Tuning the atomic and domain structure of epitaxial films of multiferroic BiFeO₃

Recent works have shown that the domain walls of room-temperature multiferroic BiFeO₃ (BFO) thin films can display distinct and promising functionalities. It is thus important to understand the mechanisms underlying domain formation in these films. High-resolution x-ray diffraction and piezoforce microscopy, combined with first-principles simulations, have allowed us to characterize both the atomic and domain structure of BFO films grown under compressive strain on (001)-SrTiO₃, as a function of thickness. The clamping of the substrate has been observed to exist in two different regimes: ultrathin, d<18 nm, and thin, d>18 nm. When this is taken into account in the calculations, an excellent agreement between the predicted and observed lattice parameters is shown. We derive a twinning model that describes the experimental observations and could explain why the 71° domain walls are the only ones showing insulating character. This understanding of the exact mechanism for domain formation provides us with a new degree of freedom to control the structure and, thus, the properties of BiFeO₃ thin films.