Displacement-type ferroelectric transition with magnetic Mn ions in perovskite Sr$_{1-x}$Ba$_{x}$MnO$_{3}$

Almost all the proper ferroelectrics with a perovskite structure discovered so far have no $d$-electrons in the off-center transition metal site, as exemplified by BaTiO$_{3}$ and Pb(Zr,Ti)O$_{3}$. This empirical $d^{0}$ rule is incompatible with the emergence of magnetism and has significantly restricted the variety of multiferroic materials. In this work, we have discovered a displacement-type ferroelectric transition originating from off-center Mn$^{4+}$ ions in antiferromagnetic Mott insulators Sr$_{1-x}$Ba$_{x}$MnO$_{3}$. As Ba concentration increases, the perovskite lattice shows the typical soft mode dynamics, and the ferroelectricity shows up for $x\!\ge\!0.45$. In addition to the large polarization and high transition temperature comparable to BaTiO$_{3}$, we demonstrate that the magnetic order suppresses the ferroelectric lattice dilation by $\sim$70\% and increases the soft-phonon energy by $\sim$50\%, indicating gigantic magnetoelectric effects [1]. This work was supported by the FIRST program on ``Quantum Science on Strong Correlation''. \\[4pt] [1] H. Sakai {\it et al}., Phys. Rev. Lett. {\bf 107}, 137601 (2011).