Giant spin-driven ferroelectric polarization and magnetoelectric effect in perovskite rare-earth maganites under high pressure

The discovery of ferroelectricity in TbMnO$_{3}$ triggered extensive studies on a type of multiferroics, ``spin-driven ferroelectrics.'' Unlike conventional ferroelectrics such as BaTiO$_{3}$, spin-driven ferroelectrics exhibit remarkable magnetoelectric (ME) effects. However, the ferroelectric polarization $P$ in spin-driven ferroelectrics ever reported (\textless 10$^{-1} \mu $C/cm$^{2}$) is much smaller than that in conventional ferroelectrics (typically 10$^{0}$ $\sim$ 10$^{1} \mu $C/cm$^{2}$). Thus, the quest for robust magnetically-controllable $P$ comparable to that in conventional ferroelectrics is still a major challenge in the research on multiferroics. In this study, we utilized the ``high-pressure'' to attain a magnetically-controllable spin-driven $P$ with its magnitude being comparable to that in conventional ferroelectrics [T. Aoyama \textit{et al.}, Nature Commun. 5, 4927 (2014)]. With a home-made high-pressure measurement system with a diamond anvil cell, we investigated high-pressure effects on ME properties of perovskite $R$MnO$_{3}$ ($R = $ Gd, Tb, and Dy). Our study revealed that these manganites exhibit a pressure-induced ME phase transition and that the high-pressure phase shows the largest $P$ (e.g., 1 $\mu $C/cm$^{2}$ in TbMnO$_{3}$) among spin-driven ferroelectrics ever reported. Moreover, $P$ is further enhanced by applying a magnetic field. Our study demonstrates that it is possible to attain giant spin-driven ferroelectric polarization which comes close to that in conventional ferroelectrics, and to control it magnetically.\\[4pt] This work has been done in collaboration with T. Aoyama, K. Yamauchi, A. Iyama, S. Picozzi, A. Miyake, and K. Shimizu.