Displacement-Type Ferroelectricity with Off-Center Magnetic Ions in PerovskiteSr1−xBaxMnO3

We report a ferroelectric transition driven by the off-centering of magnetic ${\mathrm{Mn}}^{4+}$ ions in antiferromagnetic Mott insulators ${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x{\mathrm{MnO}}_3$ with a perovskite structure. As $x$ increases, the perovskite lattice shows the typical soft-mode dynamics, as revealed by the momentum-resolved inelastic x-ray scattering and far-infrared spectroscopy, and the ferroelectricity shows up for $x\ge 0.45$. The observed polarization is comparable to that for a prototypical ferroelectric ${\mathrm{BaTiO}}_3$. We further demonstrate that the magnetic order suppresses the ferroelectric lattice dilation by $\sim 70\%$ and increases the soft-phonon energy by $\sim 50\%$, indicating the largest magnetoelectric effects yet attained.