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 polarization-flop 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. (author)