Institute of Multidisciplinary Research for Advanced Materials, Tohoku University^* Japan Atomic Energy Agency, Tokai, Ibaraki, 319-1195, Japan^** Depertment of physics, Waseda Univ., Tokyo, 169-8555 Tokai, Japan^***
○Hiroyuki Kimura^* Shuichi Wakimoto^** Youichi Kamada^* Yukio Noda^* Kazuhisa Kakurai^** Koji Kaneko^** Naoto Metoki^** Kay Kohn^***

A colossal magnetoelectric (CME) effect, has became one of the most important issues in strongly correlated electron systems. A key feature in the CME effect is a multiferroics where a magnetic and a dielectric order coexist. However microscopic relations between magnetic and dielectric properties are not fully understood yet. In this work, we performed neutron diffraction measurements under magnetic field for the multiferroic RMn₂O₅ (R = Er, Ho) to clarify the relevance between the field-induced magnetic phase and the dielectric phase. Summary of the results is shown in the Figure. In HoMn₂O₅ the field-induced magnetic phase transition from an incommensurate to a commensurate spin state occurs, where the ferroelectric phase is induced. On the contrary in ErMn₂O₅, the field-induced magnetic phase transition from the commensurate to the incommensurate spin state occurs where the electric depolarization concomitantly occurs. In both system, there are one to one correspondences between the magnetic and dielectric phase diagrams, indicates that the electric polarization is magnetically induced and the commensurate spin state is indispensable to the ferroelectricity in the multiferroic RMn₂O₅ system.