Competition of superconductivity and magnetism in MoSr₂R_1.5Ce_0.5Cu₂O_10−δ (R=rare-earth)

We have investigated the MoSr₂R_1.5Ce_0.5Cu₂O_10−δ (Mo-1222R, R=rare earth) system by several complementary experimental techniques. In contrast to the isostructural RuSr₂R_1.5Ce_0.5Cu₂O_10−δ (Ru-1222) system, in which superconductivity (SC) in the CuO₂ planes and weak ferromagnetism in the Ru sublattice coexists, in Mo-1222, displays a competition between the two states, namely, SC vanishes when the magnetic order sets in. The contraction in the R elements leads to a change of the physical states. The light R ions (Pr and Nd) are paramagnetic down to 5 K, whereas the middle R ions (Sm and Eu) are SC at T_c 18–23 K, respectively. The SC charge carriers originate from the CuO₂ planes, and annealing under oxygen pressures does not affect T_c. A simple model for the SC state is proposed. For the heavy R elements Ho-Lu and Y, the pentavalent Mo layers are antiferromagnetically (AFM) ordered at T_N ranging from 13–26 K. For R=Gd, the sample is not SC and exhibit two magnetic transitions at 11 and 184 K. Both the SC or AFM states depend strongly on the R/Ce ratio and for R/Ce=1, both states are suppressed.