Surface effects on domain switching of a ferroelectric thin film under local mechanical load: A phase-field investigation

Flexoelectricity arises from electromechanical coupling of a strain gradient and electrical polarization, which makes mechanical manipulation of ferroelectric domains possible in a ferroelectric. However, surface effects in ferroelectric ultrathin films, even at nanoscale thicknesses, cannot be ignored. In this paper, a phase-field approach that incorporates flexoelectricity and surface effects into the free energy was extended to evaluate domain switching of ferroelectric polarization in a ferroelectric thin film under local mechanical loads. The model predicted that microscopic 180° domains were formed near where the load was applied to the film surface during switching of ferroelectric polarizations. The reversal of micro-domains within the region of ferroelectric polarization was reduced over time, and it even disappeared when a final stable state was reached. The mechanism of 180° domain formation was studied by examining surface displacement and related strain gradients and flexoelectric fields. Switching of ferroelectric domains driven by mechanical loads revealed a size-dependent characteristic that becomes more apparent as the thin film thickness or the length of the load area decreases. Surface effects significantly alter the polarization field, and a narrower local load can suppress mechanical switching of ferroelectric polarization.