Structure dependence of ferroelectricity in high quality BiMnO₃ epitaxial films

Epitaxial BiMnO₃ (BMO) thin films were prepared on (001)-oriented SrTiO₃ substrates with a SrRuO₃ buffer layer using pulsed laser deposition. The correlation between ferroelectric and structural properties of BMO has been investigated in detail. By high-resolution transmission electron microscopy measurement, we observed a structural transition from pseudocubic to monoclinic structure with increasing thickness of BMO films, which can be ascribed to the intermediate layer and small composition fluctuation, i.e., Bi deficiency. The ferroelectricity of the BMO films has been characterized by piezoresponse force microscope, demonstrating that pseudocubic BMO thin films show intrinsic ferroelectricity, while the ferroelectricity is absent or very weak in the monoclinic BMO thin films at room temperature. The results of density-functional theory calculations also support the experimental results.