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Magnetic and dielectric properties of $textit{R}$Fe$$_2$$O$$_4$$, $textit{R}$Fe$textit{M}$O$$_4$$, and $textit{R}$GaCuO$$_4$$ ($textit{R}$=Yb and Lu, $textit{M}$=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 ${it R}$Fe$$_{2}$$O$$_{4}$$, ${it R}$Fe${it M}$O$$_{4}$$, and ${it R}$GaCuO$$_{4}$$ (${it R}$=Yb and Lu, ${it M}$=Co and Cu). The magnetization measurements for ${it R}$Fe$$_{2}$$O$$_{4}$$ showed ferrimagnetic ordering at $$sim$$250 K. This system also exhibited large dielectric constants of $$sim$$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 $$sim$$45-90 K for ${it R}$Fe${it M}$O$$_{4}$$. Magnetic ordering is not found for ${it R}$GaCuO$$_{4}$$. 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 K become smaller in the order of ${it R}$Fe$$_{2}$$O$$_{4}$$, ${it R}$FeCoO$$_{4}$$, ${it R}$FeCuO$$_{4}$$, and ${it R}$GaCuO$$_{4}$$. From the analysis of the dielectric dispersion, the distribution of the fluctuation time of polar regions is wider in ${it R}$FeCoO$$_{4}$$, ${it R}$FeCuO$$_{4}$$, and ${it R}$CuGaO$$_{4}$$ than in ${it R}$Fe$$_{2}$$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 3${it d}$ orbitals, consistently with the proposed novel mechanism of the ferroelectricity in ${it R}$Fe$$_{2}$$O$$_{4}$$.

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Category:Materials Science, Multidisciplinary

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