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

对YbFe2O4的磁性和介电性能进行了研究，由于YbFe2O4的介电性能尚未见报道，该材料的磁相变温度为250K，介电常数约为10000，均接近其他RFe2O4氧化物(R：稀土)的介电常数。以低成本的Yb_2O_3为原料可以制备YbFe_2O_4，因此YbFe_2O_4适合于实际应用。我们还开始了改变CO/CO_2流量比来寻找YbFe_2O_4的最佳合成条件。这项研究目前主要是通过磁化测量进行的，此外，还研究了Fe位取代氧化物的磁性和介电性能。发现介电常数按RFe_2O_4、RFeCoO_4、RFeCuO_4、RGaCuO_4的顺序减小。结合磁性测量结果，这一结果与Fe位离子之间的电子转移有关。因此，我们提出了RFE_2O_4型氧化物的介电响应具有不同于普通铁电和介电材料的介电响应的起源，其中阳离子和阴离子的位移起着中心作用。电子转移在其他一些过渡金属氧化物中可能是重要的，如R_2RuNiO_6和R_&lt；0.5&gt；Ca_&lt；0.5&gt；MnO_3(R；稀土)。在后一种体系中，介电响应中的激活能接近于Mn-3d电子转移的激活能，表明介电响应与电子转移之间存在关联，并发现这些氧化物表现出较小的转动极性反应的激活能。这一事实表明，它们的介电响应与晶格扭曲的耦合很弱，这为利用这些材料制造无疲劳器件提供了可能性。

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.