Relationship between weak ferromagnetism, superconductivity, and lattice parameter in the A2−x−y>BxCeyCuO4 (A,B≡La, Pr, Nd, Sm, Eu, Gd, Y) compounds

Abstract

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 A_2−x−yB_xCe_yCuO₄ 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≊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≊3.91 Å, regardless of the rare‐earth (R) ions present in the system. Superconductivity is observed only in the Ce‐doped compounds with a≳a₁, a₁≊3.92 Å. The maximum onset temperature T_c is found for a≊3.95 Å. 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.