Correlation Phenomena among electric, magnetic and mulitiple dipoles in ferroelectric oxides

固体中电荷和自旋之间的有效相互作用往往与电动力学的基本定律大不相同，这导致了不寻常的现象。在磁性铁电材料(所谓的多铁性材料)中观察到的静电偶极子和磁偶极子之间的耦合是一个令人着迷的效应，它的基础物理和潜在的技术应用都令人感兴趣。在我们的研究项目中，我们主要集中在探索由于电荷、自旋和轨道自由度之间的有效相互作用而导致的铁电磁性氧化物中的巨大电磁效应等新现象，并阐明这些现象的物理根源。主要研究成果如下：(1)我们发现了外加磁场对DyMn_2O_5中铁电态的诱导和修饰现象。该化合物在43K和4K之间相继发生相变，并伴随着三个铁电…更多的阶段。得到了DyMn_2O_5在温度和磁场平面上的电相图。(2)我们发现LuFe_2O_4中的电荷有序(CO)结构是以波矢Q=1/3[1-13/2]的调制结构为特征的，CO的平均尺寸约为10-20 nm。相反，在LuFe_2O_4中，Cu^&lt；2+&gt；(Co^&lt；2+&gt；)取代则严重破坏了CO结构，引起特征局域晶格扭曲，从而引起特征扩散散射。高分辨率晶格图像显示，在LuFeCuO4中存在纳米团簇，其特征是Fe~(2+)&lt；3+&gt；和Cu~(2+)&gt；离子在三角形晶格上的短程有序。磁测量表明，LuFeCu(Co)O_4在45K(80K)附近有一个磁相变，远低于LuFe_2O_4中250K的磁相变温度。(3)我们从理论上分析了层状氧化物RFE_2O_4的电子结构和介电性质。自旋受挫通过磁有序诱导这种极性CO的增强。我们还分析了铁离子的轨道模型，该模型没有显示出常规的长程有序化。较少

Effective interaction between charges and spins in solids are often quite different from the fundamental laws in electrodynamics, which gives rise to unusual phenomena. The coupling between static electric and magnetic dipoles, which are observed in magnetic ferroelectric materials (so-called multiferroic materials), is a fascinating effect interesting both for its fundamental physics and potential technological applications. In our research project, we mainly focused on the exploration of novel phenomena such as the gigantic electromagnetic effect in ferroelectric magnetic oxides, which are caused by effective interaction among charge, spin and orbital degrees of freedom, and elucidated their physical origins of the phenomena. The main research achievements are as follows.(1)We found the phenomena that an external magnetic field induces and modifies ferroelectric states in DyMn_2O_5. This compound shows successive phase transitions between 43K and 4K, accompanying three ferroelectric … More phases. We obtained the electric phase diagram for DyMn_2O_5 in the plane of temperature and magnetic field.(2)We found that the charge-ordered (CO) structure in LuFe_2O_4 is characterized by the modulated structure with the wave vector of q=1/3[1-13/2] and the average size of the CO domains can be estimated to be about 10-20nm. On the contrary, the Cu^<2+> (Co^<2+>) substitution in LuFe_2O_4 destroyed the CO structure drastically and induced characteristic local lattice distortion, which gives rise to characteristic diffuse scattering. High-resolution lattice images in LuFeCuO_4 revealed that there exist nano-scale clusters, which are characterized as the short-range ordering of the Fe^<3+> and Cu^<2+> ions on the triangular lattice. The magnetic measurements revealed that LuFeCu(Co)O_4 exhibits a magnetic transition around 45K(80K), which is much lower than the magnetic transition temperature of 250K in LuFe_2O_4.(3)We analyzed theoretically the electronic structure and dielectric property in a layered oxide RFe_2O_4. Charge frustration in paired-triangular lattices allows a charge imbalance with out inversion symmetry, i.e. the electric polarization. Spin frustration induces reinforcement of this polar CO by a magnetic ordering. We also analyze an orbital model for the Fe ion which does not show a conventional long-range order. Less