  Eu2-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 sub-lattice coexists. The hole doping in the CuO2 planes, is controlled by
appropriate variation of the Ce concentration. 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 (x=1) sample exhibits magnetic irreversibility below Tirr=125 K
and orders anti-ferromagnetically (AFM) at TM =165 K. The saturation moment at
5 K is Msat=0.89 mB /Ru close to the expected 1 mB for the low-spin state of
Ru5+. 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. Systematic magnetic studies on Eu2-xCexRuSr2Cu2O10-d show that
TM, Tirr and Msat decrease with x, and the Ce dependent magnetic-SC phase
diagram is presented. A simple model for the SC state is proposed. We interpret
the magnetic behavior in the framework of our ac and dc magnetic studies, and
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.
