Multiferroic Bi2NiMnO6 thin films: A computational prediction

We report first-principles calculations for one of the few materials that is believed to be a ferroelectric ferromagnet, ${\mathrm{Bi}}_2{\mathrm{NiMnO}}_6$. Our calculations show that, contrary to what has been reported so far, bulk ${\mathrm{Bi}}_2{\mathrm{NiMnO}}_6$ does not have a polarization. Instead, like ${\mathrm{BiMnO}}_3$, it crystallizes into a centrosymmetric structure with space group $C2/c$. We also predict that ${\mathrm{Bi}}_2{\mathrm{NiMnO}}_6$ will indeed be a ferroelectric ferromagnet if it is grown as an epitaxial film on a substrate with in-plane square symmetry and a lattice constant around 4 Å, such as ${\mathrm{BaTiO}}_3$ or ${\mathrm{PbZr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$.