RELATIONSHIP BETWEEN WEAK FERROMAGNETISM, SUPERCONDUCTIVITY, AND LATTICE-PARAMETER IN THE A2-X-YBXCEYCUO4 (A, B=LA, PR, ND, SM, EU, GD, Y) COMPOUNDS

We report the results obtained by x-ray, dc magnetization, ac magnetic susceptibility, microwave magnetic absorption, and electron paramagnetic resonance of the magnetic and superconductive properties of a large series of compounds of the form A2-x-yBxCeyCuO4 with A,B=La, Pr, Nd, Sm, Eu, Gd, and Y For y = 0 antiferromagnetic ordering of the copper moments is present at T(N) almost-equal-to 280 K for all the compounds formed in the tetragonal T' phase. Weak ferromagnetism (WF) associated with the canting of the copper moments is observed in all the compounds with a lattice parameter, a<a0, a0 almost-equal-to 3.91 angstrom regardless of the rare-earth (R) ions present in the system. Superconductivity is observed only in the Ce-doped compounds with a>a1, a1 almost-equal-to 3.92 angstrom. The maximum onset temperature T(c) is found for a almost-equal-to 3.95 angstrom. A phase diagram was obtained by varying the lattice parameter size as a function of A, B, and x. The correlation between the appearance of WF in the undoped compounds and the disappearance of superconductivity in the Ce-doped systems is discussed.