Coexistence of ferromagnetism and superconductivity in EuSr2Ru1-xNbxCu2O8

The structural, magnetic and transport properties of EuSr2Ru1-xNbxCu2O8 (x = 0, 0.1, 0.2 and 0.5) samples have been investigated by means of x-ray dif fraction, magnetization and resistance measurements. It is found that the E uSr2RuCu2O8 compound exhibits a ferromagnetic order of the Ru moments below the Curie temperature of 130.19K, and becomes superconducting at a much lo wer critical temperature of 10 K. The experimental observation that partial substitution of Nb for Ru in this system results in the decrease of the Cu rie temperature further supports the idea that the ferromagnetic order invo lves 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+ valence state while R u is in a mixed valence state between Ru4+ and Ru5+. The coexistence of fer romagnetism and superconductivity is also discussed.