Direct visualization of interfacial defect effects on polarization switching in BaTiO₃ tunnel junctions

Deterministic control of polarization switching at complex oxide interfaces is essential for high-performance ferroelectric devices, yet the microscopic competition between external fields and polarization response remains difficult to probe directly. Combining atomic-scale scanning transmission electron microscopy and electron energy loss spectroscopy with in-situ biasing, we establish an asymmetric interfacial pinning mechanism in epitaxial Pt/BaTiO₃/La_2/3Sr_1/3MnO₃ ferroelectric tunnel junctions. At the Pt/BaTiO₃ interface, an oxygen vacancy-rich pinning layer induces Ti reduction and a strong, uniform downward electric field. In contrast, the BaTiO₃/La_2/3Sr_1/3MnO₃ boundary is characterized by localized La_Mn antisite defects that generate internal fields through localized tensile strain. Under an upward external field, this competitive landscape forces the formation of a stable, head-to-head domain wall within the 3-nanometer-thick BaTiO₃ barrier, preventing the system from reaching a homogeneous polarization state. Our findings demonstrate that ferroelectric reversibility is fundamentally constrained by a mutual stabilization of cation and anion defects, providing a framework for engineering electrode interfaces at the limit of unit-cell thickness.