Abstract
We report the magnetic field dependence of dielectric constant and magnetoelectric effect of TmMn2O5 and YbMn2O5 single crystals. An anomalous change of magnetoelectric effect occurs at 5 kOe (TmMn2O5) or at 18 kOe (YbMn2O5) at 4.2 K, when an magnetic field is applied along the c-direction. The anomaly in YbMn2O5 is attributed to a transition between different magnetic structures. In TmMn2O5 an anomaly was also observed in dielectric constant. We propose a structural transition caused by a magnetic field.