Room temperature energy-efficient spin-orbit torque switching in two-dimensional van der Waals Fe₃GeTe₂ induced by topological insulators

The study addresses how to achieve low-power, room-temperature magnetization switching in spintronic devices using spin-orbit torques (SOT). While spin-transfer torque MRAM offers nonvolatility and density, it suffers from incubation delay and high write currents. SOT-based switching can overcome these drawbacks, enabling faster, more energy-efficient operation. Conventional SOT systems use heavy metals (W, Ta, Pt) as spin current sources, but interfacial limitations with 3D ferromagnets reduce efficiency. Two-dimensional van der Waals ferromagnets like Fe3GeTe2 (FGT) provide atomically flat interfaces and robust magnetism down to the 2D limit. Topological insulators (TIs) with spin-momentum-locked surface states promise higher spin-charge conversion. The research goal is to realize and quantify room-temperature, energy-efficient SOT switching in an all-vdW TI/FGT heterostructure and to elucidate the mechanisms behind the high efficiency.

Prior work has demonstrated SOT switching using heavy metals via the spin Hall and Rashba-Edelstein effects, but typically with 3D ferromagnets and at higher currents. TIs (e.g., Bi2Se3, Bi2Te3) have enabled high-efficiency SOT switching in 3D ferromagnets at room temperature due to topological surface states (TSS). However, 3D ferromagnets limit scaling and interfacial spin transparency. 2D ferromagnets such as FGT, CrI3, and Cr2Ge2Te6 challenge the Mermin–Wagner–Hohenberg theorem via magnetic anisotropy, enabling intrinsic 2D ferromagnetism. Previous FGT/Pt heterostructures showed SOT switching only below ~200 K. Literature suggests that all-vdW heterostructures can enhance spin transparency by providing clean, dangling-bond-free interfaces, and that TIs can significantly boost SOT efficiency via TSS and bulk spin Hall contributions.

Samples were grown on (0001) sapphire substrates by molecular beam epitaxy (MBE) using high-purity Bi, Fe, Ge, and Te sources in a base vacuum of 1e-10 Torr. Growth was monitored in situ by reflection high-energy electron diffraction (RHEED), including azimuthal rotation to confirm in-plane crystallinity and exclude multidomain formation. For Bi2Te3 bottom layers, the growth temperature was carefully ramped under Te-rich conditions to maintain flatness and single crystallinity. FGT surfaces were capped to prevent degradation. Surface morphology was characterized by atomic force microscopy (AFM); crystal structure by X-ray diffraction (XRD); and cross-sections by HAADF-STEM. Hall-bar devices (100 µm × 30 µm) were fabricated via photolithography and ion beam etching. Magnetic properties were measured by SQUID magnetometry and magneto-optical Kerr effect (MOKE); magnetotransport by a PPMS. SOT switching experiments applied 10-ms current pulses along the bar under in-plane fields (Hext) to break symmetry in perpendicular magnetic anisotropy (PMA) samples. Harmonic Hall measurements used an AC current (lock-in at 133.33 Hz) to extract first and second harmonic Hall voltages. The second harmonic Hall resistance R2 was analyzed to separate damping-like (DL) and thermal (ANE) contributions, fitting field dependence at large in-plane fields to determine the effective DL field HDL. The DL SOT efficiency was computed via εDL = (2e Ms t HDL)/(ħ Jac). Thermal artifacts from the anomalous Nernst effect (ANE) were mitigated by tuning FGT thickness to adjust current shunting and reduce vertical thermal gradients. Temperature- and thickness-dependent studies were performed to distinguish TSS vs bulk contributions to SOT.

• All-van der Waals Bi2Te3/Fe3GeTe2 heterostructures exhibit perpendicular magnetic anisotropy and room-temperature ferromagnetism, with Bi2Te3 interfacial exchange raising the effective Curie temperature of FGT relative to standalone films (pure FGT Tc ~220 K). • Deterministic SOT-driven magnetization switching was demonstrated with in-plane bias fields. At 200 K, complete switching occurred at Jwrite ≈ 4×10^6 A/cm^2; switching chirality reverses with Hext. At room temperature (Hext = ±2 kOe), current-induced switching was achieved with a critical current density reported ~2.2×10^6 A/cm^2. • Harmonic Hall analyses revealed HDL/Jwrite ≈ −160.2 Oe per MA/cm^2 (200 K) and a damping-like SOT efficiency εDL ≈ −5.3 for Bi2Te3(8)/FGT(3) at 200 K. • By optimizing thickness to Bi2Te3(8)/FGT(4), the room-temperature damping-like efficiency θDL ≈ −0.7 was obtained; further tuning TI thickness (6–10 nm) increased the room-temperature SOT efficiency to as high as −2.69, evidencing strong charge–spin conversion. • Temperature dependence shows θDL increases markedly upon cooling, correlating with a reduced bulk contribution and Fermi level approaching the Dirac cone, indicating that TSS dominate the SOT in this system. • Thermal contributions (ANE) can produce step-like features in second-harmonic signals; increasing FGT thickness to 4 nm suppresses ANE signatures, clarifying SOT extraction. • Compared to heavy-metal systems and previously reported TI/ferromagnet stacks, the all-vdW TI/FGT interface yields enhanced interfacial spin transparency and high SOT efficiency at room temperature.

The results directly address the challenge of room-temperature, low-power SOT switching in 2D ferromagnets by employing an all-vdW Bi2Te3/FGT heterostructure. The observed high SOT efficiencies and reduced switching current densities are attributed to two key factors: (1) strong spin–momentum locking in Bi2Te3’s topological surface states, enhancing charge–spin conversion; and (2) the clean vdW interface between TI and FGT, which improves interfacial spin transparency by minimizing spin backflow and spin memory loss. The temperature and thickness dependencies of θDL support a TSS-dominant mechanism, as efficiency increases when bulk conduction diminishes and Fermi level shifts toward the Dirac surface states. Mitigation of thermal artifacts (ANE) through thickness engineering strengthens the reliability of the harmonic analysis. Collectively, these findings establish a robust route to room-temperature SOT switching in 2D vdW ferromagnets with practical current densities and suggest that interface and band-engineering in all-vdW stacks can further optimize device performance.

The study demonstrates wafer-scale, all-vdW Bi2Te3/FGT heterostructures with room-temperature ferromagnetism and deterministic SOT switching. Harmonic Hall measurements reveal large damping-like SOT efficiencies: approximately −0.7 at room temperature for Bi2Te3(8)/FGT(4), increasing up to −2.69 by tuning TI thickness, and εDL ≈ −5.3 at 200 K for Bi2Te3(8)/FGT(3). These efficiencies and the achieved switching currents at room temperature highlight the advantages of topological charge–spin conversion and vdW interfaces with high spin transparency. Future work should focus on field-free switching strategies, further interface engineering to maximize spin transmissivity, optimizing thickness and Fermi-level position to favor TSS, minimizing thermal effects, and scaling down device dimensions toward integrated, low-power 2D spintronic technologies.

Deterministic switching required an external in-plane magnetic field; field-free operation was not demonstrated. Thermal effects, notably the anomalous Nernst effect, can contaminate harmonic signals and required thickness engineering to mitigate. Joule heating at higher current densities reduces Ms and degrades switching characteristics. Reported switching current densities vary with temperature, device geometry, and thickness, indicating sensitivity to fabrication and material parameters. Measurements were performed on micrometer-scale Hall bars; further scaling and device integration remain to be explored.