Magnetically Induced Electronic Ferroelectricity in Half-Doped Manganites

Using a joint approach of density functional theory and model calculations, we demonstrate that a prototypical charge ordered half-doped manganite La1=2Ca1=2MnO3 is multiferroic. The combination of a peculiar charge-orbital ordering and a tendency to form spin dimers breaks the inversion symmetry and leads to a ferroelectric ground state with a polarization up to several "C=cm2. The presence of improper ferroelectricity does not depend on the hotly debated structural details of this material: in the Zener- polaron structure we find a similar ferroelectric response with a large polarization of purely magnetic origin.