Multilevel granular structure and its coupling distribution in melt-textured YBa2Cu3O7-δ

We have performed AC magnetic susceptibility measurements as a function of the excitation magnetic field, h, for two YBa2Cu3O7-delta polycrystalline samples using a high-sensitivity home-made susceptometer, for fixed values of the temperature, T, and the external DC magnetic field, H, One of the samples is melt-textured and for H < 52.5 Oe it presents features that can be associated with a multilevel granular structure. Using an effective medium model, we have determined the critical current density distribution associated with the multilevel granular structure. The average critical current density in each intergranular region was then calculated by applying Bean's critical state model. (C) 1999 Elsevier Science B.V. All rights reserved.