Magnetic-field-control of electric polarization in multiferroic transition metal oxides

在没有稀土离子4f磁矩存在的情况下，研究了Eu_<0.595>Y_<0.405>MnO_3的介电和磁性能，发现了在零磁场下沿a轴和c轴极化的两个铁电相。通过施加平行于A轴的磁场，铁电极化方向由A轴转变为c轴，从而实现了铁电相从一相到另一相的磁场诱导转换。在TbMnO_3中，Tb^<3+>离子的4f矩在铁电相转换中起重要作用，而在Eu_<0.595>Y_<0.405>MnO_3中，磁场诱导的铁电相转换不是由稀土4f矩的磁跃迁引起的，而是由Mn三维自旋的磁跃迁引起的。我们还研究了4f矩对正交RMnO_3 （R=稀土离子）磁电效应的影响。为了明确4f矩的作用，我们制备了3种样品：不含4f矩的（Eu， Y）MnO_3、各向异性4f矩的TbMnO_3和各向同性4f矩的（Gd， Y）MnO_3。这些样品在零磁场下的铁电行为是不同的。（Eu， Y）MnO_3和（Gd， Y）MnO_3基态表现为沿a轴的铁电极化，而TbMnO_3基态表现为沿c轴的铁电极化。这种差异可能是由于各向异性Tb^<3+> 4f矩的影响。RMnO_3的铁电极化方向由4f矩产生的内部磁场决定。

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 place 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 also investigated the role of the 4f moment on the magnetoelectric (ME) effect of orthorhombic RMnO_3 (R=rare earth ions). In order to clarify the role of the 4f moment, we prepared three samples: (Eu, Y)MnO_3 without the 4f moment, TbMnO_3 with the anisotropic 4f moment, and (Gd, Y)MnO_3 with the isotropic 4f moment. The ferroelectric behaviors of these samples are different from each other in a zero magnetic field. (Eu, Y)MnO_3 and (Gd, Y)MnO_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^<3+> 4f moment. The direction of the ferroelectric polarization of RMnO_3 is determined by the internal magnetic field arising from the 4f moment.