Epitaxial phases of BiMnO3 from first principles

BiMnO3 is the only transition-metal perovskite oxide that is insulating and shows strong ferromagnetism in bulk. This distinctive behavior would make it a promising candidate as a magnetoelectric multiferroic if it was also a polar material, but experiments have shown that bulk BiMnO3 has either a very small polarization (below 0.1μC/cm2) or, most likely, that it is a paraelectric. There is also experimental evidence that the polarization in BiMnO3 films grown on SrTiO3 can be as high as 20μC/cm2. Despite the interest in these behaviors, the diagram of BiMnO3 as a function of epitaxial strain has remained largely unexplored. Here, we use first-principles to predict that, both under enough compressive and tensile epitaxial strain, BiMnO3 films are ferroelectric with a giant polarization around 100μC/cm2. The phases displayed by the films are similar to those experimentally found for BiFeO3 in similar conditions - at compressive strains, the film is supertetragonal with a large component of the polarization pointing out of plane, while at tensile strains the polarization points mostly in plane. As in BiFeO3 films, these phases are antiferromagnetic - the orbital ordering responsible for ferromagnetism in BiMnO3 is absent in the polar phases. Our calculations also show that the band gap of some of these BiMnO3 films is substantially smaller than gaps typically found in ferroelectric oxides, suggesting it may be a suitable material for photovoltaic applications.