Niklas Kercher

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.

A key aim in spintronics is to achieve current‐induced magnetization switching via spin‐orbit torques without external magnetic fields. For this, the focus of recent work has been on introducing controlled lateral gradients across ferromagnet/heavy‐metal devices, giving variations in thickness, composition, or interface quality. However, the shallow gradients achievable with growth techniques limit the impact of this approach and understanding of the underlying physical mechanisms. Here, crystalline phase gradients are patterned on a mesoscopic length scale in tungsten thin films using direct‐write laser annealing. Through transmission electron microscopy, resistivity, and second harmonic measurements, the continuous transformation of the phase of tungsten films from the highly spin‐orbit coupled, high‐resistivity phase to the minimally spin‐orbit coupled, low‐resistivity phase with increasing laser fluence is tracked. Gradients with different steepness and arbitrary shapes are patterned in the tungsten phase. When interfaced with CoFeB, current‐induced spin‐orbit torques resulting from tungsten with a sufficiently steep gradient can switch the magnetization without an applied magnetic field. Therefore, exploiting the unique microstructure of mixed‐phase W allows precise control of the local electronic current density and direction, as well as local spin‐orbit torque efficiency, providing a new avenue for the design of efficient spintronic devices.