Magnetic field and external pressure control of ferro electricity 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 13 K. We have confirmed that a small spontaneous polarization exists along a-axis (P-a) in the orthorhombic Pbnm setting and that P-a 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 quasi hydrostatic pressure. These results indicate that the ferroelectric transition is improper and is of the first-order displacement type. P-a was easily collapsed by the application of a magnetic field of 0.4 T parallel to the spin-canting direction (H parallel to c) while it was enhanced parallel to the easy axis (H parallel to b). (c) 2005 Elsevier B.V. All rights reserved.