Highly Efficient Spin-Orbit Torque Switching in Bi 2 Se 3 /Fe 3 GeTe 2 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 Bi 2 Se 3 and exfoliated 2D metallic ferromagnet Fe 3 GeTe 2 (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 × 10 6 A/cm 2 , the lowest reported for the switching of a perpendicular anisotropy ferromagnet using Bi 2 Se 3 . 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 Bi 2 Se 3 . Our density functional theory calculations find that the weak interlayer interactions at the Bi 2 Se 3 /FGT interface lead to a weakened dipole at the interface and suppress the proximity induced magnetic moment on Bi 2 Se 3 . 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.