Field dependence of the magnetization of granular Y-Ba-Cu-O

We have studied the field dependence of the zero-field-cooled magnetization of granular Y-Ba-Cu-O samples of different packing factors (between 10% and 80%). We discuss the competition between the demagnetizing factors of the sample, N, and that of the grains, n. Applying the Clausius-Mossotti approximation, we suggest a phenomenological model to treat the experimental results. According to the model the effective demagnetizing factors Neff depend on the fraction of shielded volume, which is itself a field-dependent quantity. At low fields, Neff approach N and n for highly compact and dilute samples, respectively. The suggested analysis enables the distinction and characterization of the different contributions to the total volume of the sample.