Influence of magnetic rare-earth ion substitution on the magnetic susceptibility of high-Tc superconductors

We report detailed magnetic-susceptibility studies for the oxygen-deficient perovskite superconductors MBa₂Cu₃O_7-δ(M=Y,Gd,Er,Ho). Normal-state behavior above T_c shows weak ferromagnetic (antiferromagnetic) tendencies for M=Y,Gd(M=Er,Ho), with 87% of the free-ion moment for M=Gd,Er,Ho. The estimated Fermi level density of states is 6.4 states/eV Cu for M=Y,Gd, which is twice the band-structure estimate. The superconducting state shows a 20% or more flux exclusion at small field. The Curie contributions for M=Gd,Er,Ho are weakened for T<T_c due to diamagnetic screening. The local moments appear to have little or no effect on the superconducting state. These effects are well described by a simple theoretical model.