Field-Free Multistate Spin-Orbit Torque Switching Enabled by Structurally Engineered Four-Fold Magnetic Anisotropy

Multiaxial magnetic states offer a route beyond binary spintronic functionalities and enable zero-magnetic-field spin orbit torque (SOT) switching via mirror symmetry breaking, yet remain unexplored. Here, we uncover field-free multistate SOT switching in a SrIrO₃/SrRuO₃ bilayer with exceptionally large monoclinic distortion. Pulsed current excitations fully map all twelve deterministic transitions among the four states, through a systematic switching protocol defined by two characteristic current densities. In-situ scanning nitrogen-vacancy (NV) center magnetometry provides direct real-space evidence for previously unobserved in-plane canted states, and spin dynamics simulations uncover a two-step switching pathway, driven by the concerted action of spin torques and the effective anisotropy field within the fourfold anisotropy landscape. Our results demonstrate that precise lattice-symmetry engineering enables the creation and control of multiaxial magnetic states for field-free multistate spintronic devices.