Deterministic polarity control of self-generated spin–orbit torque via inversion symmetry engineering

Self-generated spin–orbit torque (SOT) in magnetic materials has emerged as a promising pathway for efficient magnetization control without external spin current sources. Here, we unveil an alternative mechanism for deterministic SOT polarity control based on inversion symmetry engineering via ferromagnet (Co) thickness gradients in [Co/Pt]p multilayers with the repeating periods p. Unlike conventional approaches relying on spin Hall effect asymmetry in heavy metals, we modulate the spatial profile of spin current absorption and reflection across the stack, creating a net damping-like SOT driven by gradient-induced imbalanced spin accumulation. By inverting the Co gradient, we achieved polarity-reversed perpendicular magnetization switching, demonstrating that the SOT originates from structural symmetry breaking within the ferromagnetic layers. Second harmonic measurements confirm robust SOT switching efficiencies up to 294 Oe cm2 MA−1, along with enhanced thermal stability and scalability beyond bilayer systems. Our findings establish the symmetry-governed design principle for spin transport, enabling tunable and thickness-robust spintronic devices.