Classification and interpretation of the polarization of multiferroic RMn₂O₅

We have collected and compared temperature-dependent polarization data of multiferroic RMn₂O₅ for 11 kinds of rare earths (R) from the literature and our own measurements. Although the reported data are inconsistent, it was found that the maximum polarization values for R = Y, Dy, Ho, Er, and Tm are almost equal above 30 K except for the phase transition temperature where the polarization disappears. The findings suggest a standard polarization value for RMn₂O₅ of 100-120 nC/cm² at 30 K in a commensurate magnetic (CM) phase. We discuss the nature and issues of the measured polarization of RMn₂O₅ from the data and our detailed measurement results for YMn₂O₅. Temperature-dependent ferroelectric hysteresis loops of YMn₂O ₅ by the double-wave method reveal that an antiferroelectric-like high-temperature incommensurate magnetic (ICM) phase and a ferroelectric low-temperature ICM phase, while the intermediate CM phase is ferrielectric.