Phase-Boundary-Mediated Nonvolatile Switching of Polar Vortices in Ferroelectric Superlattices

Topological polar vortices in ferroelectric superlattices offer intriguing opportunities for nanoscale functional devices; however, achieving nonvolatile electric-field control remains a formidable challenge due to their inherent elastic recovery. Here, we demonstrate reversible nonvolatile switching of polar vortices in PbTiO3/SrTiO3 (PTO/STO) superlattices, enabled by a thickness-engineered mixed-phase state. Using in situ transmission electron microscopy, we reveal that in PTO7/STO7 superlattices, polar vortices structurally coexist with ferroelectric a-domains, forming a laterally modulated mixed-phase configuration. Under a local electric field, vortex switching proceeds via deterministic lateral propagation of vortex-a-domain phase boundaries, resulting in stable domain configurations upon field removal. In stark contrast, thicker PTO10/STO10 superlattices, which host a pure vortex phase, exhibit a volatile switching behavior that elastically relaxes to the ground state. Phase-field simulations further confirm that phase-boundary-mediated pathways provide the necessary flattened energy landscape for topological reconfiguration. These results establish mixed-phase engineering as an effective strategy for nonvolatile control of polar topological textures.