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

We report detailed magnetic-susceptibility studies for the oxygen-deficient perovskite superconductors $M{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ ($M=\mathrm{Y},\mathrm{G}\mathrm{d},\mathrm{E}\mathrm{r},\mathrm{H}\mathrm{o}$). Normal-state behavior above $T_c$ shows weak ferromagnetic (antiferromagnetic) tendencies for $M=\mathrm{Y},\mathrm{G}\mathrm{d}$ ($M=\mathrm{E}\mathrm{r},\mathrm{H}\mathrm{o}$), with 87% of the free-ion moment for $M=\mathrm{G}\mathrm{d},\mathrm{E}\mathrm{r},\mathrm{H}\mathrm{o}$. The estimated Fermi-level density of states is 6.4 states/eV Cu for $M=\mathrm{Y},\mathrm{G}\mathrm{d}$, which is twice the band-structure estimate. The superconducting state shows a 20% or more flux exclusion at small field. The Curie contributions for $M=\mathrm{G}\mathrm{d},\mathrm{E}\mathrm{r},\mathrm{H}\mathrm{o}$ are weakened for $T<\mathrm{}{T}_c$ 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.