Unfolding of Field-Free Spin-Orbit Torque Magnetization Switching in a van der Waals Heterostructure

Heterostructures of 2D materials offer high tunability of magnetic properties, including current-induced magnetization switching by unconventional spin-orbit torques. The microscopic mechanisms governing spin-torque switching in van der Waals systems, however, remain insufficiently understood. Here, we investigate current-induced magnetization reversal in an Fe3GeTe2 (FGT) and WTe2 heterostructure using scanning harmonic magneto-optical Kerr effect microscopy. Spatially resolved magnetization maps show that field-free switching unfolds from asymmetric domain nucleation at the edges of the FGT flake, followed by domain wall propagation. This switching behavior is deterministic and bipolar, consistent with spin-orbit torques arising from the injection of both in-plane and out-of-plane spins from WTe2 into FGT. The critical currents for up-to-down and down-to-up switching differ because of asymmetric domain nucleation and directional domain wall pinning effects. Micromagnetic simulations reproduce the switching behavior observed experimentally, showing how the initial domain nucleation sites are determined by the sample geometry and combined influence of in-plane and out-of-plane spins and Dzyaloshinskii-Moriya interaction. Finally, by systematically varying the field-like and damping-like torque components and sweeping the spin polarization between in-plane and out-of-plane, we identify the torque configurations that yield the most efficient switching in van der Waals magnets with different shapes.