Magnetic and dielectric properties of R Fe2 O4, RFeM O4, and R GaCu O4 (R=Yb and Lu, M=Co and Cu)

The magnetic and dielectric properties have been investigated for the isostructural oxides of R Fe2 O4, RFeM O4, and R GaCu O4 (R=Yb and Lu, M=Co and Cu). The magnetization measurements for R Fe2 O4 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 paper. The magnetic transition temperatures are lowered to ∼45-90 K for RFeM O4. Magnetic ordering is not found for RGaCu O4. 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 R Fe2 O4, RFeCo O4, RFeCu O4, and RGaCu O4. From the analysis of the dielectric dispersion, the distribution of the fluctuation time of polar regions is wider in RFeCo O4, RFeCu O4, and RGaCu O4 than in R Fe2 O4; 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 3d orbitals, consistently with the proposed mechanism of the ferroelectricity in R Fe2 O4.