Coexistence of Ferromagnetism and Superconductivity in EuSr₂Ru_1-xNb_xCu₂O₈

The structural, magnetic and transport properties of
EuSr₂Ru_1-xNb_xCu₂O₈ (x = 0, 0.1, 0.2 and 0.5) samples have been investigated by means of x-ray diffraction, magnetization and resistance measurements. It is found that the EuSr₂Ru_1-xNb_xCu₂O₈ compound exhibits a ferromagnetic order of the Ru moments below the Curie temperature 130.19 K, and becomes superconducting at a much lower critical temperature of 10 K. The experimental observation that partial substitution of Nb for Ru in this system results in the decrease of the Curie temperature further supports the idea that the ferromagnetic order involves only Ru moments in this compound. We suggest that the decrease of the superconducting critical temperature T_c due to the partial substitution of Nb for Ru originates from the decrease of the carrier concentration in the CuO₂ plane because of Nb existing always in the Nb⁵⁺ valance state while Ru is in a mixted valance state between Ru⁴⁺ and Ru⁵⁺. The coexistence of ferromagnetism and superconductivity is also discussed.