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

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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