IMF2017

The intriguing behaviors of ferroic oxides in response to external stimuli at the nanoscale have attracted extensive interest in the recent decade. In particular, mechanically-induced phenomena enabled by a scanning probe, such as ferroic ordering reversal, domain wall movement, and local phase transition, offer new possibilities for the mechanical control of oxide-based multifunctional devices. Here we demonstrate a controlled modulation of the oxygen vacancy distribution by a mechanically loaded scanning probe in a homoepitaxial SrTiO₃ thin film. The Kelvin probe force microscopy imaging illustrates distinct motions of oxygen vacancies in lateral and vertical directions under the applied mechanical pressure, suggesting a strong coupling between chemical orderings and electromechanical fields at the nanoscale. To understand the mechanism involved, we extended the phase field model of ferroelectrics to incorporate the transport theory of semiconductors, the flexoelectric effect, and the Vegard effect. The simulations reveal that under mechanical stress, localized nonuniform electric dipoles are generated in otherwise paraelectric SrTiO₃ due to flexoelectricity, alters local electrostatics and thus redistributes the oxygen vacancy. Moreover, the Vegard’s strain also interplays with the flexoelectric effect under specific circumstances and results in a downward depletion and a circumferential accumulation of vacancies under the probe tip. Tailoring oxygen vacancies distribution at the nanoscale in a controlled fashion may yield far-reaching implications for exploring defect-mediated emergent properties in multiferroic oxides.