Spin-transfer torque magnetic random access memory (STT-MRAM) faces limitations such as incubation delay and high writing currents. Spin-orbit torque (SOT) offers advantages in terms of speed, endurance, and energy efficiency, making it a promising candidate for next-generation memories. Research focuses on identifying materials and principles for high-performance SOT devices. Heavy metals like W, Ta, and Pt have been used as spin current sources, but van der Waals (vdW) topological insulators (TIs) offer potential for higher efficiency due to spin-momentum locking in their topological surface states (TSS). While demonstrated in 3D ferromagnets, using 3D ferromagnets limits size scaling and spin transparency. 2D vdW ferromagnetic materials, such as Fe₃GeTe₂ (FGT), circumvent this limitation by possessing atomically flat surfaces and maintaining magnetic ordering down to the 2D limit, overcoming the Mermin-Wagner-Hohenberg (MWH) theorem limitations through magnetic anisotropy. Previous work using Pt as a spin current source with FGT demonstrated SOT switching only at low temperatures. This study aims to create an all-vdW heterostructure to achieve room-temperature, energy-efficient SOT switching using the unique properties of a TI/FGT combination.

Extensive research explores new materials and principles for high-performance spin-orbit torque (SOT) devices. Heavy metals (W, Ta, Pt) have been employed as spin current sources due to their charge-spin conversion capabilities. However, vdW topological insulators (TIs) are now considered superior due to their spin-momentum locking in the non-trivial topological surface state (TSS), leading to high-efficiency SOT-driven magnetic switching in 3D ferromagnets at room temperature with low critical switching currents. The limitations of 3D ferromagnets, such as size scaling and reduced spin transparency due to dangling bonds, necessitate the exploration of lower-dimensional materials with superior interfaces. 2D vdW ferromagnetic materials, such as FGT, offer atomically flat surfaces and maintain magnetic ordering in the 2D limit. While FGT's SOT-driven magnetization switching has been demonstrated with Pt as a spin current source, this only works at low temperatures. Constructing an all-vdW heterostructure promises higher SOT efficiency due to clean interfaces and high interfacial spin transparency, addressing the need for room-temperature, energy-efficient SOT switching for 2D spintronic applications.

The researchers used molecular beam epitaxy (MBE) to grow thin films on a (0001) sapphire substrate. Reflection high-energy electron diffraction (RHEED) in situ monitored the surface structure during growth, and atomic force microscopy (AFM) analyzed surface morphology. To ensure high-quality wafer-scale all-vdW heterostructures, the growth temperature was carefully controlled to maintain a Te-rich environment and achieve excellent single crystallinity. RHEED was used to verify the in-plane crystallinity. A protective layer covered the FGT surface to prevent degradation. Micrometer-sized Hall-bar devices were fabricated using standard photolithography and ion beam etching. Magneto-transport measurements were performed on Bi₂Te₃, FGT, and the Bi₂Te₃/FGT heterostructure. The characterization included analysis of Hall resistance, temperature-dependent 2D carrier density, and hysteresis loops to determine magnetic properties like Curie temperature (T_c), saturation magnetization (M_s), and perpendicular magnetic anisotropy (PMA). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) confirmed the atomic structure. For SOT switching studies, a series of in-plane magnetic fields were applied with a 10-ms pulse current. Harmonic Hall measurements, involving applying a small sinusoidal current, were used to quantify SOT efficiency by analyzing the damping-like and field-like torque components. The thermal contribution from the anomalous Nernst effect (ANE) was considered and mitigated by adjusting FGT thickness. Different FGT thicknesses were investigated to optimize SOT efficiency and minimize the ANE's influence. Temperature-dependent SOT efficiency and its relation to the Fermi level (E_F) were analyzed to understand the contribution of the topological surface state (TSS) and bulk state to the SOT.

The study successfully achieved room-temperature SOT-driven magnetic switching in an MBE-grown all-vdW Bi₂Te₃/FGT heterostructure. A critical switching current density of ~2.2 × 10⁶ A/cm² was observed at 200 K. The damping-like SOT efficiency was calculated to be ~2.69 at room temperature. The high efficiency is attributed to the superior characteristics of the all-vdW heterostructure. Analysis of harmonic Hall measurements revealed a significant thermal contribution, mainly due to the anomalous Nernst effect (ANE) in FGT. By adjusting FGT thickness, the ANE's influence was minimized, resulting in a room-temperature damping-like SOT efficiency of ~0.7. Further experiments with varying Bi₂Te₃ thicknesses showed that the SOT efficiency dramatically increases to -2.69, highlighting the substantial contribution of the topological surface state at room temperature. The temperature-dependent SOT efficiency demonstrates the dominant role of the TSS in achieving high SOT efficiency, as the contribution from the bulk state decreases with decreasing temperature. The chirality of the SOT was consistent with previous findings, confirming that both the TSS and bulk state contribute to the SOT with positive spin Hall angles. Finally, room-temperature SOT switching was demonstrated in the Bi₂Te₃(8)/FGT(4) heterostructure.

The results demonstrate a significant advancement in 2D spintronics by achieving room-temperature, energy-efficient SOT switching in an all-vdW heterostructure. The high SOT efficiency, particularly the enhanced efficiency with thinner TI layers, strongly supports the significant role of the topological surface states in the Bi₂Te₃ layer. The minimization of the ANE's influence through thickness optimization further validates the approach. The findings are highly relevant to the development of low-power, high-performance spintronic devices, particularly for applications requiring room-temperature operation. The all-vdW nature of the heterostructure ensures high interfacial spin transparency and compatibility for future integration.

This work successfully demonstrated room-temperature SOT switching in an all-vdW Bi₂Te₃/Fe₃GeTe₂ heterostructure with high efficiency, primarily attributed to the topological surface states of Bi₂Te₃. This achievement paves the way for the development of energy-efficient room-temperature 2D spintronic devices. Future research could focus on exploring other 2D vdW ferromagnetic materials and TIs to further optimize SOT efficiency and explore device applications.

The study primarily focused on a specific heterostructure configuration. Further investigations with different combinations of TIs and 2D ferromagnets are needed to generalize the findings. The impact of potential interfacial defects or imperfections on SOT efficiency requires further investigation. While the ANE's effect was minimized, it was not entirely eliminated, suggesting further optimization strategies could be explored.