Magnetic field and external pressure control of ferroelectricity in multiferroic manganites

We have investigated dielectric properties in a series of crystals of RMnO3 (R is a rare-earth ion) under magnetic fields and quasihydrostatic pressure. We have found that a ferroelectric phase appeared in GdMnO3 crystal below 13K. We have confirmed that a small spontaneous polarization exists along a-axis (Pa) in the orthorhombic Pbnm setting and that Pa can be reversed by the DC electric field. The dielectric anomaly due to the ferroelectric transition accompanied thermal hysteresis and lattice striction. The ferroelectric transition temperature decreased with quasihydrostatic pressure. These results indicate that the ferroelectric transition is improper and is of the first-order displacement type. Pa was easily collapsed by the application of a magnetic field of 0.4T parallel to the spin-canting direction (H-bar c) while it was enhanced parallel to the easy axis (H-bar b)