Magnetic clusters in superconducting lightly doped YBa2Cu3O6+x

We report mu SR experiments on two lightly doped superconducting YBa2Cu3O6+x samples (0.39 <= x <= 0.41), performed with the initial muon spin polarization rotated by 47 degrees from the applied field direction. At low temperature, the muon asymmetry of both samples exhibits a dominant component with very large depolarization rates, due to the presence of frozen magnetic clusters. A two-component model, accounting for muon sites close to the clusters and far from them, respectively, is presented. The model provides a prediction for the time evolution of both the longitudinal and transverse projections of the muon polarization, and yields a good simultaneous fit to the data of each sample at all the applied fields. The best-fit parameters demonstrate that the static internal fields from the magnetic clusters are disordered, in agreement with the absence of long-range magnetic Bragg peaks in neutron diffraction.