The magnetic and dielectric properties have been investigated for the isostructural oxides of $R{\mathrm{Fe}}_2{\mathrm{O}}_4$, $R\mathrm{Fe}M{\mathrm{O}}_4$, and $R\mathrm{Ga}\mathrm{Cu}{\mathrm{O}}_4$ ($R=\mathrm{Yb}$ and Lu, $M=\mathrm{Co}$ and Cu). The magnetization measurements for $R{\mathrm{Fe}}_2{\mathrm{O}}_4$ showed ferrimagnetic ordering at $\sim 250\mathrm{K}$. This system also exhibited large dielectric constants of $\sim 10000–30000$ 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 $\sim 45–90\mathrm{K}$ for $R\mathrm{Fe}M{\mathrm{O}}_4$. Magnetic ordering is not found for $R\mathrm{Ga}\mathrm{Cu}{\mathrm{O}}_4$. 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 $300\mathrm{K}$ become smaller in the order of $R{\mathrm{Fe}}_2{\mathrm{O}}_4$, $R\mathrm{Fe}\mathrm{Co}{\mathrm{O}}_4$, $R\mathrm{Fe}\mathrm{Cu}{\mathrm{O}}_4$, and $R\mathrm{Ga}\mathrm{Cu}{\mathrm{O}}_4$. From the analysis of the dielectric dispersion, the distribution of the fluctuation time of polar regions is wider in $R\mathrm{Fe}\mathrm{Co}{\mathrm{O}}_4$, $R\mathrm{Fe}\mathrm{Cu}{\mathrm{O}}_4$, and $R\mathrm{Ga}\mathrm{Cu}{\mathrm{O}}_4$ than in $R{\mathrm{Fe}}_2{\mathrm{O}}_4$; 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 $R{\mathrm{Fe}}_2{\mathrm{O}}_4$.

• Received 28 December 2006

DOI:https://doi.org/10.1103/PhysRevB.76.024423