Anisotropy of the magnetization of single crystals and bulk high-T_c superconductors

Measurements of the angular dependence of the zero-field-cooled (ZFC), field-cooled (FC) and remanent magnetization of both high-T_c single crystals and polycrystalline samples show significant anisotropy. Comparison of the results obtained on both, prove that most anisotropy is governed by the shape of the sample via the demagnetizing factors. For ceramic samples, there is competition between the demagnetizing factors of the sample as a whole, N, and that of the grains, n. According to a suggested phenomenological model, the magnetic moment is related to the external field by a relation involving an effective demagnetizing factor, N_eff, which depends on the fraction of superconducting volume, which is itself a field dependent quantity. At low fields, N_eff approaches N and n for highly compact and dilute samples, respectively. Between these extremes, N_eff is a weighted average of N and n.