Dependence of current-induced switching threshold on composition gradient width in Si/Al-integrated perpendicular magnetic films

We report spin–orbit torque switching in perpendicularly magnetized Pt(3.1 nm)/Co(0.95 nm)/Pt(3.1 nm) trilayer using Si/Al composition-graded materials (CGMs). By systematically varying the CGM width tg, we find that the critical switching current exhibits a pronounced maximum at tg = 3.1 nm. Remarkably, the spin current from Si/Al CGMs reaches nearly 86% of that produced by a Pt(5.0 nm)/Co(0.95 nm)/Pt(3.1 nm) trilayer, despite the much smaller spin Hall angles of bulk Si and Al. Unlike conventional electrical probes such as spin-torque ferromagnetic resonance or harmonic Hall methods, our approach evaluates the spin-current generation of graded materials directly through magnetization switching. These results establish composition-graded interfaces as a versatile platform for controlling spin–orbit torques, and they highlight the potential of Si/Al CGMs for achieving energy-efficient spintronic devices without relying on strong spin–orbit interaction materials.