Magnetic and dielectric properties of the ruthenium double perovskites la ₂MRuO ₆ (M = Mg, Co, Ni, and Zn)

Magnetic and dielectric properties of the ruthenium double perovskites La ₂MRuO ₆ (M = Mg, Co, Ni, and Zn) were investigated. The magnetization measurements for M = Co and Ni showed the existence of magnetic order at 20-30 K. Though the oxides with M = Zn and Mg exhibit a deviation from the Curie-Weiss law, magnetic order was not clearly observed. The result of La ₂ZnRuO ₆ was different from that previously reported, in which a ferromagnetic transition was found at around 165 K. The AC dielectric measurements for M = Co and Ni showed large dielectric constants (typically larger than 1000) at around room temperature, suggesting both the formation of short-ranged polar regions and the magnetic origin of large dielectric constant. In addition, two peaks were found for the temperature dependence of the tan δ component for La ₂NiRuO ₆. The behavior suggests the existence of two different polar regions.