Multiferroic Perovskite Manganites with Symmetric Exchange Striction

Orthorhombic perovskite manganites have been extensively studied as a representative system hosting versatile multiferroic phases such as the cycloidal spin phase and the $E$-type antiferromagnetic phase with an exchange striction mechanism. Recently, the latter phase has been the subject of growing interest for a potentially giant polarization as large as 60000 $\mu $C/m$^{2}$, which might involve a significant contribution from the orbital polarization.\footnote{S. Picozzi \textit{et al.}, Phys. Rev. Lett. \textbf{99}, 227201 (2007).} However, while several groups have reported ferroelectricity in this phase, further experimental progress on the clarification of the multiferroic properties and the microscopic mechanism has been hampered by the difficulty in sample preparation. In this talk, we report a series of multiferroic perovskite $R$MnO$_{3}$ with $R$ = Dy-Yb, Eu$_{1-x}$Y$_{x}$ and Y$_{1-y}$Lu$_{y}$, synthesized under high pressure and show the complete phase diagram.\footnote{S. Ishiwata \textit{et al}., Phys. Rev. B \textbf{81}, 100411(R) (2010).}$^,$\footnote{Y. Takahashi \textit{et al}., Phys. Rev. B \textbf{81}, 100413(R) (2010).} The magnitude of the polarization in the $E$-type phase was estimated to be about 5000 $\mu $C/m$^{2}$ (10 times larger than that of the \textit{bc}-cycloidal phase) and an enhanced magnetoelectric response was discovered near the first-order phase boundary. Furthermore, we have succeeded in synthesizing single crystals of perovskite YMnO$_{3}$ under a high pressure and succeeded in structure refinements for the $E$-type phase with a polar space group of $P$2$_{1}$\textit{nm}.\footnote{D. Okuyama \textit{et al.}, manuscript in preparation.} This work demonstrates for the first time the quantitative estimation of ferroelectric lattice displacements induced by a magnetic order. This work was done in collaboration with D. Okuyama, Y. Kaneko, Y. Takahashi, H. Sakai, K. Sugimoto, K. Yamauchi, S. Picozzi, Y. Tokunaga, R. Shimano, Y. Taguchi, T. Arima and Y. Tokura, and in part supported by JSPS FIRST program.