Analog Spin-Orbit Torque Switching in Vertically Graded Pt/Co-Re Heterostructures for Neuromorphic Computing

Achieving gradual, analog magnetization switching in spin-orbit torque (SOT) devices is essential for neuromorphic computing. In perpendicular magnetic anisotropy (PMA)-based SOT devices, gradual switching can arise from domain nucleation, domain-wall propagation, and pinning, and controlling it through materials design remains a key goal. This study demonstrates that a vertically graded cobalt (Co)-rhenium (Re) alloy region, formed through a simple Co/Re multilayer design, enables analog SOT switching in Pt/Co-Re heterostructures. Microstructural and compositional analyses confirm that annealing transforms the discrete multilayer into a depth-dependent alloy with tunable gradient direction and magnitude, controlled by adjusting the Re layer thickness distribution. Increasing the Co/Re bilayer repetition number up to 3 yields a pronounced sigmoid-like transfer characteristic plausibly explained by the sequential reversal of sub-regions with distributed local switching thresholds, with only a small change in the effective PMA energy. We demonstrate stable, bidirectional long-term potentiation and depression under repeated pulse stimulation across Re compositions of 30-70%. System-level validation using a multilayer perceptron on MNIST (modified National Institute of Standards and Technology) Digits, Fashion-MNIST, and MNIST Characters datasets shows test accuracies within 1.21 percentage points of the ideal sigmoid baseline, with MNIST Digits accuracy varying by 0.24 percentage points across the full compositional range.