Multiferoic properties of rare-earth iron oxides with frustrated charge interactions

研究了YbFe_2O_4的磁性和介电性能。由于其介电性能至今未见报道，该材料的磁转变温度为250 K，介电常数约为10000，与其它RFe_2O_4氧化物（R：稀土）相近。本文还研究了改变CO/CO_2流量比合成YbFe_2O_4的最佳条件。本研究目前主要以磁化强度量测为主要手段，并探讨铁位取代氧化物之磁性与介电性质。结果表明，随着掺杂量的增加，介电常数的降低程度依次为RFe_2O_4、RFeCoO_4、RFeCuO_4和RGaCuO_4。考虑到从磁性测量的结果，这一结果被解释与Fe位离子之间的电子转移。因此，我们认为RFe_2O_4型氧化物的介电响应不同于一般的铁电体和介电常数，其介电响应的主要机制是阳离子和阴离子的置换，而在其它一些过渡金属氧化物中，如R_2RuNiO_6和<0.5>R_Ca_MnO_3<0.5>（R;稀土），电子转移可能是重要的。在后一种体系中，介电响应的活化能与Mn-3d电子转移的活化能接近，表明介电响应与电子转移之间存在相关性，并发现这些氧化物显示出较小的活化能来旋转极性反应。这一事实意味着它们的介电响应与晶格畸变弱耦合，并提供了使用这些材料制造无疲劳器件的可能性。

The magnetic and dielectric properties of YbFe_2O_4 have been investigated., as its dielectric properties have not been reported so far The material showed a magnetic transition temperature of 250 K and dielectric constants of about 10000, both of which are close to those of the other RFe_2O_4 oxides (R: rare earths). This oxide can be prepared from the low-cost starting material of Yb_2O_3; accordingly, YbFe_2O_4 is suitable for practical applicationWe have also started a research to find the optimum synthesis condition of YbFe_2O_4 changing a CO/CO_2 flow ratio. This study is currently under way mainly by magnetization measurements.In addition, the magnetic and dielectric properties of the Fe-site substituted oxides have been investigated. It was found that the dielectric constant was decreased in the order of RFe_2O_4, RFeCoO_4, RFeCuO_4, and RGaCuO_4. In consideration of the results from magnetic measurements, this result was explained in connection with the electron transfer between the Fe-site ions. Thus, we proposed that the dielectric response of the RFe_2O_4-type oxides has the origin different from that of ordinary ferroelectrics and dielectrics, in which the displacement of cations and anions plays a central role.Electron transfer was found to be likely important in a few other transition-metal oxides, such as R_2RuNiO_6 and R_<0.5>Ca_<0.5>MnO_3 (R; rare earths). In the latter system, the activation energy in the dielectric response was close to that of the transfer of Mn-3d electrons, suggesting a correlation between the dielectric response and the electron transfer.It was also found that these oxides showed small activation energies to rotate the polar retions. This fact implies that their dielectric response is weakly coupled to the lattice distortion, and offers a possibility of fabricating fatigue-free devices using these materials.