Charge Segregation, Cluster Spin Glass, and Superconductivity inLa1.94Sr0.06CuO4

A ${}^{63}\mathrm{Cu}$ and ${}^{139}\mathrm{La}$ NMR/NQR study of superconducting $(T_c=8\mathrm{}\mathrm{K})$ ${\mathrm{La}}_{1.94}{\mathrm{Sr}}_{0.06}{\mathrm{CuO}}_4$ single crystal is reported. Coexistence of spin-glass and superconducting phases is found below $\sim 5\mathrm{K}$ from ${}^{139}\mathrm{La}$ NMR relaxation. ${}^{63}\mathrm{Cu}$ and ${}^{139}\mathrm{La}$ NMR spectra show that, upon cooling, ${\mathrm{CuO}}_2$ planes progressively separate into two magnetic phases, one of them having enhanced antiferromagnetic correlations. These results establish the antiferromagnetic-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.