Magnetic phase transitions and magnetic-field-induced polarization flops in multiferroic YbMn₂O₅

We have measured the electric polarization along the b-and a-axes of multiferroic YbMn₂O₅ under a magnetic field along the c-axis up to 14.5 T. We found that the polarization along the b-axis disappears, while that along the a-axis appears below 5.5 K in 0 T, though the direction of the polarization switches again above 1.5 T at 4.3 K. Ferroelectric hysteresis loops along the both axes of YbMn₂O₅ were clearly observed by the double-wave method. This polarizationflop phenomenon is very similar to that previously found in TmMn₂O₅. We have also studied the magnetic phase transitions of YbMn₂O₅ by neutron diffraction, and compared the data with those of TmMn₂O₅. The low-temperature incommensurate magnetic phases relating to the polarization flop of YbMn₂O₅ and TmMn₂O₅ are similar, though YbMn₂O₅ lacks a commensurate magnetic and ferroelectric phase which exists in TmMn₂O₅ and typical similar compounds.