The Observation of Polarization Fatigue in Epitaxial BiFeO₃ Thin Films with Inhibited Ferroelastic Switching

Polarization fatigue has remained a severe problem for the application of ferroelectric oxide in memories, sensors, and electric circuits due to the limited understanding of its underlying physics. This study identifies inhibited ferroelastic switching as a primary driver of polarization fatigue in BiFeO₃ thin films deposited on SrTiO₃(110) substrates. Using voltage-biased piezo-response force microscopy, we observed polarization pinning resulting from cyclic ferroelectric domain writing. This unswitchable polarization stems from charged ferroelastic domain walls, stabilized by defect accumulation, which increases internal stress within the thin film. Elevated stress suppresses ferroelastic switching, clamping BFO unit cells away from domain walls and leading to reduced remnant polarization in ferroelectric hysteresis loops. The fatigue mechanism becomes more pronounced in high-quality thin films, as evidenced by fatigue-free BiFeO₃ films with larger full-width-at-half-maximum (fwhm) values in X-ray rocking curve analyses. Notably, the fatigue is reversible, which depends on the elimination of the ferroelastic domain wall with long-term poling that allows the removal of the charged defects from the domain boundary, followed by the release of the internal stress in the thin film system.