Ferroelectricity and Giant Magnetocapacitance in Perovskite Rare-Earth Manganites

The relationships among magnetism, lattice modulation, and dielectric properties have been investigated for $R{\mathrm{MnO}}_3$ ($R=\mathrm{Eu},\mathrm{Gd},\mathrm{Tb},\mathrm{and}\mathrm{Dy}$). These compounds show a transition to an incommensurate lattice structure below their Néel temperature, and subsequently undergo an incommensurate-commensurate (IC-C) phase transition. For ${\mathrm{TbMnO}}_3$ and ${\mathrm{DyMnO}}_3$ it was found that the IC-C transition is accompanied by a ferroelectric transition, associated with a lattice modulation in the C phase. ${\mathrm{DyMnO}}_3$ shows a gigantic magnetocapacitance with a change of dielectric constant up to $\Delta \epsilon /\epsilon \sim 500\%$.