Manipulation of spin-orbit-torque efficiency via designing gradient structure in ferrimagnetic Ta/Tb-Co/Pt multilayers

The search for material combinations that exhibit highly efficient current-induced spin-orbit torque (SOT) is crucial for the development of next-generation nonvolatile spintronics devices. Existing research results suggest that the spin Hall effect (SHE) in heavy metals with strong spin-orbit coupling or the interfacial Rashba effect at the heavy-metal--ferromagnet interface are the primary sources of spin currents. Here, we present experimental evidence that ferrimagnetic $\mathrm{Tb}$-$\mathrm{Co}$ multilayers, engineered with an artificially designed composition gradient, exhibit a bulk Rashba effect originating from gradient-related spatial inversion symmetry breaking, which provides sizable spin currents for magnetization switching. The two types of spin currents, originating separately from the SHE and the bulk Rashba effect, would cooperate or compete with each other, depending highly on the gradient direction, ultimately resulting in a weakening or enhancement of SOT efficiency. Our findings regarding the gradient-related manipulation of current-induced SOT provide valuable insights for the development of highly efficient spintronics devices based on ferrimagnetic materials.