Engineering two-dimensional material heterostructures by combining the best of different materials in one ultimate unit can offer a plethora of opportunities in condensed matter physics. Here, in the van der Waals heterostructures of the ferromagnetic insulator Cr2Ge2Te6 and graphene, our observations indicate an out-of-plane proximity-induced ferromagnetic exchange interaction in graphene. The perpendicular magnetic anisotropy of Cr2Ge2Te6 results in significant modification of the spin transport and precession in graphene, which can be ascribed to the proximity-induced exchange interaction. Furthermore, the observation of a larger lifetime for perpendicular spins in comparison to the in-plane counterpart suggests the creation of a proximity-induced anisotropic spin texture in graphene. Our experimental results and density functional theory calculations open up opportunities for the realization of proximity-induced magnetic interactions and spin filters in 2D material heterostructures and can form the basic building blocks for future spintronic and topological quantum devices.

Discovery of topological Weyl semimetals has revealed the opportunities to realize several extraordinary physical phenomena in condensed matter physics. Specifically, these semimetals with strong spin-orbit coupling, broken inversion symmetry and novel spin texture are predicted to exhibit a large spin Hall effect that can efficiently convert the charge current to a spin current. Here we report the direct experimental observation of a large spin Hall and inverse spin Hall effects in Weyl semimetal WTe2 at room temperature obeying Onsager reciprocity relation. We demonstrate the detection of the pure spin current generated by spin Hall phenomenon in WTe2 by making van der Waals heterostructures with graphene, taking advantage of its long spin coherence length and spin transmission at the heterostructure interface. These experimental findings well supported by ab initio calculations show a large charge-spin conversion efficiency in WTe2; which can pave the way for utilization of spin-orbit induced phenomena in spintronic memory and logic circuit architectures. Figure S1. Devices with WTe2-graphene heterostructure. (a) Optical micrograph of Device 1, with WTe2graphene heterostructure and ferromagnetic (TiO2 1 nm/Co 60 nm) contacts on graphene for detection and creation of (I)SHE in WTe2. The scale bar is 2 μm. (b) Atomic force microscope (AFM) picture of the heterostructure area (red mark in (a)). The inset is the thickness profile of WTe2 on few-layer graphene along the white dash line showing the WTe2 thickness is 27nm in Device 1. (c) AFM picture and thickness profile of Dev 3 (after the device burnout) showing WTe2 thickness to be 11 nm on monolayer CVD graphene.

Unique electronic spin textures in topological states of matter are promising for emerging spin-orbit driven memory and logic technologies. However, there are several challenges related to the enhancement of their performance, electrical gate-tunability, interference from trivial bulk states, and heterostructure interfaces. We address these challenges by integrating two-dimensional graphene with a three-dimensional topological insulator (TI) in van der Waals heterostructures to take advantage of their remarkable spintronic properties and engineer proximity-induced spin-charge conversion phenomena. In these heterostructures, we experimentally demonstrate a gate-tunable spin-galvanic effect (SGE) at room temperature, allowing for efficient conversion of a non-equilibrium spin polarization into a transverse charge current. Systematic measurements of SGE in various device geometries via a spin switch, spin precession, and magnetization rotation experiments establish the robustness of spin-charge conversion in the Gr-TI heterostructures. Importantly, using a gate voltage, we reveal a strong electric field tunability of both amplitude and sign of the spingalvanic signal. These findings provide an efficient route for realizing all-electrical and gatetunable spin-orbit technology using TIs and graphene in heterostructures, which can enhance the performance and reduce power dissipation in spintronic circuits.

An outstanding feature of topological quantum materials is their novel spin topology in the electronic band structures with an expected large charge‐to‐spin conversion efficiency. Here, a charge‐current‐induced spin polarization in the type‐II Weyl semimetal candidate WTe2 and efficient spin injection and detection in a graphene channel up to room temperature are reported. Contrary to the conventional spin Hall and Rashba–Edelstein effects, the measurements indicate an unconventional charge‐to‐spin conversion in WTe2, which is primarily forbidden by the crystal symmetry of the system. Such a large spin polarization can be possible in WTe2 due to a reduced crystal symmetry combined with its large spin Berry curvature, spin–orbit interaction with a novel spin‐texture of the Fermi states. A robust and practical method is demonstrated for electrical creation and detection of such a spin polarization using both charge‐to‐spin conversion and its inverse phenomenon and utilized it for efficient spin injection and detection in the graphene channel up to room temperature. These findings open opportunities for utilizing topological Weyl materials as nonmagnetic spin sources in all‐electrical van der Waals spintronic circuits and for low‐power and high‐performance nonvolatile spintronic technologies.

Dirac material–based heterostructures reveal enhanced spin-orbit coupling with an emerging spin texture.

can allow the discovery of basic new physical phenomena and the development of new device concepts. [1] The discovery of new vdW quantum materials and their heterostructures starting from graphene, insulators, semiconductors, superconductors, and topological materials has revolutionized both fundamental and applied research. [2,3] The most recent addition to this vdW family is magnets, which have offered various advantages over conventional magnets and opened new perspectives in vdW heterostructure designs. [4][5][6] In addition to the atomically thin and flat nature of vdW magnets, flexibility, gate tunability, strong proximity interactions, and twist angle between the layers can offer a unique degree of freedom and an innovative platform for device functionalities. [4,5] Recently, several vdW magnets have emerged with the discovery of insulating Cr 2 Ge 2 Te 6 , [7] semiconducting (CrI 3 , [8] CrBr 3 [9] ), and metallic Fe x GeTe 2 . [10,11] The insulating vdW magnets are useful for spin-filter tunneling [9,12] and proximityinduced magnetism, [13][14][15] whereas the metallic magnets can be used as electrodes in magnetic tunnel junctions, [16] observationThe discovery of van der Waals (vdW) magnets opened a new paradigm for condensed matter physics and spintronic technologies. However, the operations of active spintronic devices with vdW ferromagnets are limited to cryogenic temperatures, inhibiting their broader practical applications. Here, the robust room-temperature operation of lateral spin-valve devices using the vdW itinerant ferromagnet Fe 5 GeTe 2 in heterostructures with graphene is demonstrated. The room-temperature spintronic properties of Fe 5 GeTe 2 are measured at the interface with graphene with a negative spin polarization. Lateral spin-valve and spin-precession measurements provide unique insights by probing the Fe 5 GeTe 2 /graphene interface spintronic properties via spin-dynamics measurements, revealing multidirectional spin polarization. Density functional theory calculations in conjunction with Monte Carlo simulations reveal significantly canted Fe magnetic moments in Fe 5 GeTe 2 along with the presence of negative spin polarization at the Fe 5 GeTe 2 / graphene interface. These findings open opportunities for vdW interface design and applications of vdW-magnet-based spintronic devices at ambient temperatures.

The Dirac surface states of topological insulators offer a unique possibility for creating spin polarized charge currents due to the spin-momentum locking. Here we demonstrate that the control over the bulk and surface contribution is crucial to maximize the charge-to-spin conversion efficiency. We observe an enhancement of the spin signal due to surface-dominated spin polarization while freezing-out the bulk conductivity in semiconducting Bi 1.5 Sb 0.5 Te 1.7 Se 1.3 below 100 K. Detailed measurements up to room temperature exhibit a strong reduction of the magnetoresistance signal between 2 − 100 K, which we attribute to the thermal excitation of bulk carriers and to the electron-phonon coupling in the surface states. The presence and dominance of this effect up to room temperature is promising for spintronic science and technology.

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