Dynamic Mechanical Writing of Ferroelectric Bubbles

Topological ferroelectric structures offering novel functionalities due to peculiar distributions of polarization and charge with long retention have been increasingly explored in a variety of dielectric systems, such as superlattices, strained films, and nanoscale islands. The polar structures, protected by topological constraints such as boundary conditions, appear as metastable states due to the interplay between strain, depolarization and gradient energies, but their precise control is still challenging. In this study, we demonstrate the observation of skyrmion-like centre-type polar bubble domains in supertetragonal BiFeO₃ thin films. These polar textures are mechanically generated by dynamic elastic force of vibrational tapping using scanning probe microscope tips. The formation of bubble domains is accompanied by emergence of strain-driven morphotropic phase boundaries wherein two competing structural phases coexist. The bubble domains can be accurately written, mechanically or electrically erased, and remain stable for longer than ~500 days. Vibrational tapping can bidirectionally switch out-of-plane polarization by exerting strong tapping force onto the elastically soft surface driven by the morphotropic phase transition, which may be attributed to non-linear flexoelectric effects in the large strain-gradient regime beyond the conventional flexoelectrical concept. Our study presents insights into dynamic mechanical switching of polarization and provides a unique pathway into topological polar structures for next-generation energy-efficient electronic applications.