Emergent Antipolar Phase in BiFeO₃-La_0.7Sr_0.3MnO₃ Superlattice

Ferroelectric-paraelectric superlattices show emerging new states, such as polar vortices, through the interplay and different energy scales of various thermodynamic constraints. By introducing magnetic coupling at BiFeO₃-La_0.7Sr_0.3MnO₃ interfaces epitaxially grown on SrTiO₃ substrate, we find, for the first time in thin films, a sub-nanometer thick lamella-like BiFeO₃. The emergent phase is characterized by an arrangement of a two unit cell thick lamella-like structure featuring antiparallel polarization, resulting an antiferroelectric-like structure typically associated with a morphotropic phase transition. The antipolar phase is embedded within a nominal R3c structure and is independent of the BiFeO₃ thickness (4-30 unit cells). Moreover, the superlattice structure with the morphotropic phase demonstrates azimuth-independent second harmonic generation responses, indicating a change of overall symmetry mediated by a delicate spatial distribution of the emergent phase. This work enriches the understanding of a metastable state manipulated by thermodynamic constraints by lattice strain and magnetic coupling.