  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
piezo-force 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. We
derive a twining model that describes the experimental observations and
explains why the 71o domain walls are the ones commonly observed in these
films. This understanding provides us with a new degree of freedom to control
the structure and, thus, the properties of BiFeO3 thin films.
