Spin–Orbit Torque Switching in Van der Waals Heterostructures of Topological Materials

In our team, we've been working on a Van der Waals (vdW) type of topological materials for many years. The works of identifying the excellence and mysterious properties of the materials are very interesting as a researcher. The work so far has been mainly linked to the order of excellent materials excavating, materials synthesizing, property predicting, anlayzing, and device manufacturing. In the case of devices, they are manufactured using vdW Heterostructures to conduct basic evaluations to confirm their applicability to semiconductor devices such as a memory and a logic. We've been trying to provide clues and evidence to apply fundamental physics-derived materials to real-world industries. I'm going to talk about three main topics.

The first one is about all-vdW-heterostructure using WTe₂ and Fe₃GeTe₂ that we synthesized ourselves. Energy-efficient spintronic devices require a spin-current source with a large spin-orbit torque (SOT) efficiency (ξ) and electrical conductivity (σ), and an efficient spin injection across a transparent interface. single crystals of the vdW topological semimetal WTe2 and vdW ferromagnet Fe3GeTe2 are used to satisfy the requirements in their all-vdW-heterostructure with an atomically sharp interface. The results exhibit values of ξ ≈ 4.6 and σ ≈ 2.25 × 10⁵ Ω^-1m^-1 for WTe₂. Moreover, the significantly reduced switching current density of 3.90 × 10⁶ Acm⁻² at 150 K is obtained, which is an order of magnitude smaller than those of conventional heavy-metal/ferromagnet thin films.

The second challenge is to increase the energy efficiency and expand the driving temperature range compared to the previous study. One of the key components is vdW topological insulators (TIs), which can produce a strong SOT through the spin-momentum locking of their topological surface state (TSS). However, the relatively low conductance of the TSS introduces a current leakage problem through the bulk states of the TI or the adjacent ferromagnetic metal layers, reducing the interfacial charge-to-spin conversion efficiency (q_ICS). Here, a vdW heterostructure is used consisting of atomically-thin layers of a bulk-insulating TI Sn-doped Bi_1.1Sb_0.9Te₂S₁ and a room-temperature ferromagnet Fe₃GaTe₂, to enhance the relative current ratio on the TSS up to ≈ 20%. The resulting q_ICS reaches ≈ 1.65 nm⁻¹ and the critical current density J_c ≈ 0.9 × 10⁶ Acm⁻² at 300 K, surpassing the performance of TI-based and heavy-metal-based SOT devices. These findings demonstrate that an all-vdW heterostructure with thickness optimization offers a promising platform for efficient current-controlled magnetization switching at room temperature.

As in the previous two studies, spin hall layer and free layer are essential to observe the SOT phenomenon. The third was the challenge of design and synthesis to reduce these two layers into one layer. Here, we demonstrate external-magnetic-field-free switching of perpendicular magnetization in a single-phase ferromagnetic and spin Hall oxide SrRuO₃. We delicately altered the local lattices of the top and bottom surface layers of SrRuO₃, while retaining a quasi-homogeneous, single-crystalline nature of the SrRuO₃ bulk. This leads to unbalanced spin Hall effects between the top and bottom layers, enabling net SOT performance within single-layer ferromagnetic SrRuO₃. Notably, our SrRuO₃ exhibits the highest SOT efficiency and lowest power consumption among all known single-layer systems under field-free conditions. Our method of artificially manipulating the local atomic structures will pave the way for advances in spin-orbitronics and the exploration of new SOT materials.

In this presentation, we would like to share the current progress and receive opinions from many researchers for better progress and development. And I would be honored to contribute to the program and share my recent work on van der Waals Heterostructures and their progress.