is used by replacing the spin transport ing an electric current or for measuring the voltage along the Hall channel by graphene (see hybrid systems in Table I), providing a bar. FMs (typically Co) are used to both inject and detect the spin highly efficient spin injection that results in enhanced measured SCI current. Efficient spin injection and detection require the insertion efficiencies. of thin insulating barriers between the FM contacts and graphene to Over the last few years, SCI in lateral spin devices regained alleviate the conductance mismatch.168,169 attention. The rich variety of vdW materials and possible combina- SOT devices are made of bilayer heterostructures of FMs and tions opened up the possibility to realize SCI in all-vdW systems. large-SOC materials, the device geometry depending on the mea- When combining graphene, which possesses excellent spin trans- surement technique [Figs. 3(b) and 3(c)]. In the case of SOT ferro- port properties, with a non-conducting vdW material with large magnetic resonance (FMR) experiments [Fig. 3(c)], large-SOC/FM SOC, typically a semiconducting TMD, SCI occurs in graphene bilayers are embedded into high-frequency coplanar waveguides, due to proximity effects. In the graphene/TMD region, the SOC of whereas for harmonic Hall voltage measurements, the bilayer is graphene is enhanced and the electronic band structure is modified patterned in a Hall bar geometry [Fig. 3(b)]. Most of the vdW- significantly, with the most prominent features being the opening based SOT devices studied to date feature heterostructures com- of an energy gap in the meV range at the K and K′ points, lifting bining vdW materials with well-known bulk systems. Various stud- of spin degeneracy, and imprinting of an out-of-plane spin texture ies considered mechanically exfoliated TMD flakes (under vacuum (with the spin orientation depending on the valley and a winding or flowing N2 , or in a glovebox with inert Ar atmosphere) with a in-plane component far from the charge neutrality point).179 thin film of a FM (typically NiFe due to its low magnetic damping) The device geometry [as sketched in Fig. 3(a)] is slightly dif- deposited on top. Further studies spanned from exfoliated crystals ferent and consists of a graphene Hall cross with one of its arms to materials grown by CVD (TMDs such as MoS2 or WSe2 74,170 ) or shrouded with a TMD. The pristine graphene region (not covered MBE [TIs such as Bi2 Se69,70 and (Bi1−x Sbx )2 Te3 171,172 ]. The FM is by the TMD) is used to transport spin information from the spin deposited using grazing angle sputtering in various reports, to avoid injector to the Hall arm and vice versa. A spin current is injected damaging the underlying TMD, and capped to prevent oxidation. into graphene by means of a planar FM contact with magnetization Similarly, vdW FMs, such as Fe3 GeTe2 and Cr2 Ge2 Te6 , obtained along its long axis.10 When diffusing toward the Hall arm region, the from exfoliation methods can be combined with heavy-metal layers spins experience precession under an external magnetic field. The of Pt and Ta. Due to the high sensitivity of these 2D FMs to ambi- relative direction between the magnetic field and the injected spins ent conditions, they are typically rapidly transferred to a deposition is key to distinguish between the different spin-to-charge conversion chamber and the heavy metal layer is sputtered on top173,174 (in mechanisms (SGE or iSHE). In the case of the SGE, an out-of-plane APL Mater. 9, 100901 (2021); doi: 10.1063/5.0054865 9, 100901-5 © Author(s) 2021 TABLE I. SCI efficiencies in non-local devices based on metallic, hybrid, and all-vdW systems. The parameters are defined as follows: γSH ≡ jcy /jsz ; αSGE ≡ jcy /jsx (see x, y, and z directions in Fig. 3); jcy is the charge current flowing along the y direction, i.e., along the Hall cross; jsz is the spin current along the z direction (arising from the iSHE); jsx is the spin current along the x direction (arising from the SGE); λ–s and ∥ λs are the spin relaxation lengths of spins precessing in-plane and out-of-plane, respectively; and α∗SGE ≡ 2REv F ρτ ∥ , with RE being the strength of the SGE, ρ being the resistivity of the graphene beneath the © Author(s) 2021 TMD, v F being the Fermi velocity, and τ ∥ being the spin lifetime of the in-plane spins. SCI SCI SCI efficiency APL Materials Comments mechanism (%) (nm) References Metallic Cu/Ta ρ (Ta) = 333 μΩ cm (10 K) iSHE γSH (Ta) = −0.37 ± 0.20 (10 K) γSH ⋅λs (Ta) = 0.01 (10 K) 180 λs (Ta) = 2.7 nm (10 K) Cu/Pt ρ (Pt) = 17.9 μΩ cm (RT) iSHE γSH (Pt) = 0.9 (RT) γSH ⋅λs (Pt) = 0.06 (RT) 181 APL Mater. 9, 100901 (2021); doi: 10.1063/5.0054865 λs (Pt) = 7 nm (RT) Cu/Pt ρ (Pt) = 44.1 μΩ cm (10 K) iSHE γSH (Pt) = 8.5 ± 1.3 (10 K) γSH ⋅λs (Pt) = 0.06 (10 K) 182 λs (Pt) = 0.75 nm (10 K) Hybrid Bi2 O3 /graphene Bilayer graphene iSHE (in proximitized γSH = 0.10 ± 0.05 (RT) 183 λs not measured in the SCI region graphene) γSH = 0.6 ± 0.1 (10K) Pt/graphene Single-layer graphene iSHE (in Pt) γSH (Pt) = 15 ± 1 (RT) γSH λs (Pt) = 0.75 (RT) 184 ρ (Pt) = 46 μΩ cm (RT) λs (Pt) = 5 nm (RT; taken from the literature) Pt/graphene Few-layer graphene iSHE (in Pt) γSH (Pt) = 23.4 ± 2.5 (RT) γSH λs (Pt) = 0.50 ± 0.02 (RT) 185 ρ (Pt) = 134 μΩ cm (RT) λs (Pt) = (2.1 ± 0.4) nm (RT) (taken from the literature) All-vdW MoS2 /graphene Few-layer graphene iSHE (in proximitized γSH = −0.33 ± 0.04 (RT) 186 PERSPECTIVE λs not measured in the SCI region Spin absorption by MoS2 graphene and MoS2 ) γSH = −4.5 ± 0.9 (10 K) WS2 /graphene Single-layer graphene SHE–iSHE (in proximitized γSH = 0.3 (RT) γSH ⋅ λ–s (gr/WS2 ) = 3.75 (RT) 187 graphene) ∥ (Non-conducting) multilayer WS2 SGE–iSGE (in proximitized αISGE = 0.1 (RT) αISGE ⋅ λs (gr/WS2 ) = 0.42 (RT) graphene) – λs (gr/WS2 ) = 1.25 μm ∥ λs (gr/WS2 ) = 420 nm scitation.org/journal/apm 9, 100901-6 TABLE I. (Continued.) SCI SCI SCI efficiency © Author(s) 2021 Comments mechanism (%) (nm) References WS2 /graphene Single-layer graphene SHE–iSHE (in proximitized γSH = 0.13 (4 K), vanishes at Different definition of α∗SGE (see the 188 APL Materials graphene) T > 20K table caption) (Non-conducting) SGE–iSGE (in proximitized α∗SGE = 2.8 (4 K) single-layer WS2 graphene) λ–s (gr/WS2 ) = 1.58 α∗SGE = 0.56 (RT) μm ∥ λs (gr/WS2 ) = 313 nm APL Mater. 9, 100901 (2021); doi: 10.1063/5.0054865 WSe2 /graphene Few-layer graphene iSHE (in proximitized graphene) γSH = 1.7 ± 0.2 (RT) λs not measured in the SCI region 189 Spin absorption by γSH = 2.8 ± 0.3 (10 K) WSe2 2H–TaS2 /graphene Few-layer graphene SGE–iSGE (in proximitized αSGE = −1.4–4.3 (RT) 190 λs not measured in the SCI region Multilayer TaS2 graphene and TaS2 ) (Gate-tunable) (metallic) (BiSb)2 Te3 /graphene Single-layer graphene SGE (in proximitized graphene) αSGE = 0.17; 2.5; 1; 4.8 (RT) λs not measured in the SCI region 191 Multilayer BST MoTe2 /graphene Few-layer graphene Unconventional SCI (in MoTe2 ) αSCI ≥ 0.21 (RT) (conven- λs not measured in the SCI region 192 tional) Multilayer MoTe2 αSCI ≥ 0.10 (RT) (uncon- ventional) WTe2 /graphene Single-layer graphene Unconventional SCI (in WTe2 ) αSCI = 9 (RT) αSCI ⋅ λs (WTe2 ) = 0.72 (RT) 193 Multilayer WTe2 λs (WTe2 ) = 8 nm PERSPECTIVE scitation.org/journal/apm 9, 100901-7 APL Materials PERSPECTIVE scitation.org/journal/apm magnetic field causes an in-plane spin precession leading to a volt- experiments, the current-driven SOT induces precession of the mag- age along the Hall arm driven exclusively by the SGE. In the case netization, which results in time-dependent changes in the longi- of the iSHE, a magnetic field is applied to graphene along the spin tudinal and transverse resistances. Frequency mixing between the diffusion channel. The spins diffusing toward the Hall cross precess resistance and current oscillations leads to measurable direct cur- out-of-plane, generating a voltage driven exclusively by the iSHE rent and second-harmonic voltages [Figs. 3(e) and 3(f)]. The magni- [Fig. 3(d)]. tude, direction, and nature (conservative/dissipative) of the SOT are For the reciprocal effect, i.e., charge-to-spin conversion, a deduced from the field and angle dependences of these voltages. In charge current flowing along the Hall arm generates a transversal spin pumping experiments, magnetization precession is driven by spin current and a non-equilibrium in-plane spin density due to an external microwave magnetic field. Spins aligned with the aver- the SHE and the iSGE, respectively. In the case of the SHE, the age magnetization are pumped into the adjacent large-SOC material, spins point out-of-plane, whereas in the case of the iSGE, a non- where spin-to-charge conversion gives rise to a measurable direct equilibrium spin density with the spins pointing in-plane and per- charge current. pendicular to the charge current is generated at the graphene/TMD Because of the spin-polarized surface states of TIs, the TI/3D- interface. These two spin components (perpendicular to each other) FM bilayers are among the most widely used SOT systems that can be probed by a FM acting as a spin detector, with the magneti- include vdW materials. Seminal works showed very large SCI effi- zation perpendicular to both spin components, and are disentangled ciencies for Bi2 Se3 /NiFe69 and Bi2 Se3 /CoFeB.70 A strong Fermi level by varying the direction of the external magnetic field. dependence of the SOT was found in Bix Sb2−x Te3 /NiFe,171 and elec- SCI experiments in lateral spin devices were initially reported trostatic gating of the SOT was demonstrated in magnetic Cr-doped for multilayer graphene combined with MoS2 .186 Shortly after, Bix Sb2−x Te3 .196 A large in-plane damping-like SOT is generally spin–charge interconversions driven by both the SHE and SGE observed, enabling efficient current-driven magnetization switch- were probed in the graphene-WS2 heterostructure at room tempera- ing.196,197 Noteworthy, there is less evidence of large field-like SOT, ture.187,188 Remarkably, both SCI mechanisms exhibit a gate-tunable expected to be a clear signature of the iSGE. The reported damping- magnitude that can be controlled by electrostatic gating. This knob like SOT efficiency is typically 0.1–1, which is significantly larger is, in fact, essential to determine whether the SCI occurs exclu- than in heavy metals.41 The corresponding Edelstein inverse length sively in graphene. The electrostatic gating not only varies the carrier q is in the range 0.1–1 nm−1 .171,172 On the other hand, the Edelstein density in the graphene layer but also shifts the Fermi energy in length λ measured with spin pumping is typically 0.01–0.1 nm,198 the TMD. The latter may alter the insulating character, resulting which is rather low in comparison to other 2D systems (0.2–0.4 nm in spin absorption and related SCI within the TMD if the Fermi for the Bi/Ag Rashba interface199 and 2.1 nm for the surface of the level shifts toward the conduction (or valence) band. More recently, α-Sn TI200 ). SCI driven only by the SHE was reported for the graphene-WSe2 SOT measurements were reported for various TMD/3D-FM heterostructure.189 bilayers.201 Semiconducting (MoS2 , WS2 , and WSe2 ), semimetal- Beyond semiconducting TMDs, SCI experiments combin- lic (WTe2 , β-MoTe2 , TaTe2 , and PtTe2 ), and metallic (NbSe2 and ing graphene with other large-SOC materials were also reported TaS2 ) TMDs were interfaced with NiFe, Co, or CoFeB. Salient results for Bi2 O3 ,183 metallic layered 2H–TaSe2 ,190 the layered p-type are the gate-modulation of the SOT with WS2 202 and the genera- TI (Bi0.15 Sb0.85 )2 Te3 ,191 and the semimetals 1T′ -MoTe2 192 and tion of unconventional SOT with low-symmetry TMDs.203,204 ST- WTe2 .193 Overall, the SCI efficiency in all-vdW heterostructures, FMR measurements of 1T′ -WTe2 /NiFe devices revealed an unusual defined as γSH ⋅λs with λs being the spin relaxation length in out-of-plane damping-like SOT whose magnitude depends on the the region where the SCI takes place, compares favorably with direction of the current with respect to the crystallographic axes, fully metallic or hybrid systems based on heavy metals, such reflecting the reduced symmetry of the bilayer.203,204 2ω-Hall mea- as Pt and Ta. Table I summarizes experimental measurements surements in similar bilayers showed an anisotropic field-like SOT, of the SCI efficiency for metallic systems, hybrid systems, and which was attributed to the contribution of topological Fermi all-vdW heterostructures, specifying which SCI mechanism takes arcs.205 The reported SOTs are generally weaker with TMDs than place and where. We note that some studies consider an effec- with TIs. Nonetheless, efficient and field-free current-driven magne- tive λeff s of the whole channel (not only the SCI region, i.e., tization switching was recently achieved in WTe2 /NiFe.206 Further include a large contribution of regular graphene). These values work is required to clarify the SCI mechanisms at play with TMDs, are not included in Table I, as they could overestimate the SCI as contrasting results were reported about the damping-like/field- efficiency. like SOT ratios and the bulk or interfacial origin of the SOT. Spin pumping studies of TMD/FM systems are still scarce. Spin pump- ing measurements of MoS2 /YIG yielded an Edelstein length λ of B. SOT devices 0.4 nm.207 Aside from the non-local Hall cross, two other methods are More recently, the SOT was studied in large-SOC/2D-FM het- commonly employed to study the SCI in TMD/FM and TI/FM bilay- erostructures, with significant efficiencies and magnetization switch- ers [Figs. 3(b) and 3(c)]. The second-harmonic Hall (2ω-Hall)194 and ing reported in Fe3 GeTe2 /Pt.173,175 Single 2D-FM layers are also SOT ferromagnetic resonance (SOT-FMR)37 techniques both mea- interesting candidates for the generation of unconventional SOT. sure the current-driven SOT in the FM, from which the charge-to- For example, the low symmetry of monolayer Fe3 GeTe2 allows for spin conversion efficiency is inferred. Additionally, the spin pump- a SOT akin to a current-driven magnetic anisotropy.208,209 Exper- ing effect, based on the reciprocal effect of spin transfer, can be imental hints of such a SOT were also reported for multilayer used to measure the SCI efficiency.195 In 2ω-Hall and SOT-FMR Fe3 GeTe2 .210 APL Mater. 9, 100901 (2021); doi: 10.1063/5.0054865 9, 100901-8 © Author(s) 2021 APL Materials PERSPECTIVE scitation.org/journal/apm In contrast to lateral spin devices in which the spin injection, However, although computationally inefficient, Wannier Hamilto- transport, and detection are spatially well separated, these processes nians are the preferred choice for more complex vdW heterostruc- occur in vertical bilayers within a couple of nanometers near the tures.223,224 On the other hand, linear scaling quantum transport interface. As recently shown for TI/FM bilayers,172 the SOT is thus methodologies are the only computational approaches capable of extremely sensitive to the structural, chemical, and electronic prop- addressing the diffusive regime, since the systems involved demand erties of the interface, where band bending, orbital hybridization, for models containing many millions of atoms.225 Most of these spin memory loss, material intermixing, and magnetic dead layers methods are numerical implementations of the Kubo formula, either were reported.211–215 These phenomena are generally detrimental in the time or energy domain, allowing us to determine a system’s to the SOT, complicate the analysis of the SCI, and are probably properties for different transport regimes. responsible for the large experimental spread in the SCI efficien- For honeycomb-based vdW heterostructures, spin–valley cou- cies. It is expected that all-vdW heterostructures combining 2D- pling and broken inversion symmetry make the two valleys inequiv- FMs with TIs or TMDs will display enhanced SOT and will con- alent in terms of the spin and Berry curvature, providing a unique stitute model systems, thanks to their sharp and weakly interacting platform for efficient SCI.186,187,222,226,227 In Fig. 4, we present a interfaces. Indeed, a recent study found a SOT efficiency of 4.6 in sketch of how lowering the symmetries enables modifications in WTe2 /Fe3 GeTe2 .178 the band structure that enable different spin-related effects. In a Apart from the novel spin phenomena due to proximity effects highly symmetric system, such as graphene, combination of the between different vdW materials, there is promise to solve a chal- inversion and time-reversal symmetries does not allow for spin- lenge that exists in today’s magneto-resistive random access mem- momentum locking and, therefore, prevents the Rashba–Edelstein ory technology. Here, perpendicular magnetization switching is effect and its associated SOTs. In TMDs, the presence of chalco- more attractive because it is faster and more scalable. Efficiently gen and metal atoms lifts the inversion symmetry and enables switching a perpendicular magnetization using SOT requires the band-splitting due to the SOC. However, since there is a horizon- spins to have a large out-of-plane component (out-of-plane anti- tal mirror plane, the system cannot host a vertical electrical field, damping SOT), which is very difficult to realize by common which prevents the typical in-plane spin-momentum locking. Yet, materials. Recent evidence shows that vdW materials with low- the broken inversion symmetry enables a novel torque not related crystalline symmetry, in principle, can be used to control the direc- to the Rashba effect that acts as an electrically controlled magnetic tion of the current-induced SOT.204,216 A summary of experimental anisotropy, which is a promising gateway to magnet-free switch- parameters describing the SCI in vdW-based SOT devices is given ing of perpendicular magnetizations.228 In honeycomb-based het- in Table II. erostructures, the presence of dissimilar layers finally removes the horizontal mirror plane and reduces the symmetry to a C3v point group, which supports the conventional field-like and damping-like V. SCI: THEORETICAL CONTRIBUTIONS torques, as well as an electrically controlled magnetic anisotropy,228 Successful integration of vdW heterostructures in the techno- as demonstrated experimentally in L11 CoPt/CuPt bilayers.229 How- logical workflow relies on achieving efficient electrical control. For ever, this switching depends on the threefold rotation, and systems low-power applications, where a small current flow is essential, lin- such as graphene and TMDs, where the Fermi level lies on an ear response theory provides a general framework to determine any isotropic surface, are not expected to display the effect. Recently, observable electrical response. The central figure of merit of the SCI tremendous efforts focused on synthesizing a new class of mate- is the spin Hall angle γsH , which determines the amount of pure spin rials, the magnetic Janus TMDs, built by replacing the chalcogen current generated for an incoming charge current.33 For quantify- atoms on one side of a TMD with different ones. Janus materi- ing the SOT efficiency, it is usual to evaluate the torque efficiency als show simultaneously spin–valley coupling and a large Rashba χ τ , which measures the amount of torque felt by a magnet per unit effect due to their intrinsic electrical dipole178 and magnetism due of current density and can be rewritten as an effective spin Hall to the presence of magnetic metal atoms.224 Although Janus mate- angle.41 The crystal symmetries prescribe the spin orientation via rials are still in their infancy, they combine multiple desirable the SOC. In combination with group theory, the theory of invari- properties for technological applications and deserve scrutiny. For ants enables a relatively straightforward method to determine the instance, they are ideal for achieving all-in-one SOT, where the allowed directions.217,218 First, one identifies the minimum set of magnetization is electrically switchable in a single material via a point symmetries capable of expanding the whole point group (gen- current.224 erating elements) and then imposes invariance of the linear response The valley-driven spin Hall effect is a phenomenon where tensor with respect to those symmetries. By doing so, one finds a set the electron motion is monitored by a spin-dependent Lorentz of constraints limiting the number of finite elements and fixes the force opposite in each valley.186,187,227 The broken inversion sym- allowed directions for a given current. metry of heterostructures can also generate a Rashba interaction, The combination of disorder, exchange interaction, and SOC which, when combined with spin–valley coupling, leads to valley- defines the efficiencies. Therefore, it is essential to properly describe dependent SOT228 and yields optimal SCI via the Rashba–Edelstein the band structure, spin texture, and exchange splitting of the con- effect.227 The interplay between spin–valley coupling, Rashba inter- sidered vdW system and evaluate the role of disorder in the diffusive action, and magnetism is not extensively studied to date. In mag- regime, which is the most common experimental situation.219–221 netic systems with a Zeeman splitting, the valley-Zeeman effect221 Recent reports suggest that the use of symmetry-based models leads to valley polarization, as predicted for TMD/2D-magnet het- derived from ab initio methods provides the most cost-effective erostructures230,231 and recently realized in WSe2 /CrI3 .232 Never- way to describe simple systems, such as graphene220 and WTe2 .222 theless, understanding the combined impact of the valley-Zeeman APL Mater. 9, 100901 (2021); doi: 10.1063/5.0054865 9, 100901-9 © Author(s) 2021 TABLE II. SCI in vdW-based SOT devices. The parameters are defined in Ref. 41, where additional non-vdW structures are reviewed. For TMDs, a complementary comparative study can be found in Ref. 201. j j E E BDL /j BFL /j ξDL ξFL ξDL ξFL qREE λIREE © Author(s) 2021 11 2 5 −1 −1 Other Fabrication Method mT/(10 A/m ) Dimensionless ̵ (Ωm) 10 h/2e nm nm torques References APL Materials Non-vdW Pt(3)/Co(0.6)/AlOx (1.6) Sputtering 2ω-Hall −6.9 4 0.13 −0.073 3.5 −2 41 Ta(1.5–5)/CoFeB Sputtering 2ω-Hall 1.3–4.4 −(2–19) −(0.03–0.11) 0.04–0.47 −(0.14–0.68) 0.22–0.67 (0.8–1.1)/MgO(2) Bi(8)/Ag(5–20)/Py(15) Evaporation Spin 0.2–0.4 199 pumping α-Sn(30 Ml)/Ag(2)/Fe(5) MBE Spin 2.1 200 APL Mater. 9, 100901 (2021); doi: 10.1063/5.0054865 pumping TI Bi2 Se3 (8)/Py(8,16)/ MBE ST-FMR 2.0–3.5a 2.5–2.8b 1.1–2.0 1.4–1.6 69 AlOx (2) Bi2 Se3 (20)/CoFeB(5)/ MBE ST-FMR 70 MgO(1)/SiO2 (3) RT 0.05–0.11 0–0.1 ∼20 K ∼0.4 ∼0.2 Bi2 Se3 (10)/Ag(5)/CoFeB(7)/ MBE ST-FMR 5.3 3.2 0.5c 0.3b 233 MgO(2)/SiO2 (4) Bi2 Se3 (7.4)/ MBE Coercivity 6.1 0.16 71 CoTb(4.6)/SiNx (3) Bi2 Se3 (8)/Py(6)/ MBE Coercivity 1.71 72 MgO(1)/SiO2 (4) Bi2 Se3 (5)/CoFeB(7)/ MBE ST-FMR 1.75 0.82 72 MgO(2)/Al2 O3 (3) Bi2 Se3 (20)/CoFeB(7)/ 0.3c ∼0.1c MgO(2)/Al2 O3 (3) Bi2 Se3 (5–35)/ MBE Spin 0.0093 234 PERSPECTIVE Py(20)/SiO2 (30) + Ar milling pumping Bi2 Se3 (5,10)/ MBE Spin 0.021–0.43 235 CoFeB(5)/MgO(2) pumping Bi2 Se3 (10)/CoFeB(5)/MgO(2) MBE ST-FMR 0.3–1.7 235 c Ge/Bi2 Se3 (10) MBE Non-local −0.026 236 Bix Se1-x (4)/CoFeB(5)/ Sputtering 2ω-Hall 99 18.6 1.45 73 MgO(2)/Ta(5) ST-FMR 8.7 YIG/Bi2 Se3 (6–60) MBE Spin 0.035 237 pumping scitation.org/journal/apm 9, 100901-10 TABLE II. (Continued.) j j E E BDL /j BFL /j ξDL ξFL ξDL ξFL qREE λIREE © Author(s) 2021 11 2 5 −1 −1 Other Fabrication Method mT/(10 A/m ) Dimensionless ̵ (Ωm) 10 h/2e nm nm torques References MBE Spin 0.017 APL Materials YIG/(Bi,Sb)2 Te3 (6) pumping MgO(2)/Bi2 Te3 (8)/ Sputtering 2ω-Hall 87 ∼0 3.3 238 CoTb(6)/TaOx (1.5) (Bi1-x Sbx )2 Te3 (8)/ MBE ST-FMR 171 Cu(8)/Py(10) x = 0.5, 0.7, 0.9 ∼0.5c 0.45–0.57 0.8 < x < 0.9 (1–5)a 0.1–0.2 APL Mater. 9, 100901 (2021); doi: 10.1063/5.0054865 (EF near Dirac point) (Bi0.4 Sb0.6 )2 Te3 (9)/ MBE ST-FMR 0.02–0.1 0 < −BFL < BOe c (0.88)a 0.19–0.88 172 Py(5)/AlOx (2) mT/(A/m)d (Bi0.4 Sb0.6 )2 Te3 (9)/Ag(7)/ MBE ST-FMR 0.01–0.07 BFL ≈ −BOe c 0.12–0.53 172 d Py(5)/AlOx (2) mT/(A/m) (Bi0.4 Sb0.6 )2 Te3 (9)/Al(6)/ MBE ST-FMR 0.01–0.06 0 < BOe < −BFL c 0.11–0.48 172 Py(5)/AlOx (2) mT/(A/m)d Cr0.16 (Bi0.54 Sb0.38 )2 Te3 (6)/ MBE 2ω-Hall 48 000 1.9 K –146 000 140–425e 177 (Bi0.5 Sb0.5 )2 Te3 (3) Cr0.16 (Bi0.5 Sb0.42 )2 Te3 (7)/ MBE 2ω-Hall 46 900b 116e 196 Al2 O3 (20) 1.9 K (Bi0.22 Sb0.78 )2 Te3 (6)/ MBE Spin 0.075 198 Py(12) pumping (Bi0.22 Sb0.78 )2 Te3 (6)/ MBE Spin- 0.076 198 NiO(5)/Py(12) Seebeck effect TMD MoS2 (0.8)/CoFeB(3)/ CVD 2ω-Hall ∼0 0.078b ∼0 −0.14 ∼0 0.0288 74 TaOx (3) PERSPECTIVE WSe2 (0.8)/CoFeB(3)/ CVD 2ω-Hall ∼0 0.114b) ∼0 ∼0 0.0552 TaOx (3) MoS2 (0.8)/Py(5) CVD ST-FMR Large 170 WTe2 (1.8–15)/Py(6)/ Exfoliation ST-FMR/ 0.03∗ ∼0 0.08 0.09 Out-of- 204 AlOx (2) 2ω-Hall plane DL WTe2 (5.6–7)/Py(6)/Ru(4) Exfoliation 2ω-Hall ∼1f 205 RT BFL ≫ BOe <50 K ∼2.5f WTe2 (20–31)/Py(6)/Ru(4) Exfoliation 2ω-Hall ∼0 205 RT <50 K ∼4f scitation.org/journal/apm 9, 100901-11 TABLE II. (Continued.) j j E E BDL /j BFL /j ξDL ξFL ξDL ξFL qREE λIREE © Author(s) 2021 11 2 5 −1 −1 Other Fabrication Method mT/(10 A/m ) Dimensionless ̵ (Ωm) 10 h/2e nm nm torques References Exfoliation ST-FMR 0.09–0.79 ∼0 206 APL Materials WTe2 (5.8–122)/Py(6) coercivity 0.02–0.6f Out-of-plane DL 0.15–0.65 YIG/MoS2 (2.4) Exfoliation Spin 0.32 0.4 207 pumping TaTe2 (4.5–19.7)/Py(6)/ Exfoliation ST- ∼0 ∼0 ∼0 ∼0 ∼0 ∼0 239 AlOx (2) FMR/2ω- Hall APL Mater. 9, 100901 (2021); doi: 10.1063/5.0054865 MoTe2 (0.7–14.2)/ Exfoliation ST-FMR 0.058 0.15 Out-of-plane DL 240 Py(6)/AlOx (2) PtTe2 (3–20)/Py(2.5–10) CVD ST-FMR 0.05–0.15 −0.004 0.2–1.6 241 NbSe2 (0.6–6)/ Exfoliation ST-FMR 0.03 0.40 Out-of-plane DL 242 Py(6)/AlOx (2) TaS2 (0.88)/Py(7)/AlOx (3) Plasma- ST-FMR 0.25 ∼0 14.9 ∼0 243 assisted sulfuriza- tion WTe2 (10)/CoTb(6)/Ta(2) Sputtering Magnetic 0.20 244 loop shift 2D FM Fe3 GeTe2 (4)/Pt(6) Exfoliation 2ω-Hall 53.4 24.3 173 low T Cr2 Ge2 Te6 (19.6)/Pt(10) Exfoliation 2ω-Hall 2.0 ∼0 0.25 ∼0 174 5K Fe3 GeTe2 (23)/Pt(5) Exfoliation 2ω-Hall 0.14 175 180 K Cr2 Ge2 Te6 (8)/Ta(5) Exfoliation Coercivity ∼80 176 PERSPECTIVE WTe2 (12.6)/Fe3 GeTe2 (7.3)/ Exfoliation Coercivity 4.6 2.25 178 AlOx (2.6) 160 K Fe3 GeTe2 (6–21) Exfoliation Coercivity Anisotropy torque 210 2K a Assuming 3D carriers. b Inferred from other quantities. c Contribution of non-topological Rashba states included in the interpretation. d Assuming 2D carriers. e j Reference 197 later showed that ξDL is largely overestimated in magnetic TIs when the asymmetric magnon scattering is disregarded in 2ω-Hall measurements. f Anisotropic. scitation.org/journal/apm 9, 100901-12 APL Materials PERSPECTIVE scitation.org/journal/apm based on first-principles. For instance, Wannier interpolation tech- niques, which boil down to fitting the band structure using maxi- mally localized Wannier functions, have proven to be very accurate in realistic charge transport simulations. Nonetheless, their exten- sion to the investigation of non-equilibrium spin properties can be problematic, as the Wannier functions do not necessarily map the crystal structure accurately. This limitation is particularly serious for SOT simulations. Indeed, the interplay between magnetism and SOC in vdW heterostructures of interest demands special care in the study of resulting ground state spin textures in the reciprocal space as well as their modification under external electric or mag- netic fields. On the experimental side, current fabrication techniques demand specific material treatment to prevent rapid degradation of produced devices, which embed some materials sensitive to oxi- dation effects. Efforts to synthesize materials and fabricate devices in situ are under way, but experimental methods to produce more air-stable heterostructures are an important task to be dealt with in the near future. As a conclusion, the field of SCI in vdW heterostruc- FIG. 4. Transition of the band structure and spin orientation of Dirac materials tures is attracting more and more attention from various scientific while lowering the symmetries. The quantization axis is assumed to be out-of- plane. Red and blue colors represent the bands with up and down spin. The D6h communities, which gives hope that progress will be achieved in point group contains 24 symmetries, including the inversion and time-reversal sym- the years to come in the design of ultra-compact spin devices of metries, which prevents spin splitting and only allows for the iSHE as the SCI relevance for non-volatile technology and spin logics. mechanism due to the SOC. The D3d point group contains 12 symmetries, with the inversion symmetry absent, allowing for the SHE and spin-splitting and anisotropy- like SOT. Finally, the C3v point group contains six symmetries and allows for the ACKNOWLEDGMENTS SHE, anisotropy-like SOT, and Rashba–Edelstein effect. The authors thank H. Okuno for the images in Figs. 2(d)–2(h). All authors acknowledge financial support from the King Abdullah University of Science and Technology under Grant No. ORS-2018- and Rashba interactions on the magnetization dynamics remains CRG7-3717. The ICN2 authors were also supported by the European to be achieved and, beyond resolving some inconsistent reports Union Horizon 2020 research and innovation program under Grant of its symmetries,201 can lead to unprecedented features for effi- Agreement Nos. 881603 (Graphene Flagship), 824140 (TOCHA, cient SOT applications such as zero magnetic field switching, a H2020-FETPROACT-01-2018), and 840588 (GRISOTO, Marie long sought-after mechanism for improving non-volatile memory Sklodowska-Curie fellowship). ICN2 is also funded by the CERCA technologies. Programme/Generalitat de Catalunya and is supported by the Severo Ochoa program from Spanish MINECO (Grant Nos. SEV- VI. CONCLUSIONS AND OUTLOOK 2017-0706, PID2019-111773RB-I00/AEI/10.13039/501100011033, In this Perspective, we discussed the main challenges and and RYC2019-028368-I/AEI/10.13039/501100011033). The CNRS- recent results concerning materials synthesis, device fabrication, CEA authors acknowledge financial support from the European and optimization for eventually achieving the highest figures of Union Horizon 2020 research and innovation program under Grant merit for SCI in vdW heterostructures. On the material growth Agreement No. 881603 (Graphene Flagship), the French ANR side, MBE is becoming an essential technique to obtain magnetic projects MAGICVALLEY (Grant No. ANR-18-CE24-0007), and layered materials, although CVD-growth approaches and integra- ELMAX (Grant No. ANR-20-CE24-0015) and from the UGA IDEX tion of two-dimensional materials in nano-electronics were already IRS/EVASPIN. demonstrated. The next challenge is to demonstrate efficient inte- gration of vdW materials in spintronic building blocks, such as in DATA AVAILABILITY magnetic tunneling junctions, stimulating further research toward industry-oriented activities, for instance, in the fields of spin-transfer Data sharing is not applicable to this article as no new data were torque and SOT magneto-resistive random access memories. Cur- created or analyzed in this study. rent results concerning the SCI in vdW heterostructures combining 2D systems and topological materials are already encouraging, since REFERENCES they evidence high figures of merit even at room temperature in 1 A. C. Ferrari, F. Bonaccorso, V. Fal’ko, K. S. Novoselov, S. Roche, P. Bøggild, scalable materials. Besides these encouraging experimental results, S. Borini, F. H. L. Koppens, V. Palermo, N. Pugno, J. A. Garrido, R. Sordan, A. advanced modeling techniques make it possible to reach a deep Bianco, L. Ballerini, M. Prato, E. Lidorikis, J. 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