  Cross-correlation between magnetic and dielectric properties has been
attracting renewed interest because of the fundamental as well as technological
importance of controlling the electric (magnetic) polarization by an external
magnetic (electric) field. Here, we report the novel phenomenon that an
external magnetic field induces and/or modifies ferroelectric states in a
magnetic material, DyMn2O5. Measurements of temperature dependence of
hysteretic polarization curves, pyroelectric current, specific heat, optical
second harmonic generation, and x-ray superlattice peaks have revealed
successive phase transitions between 43 K and 4 K, accompanying three
ferroelectric phases. The zero-field lowest-temperature phase (<8 K) induced by
the Dy-moment ordering is a reentrant paraelectric state, but is turned to a
ferroelectric state with increasing the magnetic field. The phenomenon is
closely related to the metamagnetic transitions of the Dy f-moment, indicating
that all the ferroelectric phases of this material are strongly tied to the
antiferromagnetic Mn spin structure affected by the f-d exchange interaction.
The electric phase diagram for DyMn2O5 is presented in the plane of temperature
and magnetic field.
