Generating Unconventional Spin-Orbit Torques With Patterned Phase Gradients in Tungsten Thin Films

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 $\beta$ phase to the minimally spin-orbit coupled, low-resistivity $\alpha$ 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.