Coexistence or competition of magnetism and electricity due to orbital ordering in transition-metal oxides

在这学期里，我们研究了几个可能具有铁磁性和铁电性的体系。钙钛矿型锰氧化物TbMn_3就是其中之一。TbMnO_3是空穴掺杂钙钛矿型锰氧化物的终端材料，具有庞磁电阻效应，是Mott绝缘体。在这种化合物中，正交晶格畸变很大，Mn自旋之间的次近邻相互作用与最近邻相互作用相当。这些相互作用导致在低温下的公度/非公度磁有序。我们用浮区法成功地生长了TbMnO_3单晶。我们用同步辐射S射线和TbMnO_3单晶进行了X射线分析。结果表明，该材料经历了轨道和晶格的公度/非公度有序以及磁性有序。在相称有序相中，铁电极化出现沿着b轴。磁场改变铁电极化的方向。我们称这种新现象为磁场控制的铁电极化。磁场会导致Tb^3^+磁矩在低温下发生翻转。Tb^3^+磁矩的翻转通过Tb-Mn自旋之间的相互作用诱导Mn^3^+自旋的翻转。最后，Mn离子中的自旋-轨道-晶格相互作用导致磁场控制的铁电极化。

In this term, we have investigated several systems, which have possibility of ferromagnetism and ferroelectlicity. Perovskite-type manganese oxide, TbMn_3 is one of such compounds. TbMnO_3 is an end material of hole-doped perobskite-type manganites, which show colossal magnetoresistance effect, and Mott Insulator. I this compound, orthorhombic lattice distortion is large, and the next-nearest-neighbor interaction between Mn spins is comparable to the nearest-neighbor one. These interactions cause the commensurate/incommensurate magnetic ordering at low temperatures. We succeeded in the crystal growth of a single-crystal of TbMnO_3 by using a floating zone method. We performed the X-ray analysis by using synchrotron S-ray and the single crystal of TbMnO_3. The present results indicate that this material undergoes the commensurate/incommensurate ordering of the orbital and lattice as well as the magnetic one. In the commensurate ordered phase, the ferroelectric polarization appears along b-axis. The magnetic field changes the direction of the ferroelectric polarization. We call the new phenomenon magnetic-field-controlled ferroelectic polarization. The magnetic field causes the flop of the Tb^3^+moment at low temperatures. The flop of Tb^3^+ moment induces that of Mn^3^+ spin via the interaction between Tb-Mn spins. Finally, the spin-orbital-lattice interplay in Mn ion causes the magnetic-field-controlled ferroelectic polarization.