Epitaxial Strain Induced Robust Multiferroicity in BiMnO$_{3}$

By performing first principles calculations, we investigate the effects of the epitaxial strain on the properties of BiMnO$_{\mathrm{3}}$ films grown along the pseudocubic [001] direction. Unlike the ground state with the centrosymmetric $C$2/$c$ space group in bulk, two previously unreported phases, namely, paraelectric \textit{Pnma} and ferroelectric \textit{Cc} phases, are stabilized by epitaxial strain. Several surprising and interesting phenomena are revealed. In particular, we find a metal-insulator transition between the ferromagnetic metallic state and antiferromagnetic insulating ferroelectric state under compressive epitaxial strain. On the other hand, the tensile epitaxial strain stabilizes the ferromagnetic and ferroelectric \textit{Cc} state with the large polarization ($P$ \textgreater 80 $\mu $C/cm$^{2})$ and high Curie temperature (estimated $T_{c}$ $\sim$ 395 K). Moreover, there is a novel intrinsic magnetoelectric coupling in the multiferroic \textit{Cc} state with the easy magnetization axis tunable by the external electric field.