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

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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.