We have investigated dielectric properties in Mott insulators GdMnO3 and TbMnO3 under magnetic fields and external quasihydrostatic pressures. In the case of GdMnO3, thermal hysteresis for dielectric constant ε and discontinuous lattice distortion were observed at ferroelectric transition temperature (TC ), and ferroelectric spontaneous polarization was suppressed by application of external pressure. These results indicate that the ferroelectric transition in GdMnO3 is a first-order displacive-type one. On the other hand, the thermal hysteresis and discontinuous lattice striction were not observed at TC in TbMnO3. The peak of ε corresponding to ferroelectric transition was shifted toward higher temperatures by application of external pressure in TbMnO3. The ferroelectric transition of TbMnO3 was thought to be a second-order order-disorder-type one.

We have investigated the dielectric and magnetic properties of Eu 0.595 Y 0.405 MnO 3 without the presence of the 4f magnetic moments of the rare earth ions, and have found two ferroelectric phases with polarization along the a and c axes in a zero magnetic field. A magnetic field induced switching from one to the other ferroelectric phase took plase in which the direction of ferroelectric polarization changed from the a axis to the c axis by the application of magnetic fields parallel to the a axis. In contrast to the case of TbMnO 3 , in which the 4f moments of Tb 3+ ions play an important role in such a ferroelectric phase switching, the magnetic-field-induced switching between ferroelectric phases in Eu 0.595 Y 0.405 MnO 3 does not originate from the magnetic transition of the rare-earth 4f moments, but from that of the Mn 3d spins.

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 ferroelectric phase appeared in GdMnO3 crystal below 13K. We have confirmed that a small spontaneous polarization exists along a axis (Pa) in the orthorhombic P bnm 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 one. Pa was easily collapsed by application of magnetic field of 0.4T parallel to the spin-canting direction (H c) while it was enhanced parallel to the easy axis (H b).

Magnetization and magnetoresistance were measured in A-type antiferromagnet Nd 0.45 Sr 0.55 MnO 3 utilizing pulsed magnetic fields up to 45 T. We have observed a ferromagnetic transition accompanied by a discontinuous decrease of resistivity. The temperature dependence of the resistivity in the ferromagnetic state showed characteristics of a three-dimensional metal. The observed phenomena are explained in terms of simultaneous destruction of the d x 2 Ϫy 2 orbital ordering and the A-type antiferromagnetic spin ordering by magnetic field.

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