Highly Efficient Spin-Orbit Torque Switching in Bi₂Se₃/Fe₃GeTe₂ van der Waals Heterostructures

Topological insulators (TIs) have shown promise as a spin-generating layer to switch the magnetization state of ferromagnets via spin-orbit torque (SOT) due to charge-to-spin conversion efficiency of the TI surface states that arises from spin-momentum locking. However, when TIs are interfaced with conventional bulk ferromagnetic metals, the combination of charge transfer and hybridization can potentially destroy the spin texture and hamper the possibility of accessing the TI surface states. Here, we fabricate an all van der Waals (vdW) heterostructure consisting of molecular beam epitaxy grown bulk-insulating Bi₂Se₃ and exfoliated 2D metallic ferromagnet Fe₃GeTe₂ (FGT) with perpendicular anisotropy. By detecting the magnetization state of the FGT via anomalous Hall effect and magneto-optical Kerr effect measurements, we determine the critical switching current density for magnetization switching to be J_c ≈ 1.2 × 10⁶ A/cm², the lowest reported for the switching of a perpendicular anisotropy ferromagnet using Bi₂Se₃. From second harmonic Hall measurements, we further determine the SOT efficiency (ξ_DL) to be in the range of 1.8 ± 0.3 and 1.4 ± 0.08 between 5 and 150 K, comparable to the highest values reported for Bi₂Se₃. Our density functional theory calculations find that the weak interlayer interactions at the Bi₂Se₃/FGT interface lead to a weakened dipole at the interface and suppress the proximity induced magnetic moment on Bi₂Se₃. This enables direct access to the TI surface states contributed by the first quintuple layer, where the spins are singly degenerate with significant net in-plane spin polarization. Our results highlight the clear advantage of all-vdW heterostructures with weak interlayer interactions that can enhance SOT efficiency and minimize critical current density, an important step toward realizing next generation low-power nonvolatile memory and spintronic devices.