Magnetic and dielectric properties of FeO, FeO, and GaCuO (=Yb and Lu, =Co and Cu)

Yoshii, Kenji  ; Ikeda, Naoshi*; Matsuo, Yoji*; Horibe, Yoichi*; Mori, Shigeo*

The magnetic and dielectric properties have been investigated for the isostructural oxides of FeO, FeO, and GaCuO (=Yb and Lu, =Co and Cu). The magnetization measurements for FeO showed ferrimagnetic ordering at 250 K. This system also exhibited large dielectric constants of 10,000-30,000 at around room temperature, which is attributable to the charge-ordering-induced ferroelectricity, as was proposed in our recent report. The magnetic transition temperatures are lowered to 45-90 K for FeO. Magnetic ordering is not found for GaCuO. AC magnetic susceptibility measurements indicate that magnetic ordering becomes short-ranged by the substitution at the Fe site. The overall characteristic behavior of the magnetic properties is explained in terms of the change of a spin value as well as the dilution of magnetic interactions. Although the AC dielectric measurements show the existence of polar regions in each material, the dielectric constants below 300 K become smaller in the order of FeO, FeCoO, FeCuO, and GaCuO. From the analysis of the dielectric dispersion, the distribution of the fluctuation time of polar regions is wider in FeCoO, FeCuO, and CuGaO than in FeO; a coherent motion of polar regions is suppressed in the substituted systems. By comparison to the results from the magnetic measurements, the dielectric properties are discussed in connection with a charge transfer between the transition-metal 3 orbitals, consistently with the proposed novel mechanism of the ferroelectricity in FeO.