  Electric and magnetic properties of multiferroic GdMn2O5 in external magnetic
fields were investigated to map out the magnetoelectric phases in this
material. Due to strong magnetoelectric coupling, the dielectric permittivity
is highly sensitive to phase boundaries in GdMn2O5, which allowed to construct
the field-temperature phase diagrams. Several phase transitions are observed
which are strongly field-dependent with respect to field orientation and
strength. The phase diagram for a magnetic field along the crystallographic
a-axis corresponds well to a polarization step, as induced by 90 degree
rotation of Gd magnetic moments. Our results support the model of two
ferroelectric sublattices, Mn-Mn and Gd-Mn with strong R-Mn (4f-3d) interaction
for the polarization in RMn2O5.
