Engineering Symmetry Breaking Enables Efficient Bulk Spin-Orbit Torque-Driven Perpendicular Magnetization Switching

To overcome the interfacial nature of spin-orbit torque (SOT) in bilayers, novel bulk SOT (BSOT) is widely investigated to implement high-density and low-power spintronic devices. However, the underlying mechanism of efficient BSOT switching remains unclear, especially the anomalously enhanced effective spin Hall angle (θ_SH) with increasing ferromagnet thickness (t_FM), due to lacking simple and high-tunable material systems. Here, a series of Pt/Co multilayers with invariable thickness gradient and varying stacking numbers is designed to systematically explore BSOT origin and enable efficient switching via engineering symmetry breaking. As t_FM increases, the critical current density decreases while the switching efficiency and θ_SH build up. Comparative experiments directly demonstrate that gradient-induced local spin current is more efficient than that in the bilayer. Moreover, x-ray absorption spectroscopy (XAS) results reveal that the increasing stacking number can effectively engineer the symmetry breaking at Pt/Co interface to induce strong interfacial spin-orbit coupling. On this basis, it is concluded that the BSOT effect, as well as the anomalously enhanced switching efficiency, and θ_SH arises from gradient-induced bulk and interface symmetry breaking. These findings clarify the underlying mechanism of BSOT, and broaden the scope of material engineering to improve switching efficiency and inspire more memory and computing applications.