Coexistence of superconductivity and weak-ferromagnetism in Eu_2-xCe_xRuSr₂Cu₂ O_10-δ

Eu_2-xCe_xRuSr₂Cu₂ O_10-δ(Ru-2122) is the first Cu-O based system in which superconductivity (SC) in the CuO₂ planes and weak-ferromagnetism (W-FM) in the Ru sublattice coexist. The hole doping in the CuO₂ planes, is controlled by appropriate variation of the Ce concentration and/or increasing the oxygen concentration. SC occurs for Ce contents of 0.4-0.8, with the highest T_C=35 K for Ce=0.6. Due to the granular nature of the materials the magneto-resistance R(H) below T_C is positive and unexpected hysteresis loops are observed. The R(H) curve on decreasing the applied field (H_ext) is much smaller than the R(H) curve for increasing (H_ext). The width of the loops depends strongly on the weak-link properties. The as-prepared non-SC EuCeRuSr₂Cu₂O₁₀ (x=1) sample exhibits magnetic irreversibility below T_irr=125 K and orders anti-ferromagnetically (AFM) at T_M = 165 K. Systematic magnetic studies on Eu_2-xCe_xRuSr₂Cu₂ O_10-δ show that T_M, T_irr and M_sat decrease with x, and the full Ce dependent magnetic-SC phase diagram is presented. A simple model for the SC state is proposed. The interpretation of the magnetic behavior is: (i) the system becomes antiferromagnetically (AFM) ordered at T_M; (b) at T_irr < T_M, W-FM is induced by the canting of the Ru moments, and (c), at lower temperatures the appropriate samples become SC at T_C. The magnetic features are not affected by the SC state, and the two states coexist.