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 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).
I ) Sophia Univ.. i -I fioi-cho, Chiyoda-ku, Tukyu 102-8554, J a y a~~
IntroductionThe strong correlation between magnetic and electric properties has been attracted revived interest since the discovery of colossal magnetoresistive manganites (CMR). In a localized system, effects of the strong correlation are observed through a magnetocapacitance or magnetodielectric response in several materials such as BiMn03 111. A series of RMn03 (R is rare earth ion) is famous as a parent material of CMR manganites. In this system, a rich magnetic phase diagram as a function of the ionic radius of R (so-called Devil's flower) was reported, in which incommensurate (IC) and commensurate (C) antiferromagnetic (AF) phase transitions were observed between A-type AF and E-type AF phases [2]. Such curious AF states were explained in a scenario of the spin frustration caused by the combination of significant orthorhombic distortion and staggered orbital order. Furthermore, the magnetic ferroelectricity has been reported at the IC-C (lock-in) magnetic transition temperature in TbMn03 [3]. Recently, we have found that the ferroelectric phase exists below 13K in the A-type AF GdMn03 crystal [4][5]. The spontaneous polarization was confirmed to be present along a axis in the orthorhombic Pbnna setting. GdMn03 crystal shows IC transition at 42K, A-type AF transition of Mn 3d spins at 20K, and long-range order transition of Gd 4fspins at 13K in warming scan. (A-type AF and Gd order transitions have thermal hystereses and lattice strictions.) The ferroelectric transition was due to the lattice modulation connected
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