Coexistence of Superconductivity and Magnetism in R2-xcexrusr2cu 2o10-?   (R=Eu and Gd)
  R2-xCexRuSr2Cu2O10-d(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 coexists. The hole doping in the CuO2 planes, is controlled by appropriate variation of the Ce concentration and/or increasing the oxygen concentration. In Eu2-xCexRuSr2Cu2O10, SC occurs for Ce contents of 0.4-0.8, with the highest TC=35 K for Ce=0.6. The as-prepared non-SC EuCeRuSr2Cu2O10 sample exhibits magnetic irreversibility below Tirr=125 K and long range o anti-ferromagnetic (AFM) order at TM =165 K. The saturation moment at 5 K is Msat=0.89 mB /Ru. Annealing under oxygen pressures, does not affect these parameters, whereas depletion of oxygen shifts both Tirr and TM up to 169 and 215 K respectively. TM, Tirr and Msat decrease with x, and the Ce dependent magnetic-SC phase diagram is presented. A simple model for the SC and the long-range W-FM states is proposed. We argue that: (i) the system becomes AFM ordered at TM; (b) at Tirr < TM, W-FM is induced by the canting of the Ru moments, and (c), at lower temperatures the appropriate samples become SC at TC. The magnetic features are not affected by the SC state, and the two states coexist. 
I. Felner