Stability and phase transition of polar topological defects in multiferroic superlattices

The recent observation of topologically protected polar soliton textures in multiferroic BiFeO₃/SrTiO₃ (BFO/STO) superlattices has given rise to new prospects for next-generation electronic devices, with the inherent room-temperature polar and spin coupling. Herein, we employed phase-field simulations to explore the phase diagrams of the BFO/STO superlattice under various thermodynamic conditions and experimentally validated them using piezoresponse force microscopy. It is revealed that by modulating external factors such as epitaxial strain, film thickness, and external temperature, different phases including polar solitons, polar skyrmions, and trivial domains can be designed within the BFO/STO superlattice system. For instance, reducing the magnitude of compressive strain can induce transitions from polar skyrmions to polar solitons and finally to a multidomain state. On the other hand, increasing the superlattice periodicity can shift the system from a single domain like state to polar solitons and a multidomain state. Furthermore, it is predicted that heating can transform the polar soliton state into a polar skyrmion state. This research offers deep insights into the stability of polar topologies in BFO-based systems and provides practical guidelines for the manipulation and design of polar solitons in multiferroic materials.