Reinvestigation of dielectric anomalies of RMn₂O₅ associated with successive magnetic phase transitions

A ferroelectric phase transition of RMn₂O₅ (R = rare earth, Bi, Y) has been systematically reinvestigated by dielectric measurements. In addition to a sharp peak of the dielectric constant along the b axis at the temperature of ferroelectric phase transition, T_C1, we observed a small kink at T_S slightly below T_C1 for R. = Er, Y, Tb, Tm. Correspondingly, two-step successive magnetic phase transitions were found for R. = Er, Y, Tm in a narrow temperature range of l K around T_C1 by neutron diffraction measurements. The comparison between temperatures of dielectric anomalies and magnetic phase transitions indicates that the paraelectric-ferroelectric phase transition at T_C1 is induced by a commensurate magnetic ordering along the c axis in RMn₂O₅.