Unraveling the Effects of Depolarization in Ultrathin Barium Titanate

Depolarization fields pose a critical obstacle to realizing ultrathin ferroelectric devices with low-voltage operation. Although the phenomenology of depolarization has been observed for decades, whether polarization loss proceeds through suppression of ferroelectric order or reverse-domain formation has remained unresolved. Here, a combination of experiment and simulation resolves this question and establishes the electrostatic origin of polarization instability in the ultrathin limit for BaTiO₃. Synchrotron X-ray absorption spectroscopy confirms that the polar structural distortion in ${\mathrm{BaTiO}}_3$ persists down to (at least) 3 nm even as the remanent polarization falls sharply, identifying reverse-domain formation as the primary mechanism of polarization loss. Time-resolved pulsed-switching measurements reveal dynamically evolving depolarization fields that converge to a thickness-independent equilibrium value. An electrostatic model attributes these fields to the effective series capacitance of the electrode interfaces, validated through thickness scaling and dielectric-buffer experiments. Phase-field simulations reveal that modifying this series capacitance reshapes switching dynamics by redistributing internal fields without altering the intrinsic ferroelectric energy landscape. Defects play a consequential secondary role, independently controlling relaxation pathways within this electrostatically constrained regime. These results establish depolarization as a system-level effect governed by heterostructure boundary conditions, and identify interfacial design and defect management as complementary strategies for stabilizing polarization in ultrathin ferroelectrics.