Domain morphology and vertical polarization at vortex cores in c-oriented epitaxial SrBi₂Nb₂O₉ thin films

Ferroelectric domain configurations in epitaxial thin films play a critical role in determining their switching and electromechanical behavior. In particular, vortex-like domain structures have attracted attention because they can host localized polarization states that differ from the surrounding matrix. Here, epitaxial c-oriented SrBi₂Nb₂O₉ (SBN) thin films were investigated using atomic force microscopy and piezoresponse force microscopy to resolve in-plane domain variants and vortex-like junctions at the nanoscale. The SBN film exhibits a low out-of-plane remanent polarization (∼2.5 µC cm^-2), consistent with its c-axis orientation and predominantly in-plane spontaneous polarization. In the ferroelectric domain structure of the SBN thin film, a vortex domain was formed by domains formed within the in-plane. Local switching spectroscopy performed at the vortex cores reveals an enhanced out-of-plane electromechanical response compared to the surrounding in-plane domains, indicating a localized out-of-plane electromechanical response that may be associated with a vertical polarization component. These results provide insight into flux-closure-like domain configurations in c-oriented SBN thin films and their localized electromechanical behavior, which may provide useful insight for future nanoscale functional devices, although further verification is required.