Percolation effects and oxygen inhomogeneities in YBa2Cu3O7-' crystals

The magnetization curves of YBa2Cu3O7-' crystals are widely observed to exhibit a pronounced low-field minimum in the temperature range 30 K<T<Tc. We demonstrate that these magnetization anomalies correlate with the c-axis lattice parameter, and hence with the specific oxygen deficiency '. We interpret this low-field feature in terms of a field-induced decoupling of regions of oxygen-rich material by boundaries of oxygen-poor material, a phenomenon reminiscent of the behavior in granular superconductors. A phase diagram that demarcates the multigrain onset as a function of temperature and ' is then constructed. The shape of the phase boundaries is described using a percolation model. This analysis demonstrates that magnetization is a very sensitive measure of crystal quality.