  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.
