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.
We have investigated the role of the 4f moment on the magnetoelectric (ME) effect of orthorhombic RMnO 3 (R ¼ rare earth ions). In order to clarify the role of the 4f moment, we prepared three samples: ðEu; YÞMnO 3 without the 4f moment, TbMnO 3 with the anisotropic 4f moment, and ðGd; YÞMnO 3 with the isotropic 4f moment. The ferroelectric behaviors of these samples are different from each other in a zero magnetic field. ðEu; YÞMnO 3 and ðGd; YÞMnO 3 show the ferroelectric polarization along the a axis in the ground state, while TbMnO 3 shows it along the c axis. Such difference may arise from the influence of the anisotropic Tb 3þ 4f moment. The direction of the ferroelectric polarization of RMnO 3 is determined by the internal magnetic field arising from the 4f moment. r 2006 Published by Elsevier B.V.A strong correlation between dielectric and magnetic properties, so-called magnetoelectric (ME) effect, has attracted a revived interest. The series of orthorhombic RMnO 3 (R ¼ rare earth ions) containing TbMnO 3 famous as the ''magnetic-field-induced electric polarization flop'' [1] is a parent material of the colossal magnetoresistance (CMR) manganite. Recently, the noncollinear transverse spiral antiferromagnetic (AF) order of Mn 3d spins is observed between the A-type (EuMnO 3 ) and the E-type (HoMnO 3 ) AF order [2][3][4]. Such unconventional magnetic order is caused by the competition between the ferromagnetic nearest neighbor (NN) interaction and the AF next NN interaction due to the relatively large orthorhombic distortion. Such noncollinear transverse spiral magnetic structure causes the ferroelectric polarization of RMnO 3 [3-6].In previous work, we have investigated the ME effect in (Eu 0:595 Y 0:405 ÞMnO 3 (ðEu; YÞMnO 3 ) without 4f moment [7]. Even if the average ionic radius of R site in ðEu; YÞMnO 3 is the same as that of TbMnO 3 , the direction of the ferroelectric polarization of the ground state of ðEu; YÞMnO 3 is different from that of TbMnO 3 in a zero magnetic field: ðEu; YÞMnO 3 shows the ferroelectric polarization along the a axis (P a ), while TbMnO 3 shows P c . It is considered that this difference is attributed to the magnetic property of each R ion.In this paper, we have revealed the role of the rare-earth 4f moment on the ME effect of orthorhombic RMnO 3 . In order to investigate the role of the 4f moment, we prepared three samples: ðEu; YÞMnO 3 without the 4f moment, TbMnO 3 with the anisotropic 4f moment, and (Gd 0:69 Y 0:31 ÞMnO 3 (ðGd; YÞMnO 3 ) with the isotropic 4f moment. The average ionic radius of R site in these samples is fixed to be same as that of TbMnO 3 .The compositional ratio of the Eu 3þ to Y 3þ ions and that of Gd 3þ to Y 3þ were obtained by a calculation based on the Shannon's ionic radius table [8]. The single crystal sample was grown by the floating zone method. We performed X-ray diffraction and rocking curve measurements on the resulting crystal at room temperature, and confirmed that the samples have the orthorhombic Pbnm structure without any impurity phases ...
We have investigated orthorhombic RMnO 3 (R=(Gd 1−y Tb y )) crystals near the phase boundary between the paraelectric A-type-antiferromagnetic (AF) phase (PA) and the ferroelectric transverse-spiral-AF one (FS). The spiral AF structure breaks inversion symmetry and induces the ferroelectric polarization through the inverse Dzyaloshinskii-Moriya (DM) interaction. We have found that the PA-FS phase boundary is located at around 0.15
We have investigated the dielectric properties of Gd1−xTbxMnO3 crystals in magnetic fields to clarify the crossover between two distinct ferroelectric phases, GdMnO3type (first-order displacive) and TbMnO3-type (second-order order-disorder). We have found that the compositional phase boundary between the phases exists around 0.3
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