Doping the high- T(c) superconductor YBa2Cu3O6.7 with 1.5% of nonmagnetic Zn impurities in CuO2 planes is shown to produce a considerable broadening of 63Cu NMR spectra, as well as an increase of low-energy magnetic fluctuations detected in 63Cu spin-lattice relaxation measurements. A model-independent analysis demonstrates that these effects are due to the development of staggered magnetic moments on many Cu sites around each Zn and that the Zn-induced moment in the bulk susceptibility might be explained by this staggered magnetization. Several implications of these enhanced antiferromagnetic correlations are discussed.

A 63 Cu and 139 La NMR/NQR study of superconducting (Tc=7 K) La1.94Sr0.06CuO4 single crystal is reported. Coexistence of spin-glass and superconducting phases is found below ∼5 K from 139 La NMR relaxation. 63 Cu and 139 La NMR spectra show that, upon cooling, CuO2 planes progressively separate into two magnetic phases, one of them having enhanced antiferromagnetic correlations. These results establish the AF-cluster nature of the spin-glass. We discuss how this phase can be related to the microsegregation of mobile holes and to the possible pinning of charge-stripes.