Atomic-scale visualization of polarization pinning and relaxation at coherent BiFeO₃/LaAlO₃ interfaces

Complex oxide heterointerfaces, which play host to an incredible variety of interface physical phenomena, are of great current interest in introducing new functionalities to systems. Here, coherent super-tetragonal BiFeO ₃/LaAlO₃ and rhombohedral BiFeO₃/LaAlO ₃ heterointerfaces are investigated by using a combination of high-angle annular dark-field (HAADF) imaging and annular bright-field (ABF) imaging in a spherical aberration (Cs) corrected scanning transmission electron microscope (STEM), and first-principles calculations. The complicated ferroelectric polarization pinning and relaxation that occurs at both interfaces is revealed with atomic resolution, with a dramatic change in structure of BiFeO₃, from cubic to super-tetragonal-like. The results enable a detailed explanation to be given of how non-bulk phase structures are stabilized in thin films of this material. Changes in lattice strain and symmetry in BiFeO₃ (BFO) crystals at coherent T-BFO/LaAlO₃ (LAO) and rhombohedral-BFO/LAO interfaces are imaged with atomic resolution using annular bright-field scanning transmission electron microscopy and first-principles calculations. A common pinned BFO layer and polarization relaxation at the interfaces are identified, providing insight into the ferroelectric behavior of thin films and the complex oxide heterointerfaces between them.