Low-field magnetic susceptibility studies of high-T_c superconductors

AC susceptibility measurements of various kinds of high-T_c superconductors in low magnetic fields were made and analyzed on the basis of the Anderson-Kim critical state model. Explicit expressions were derived to account for the temperature at the peak of the out-of-phase susceptibility as a function of AC and DC magnetic fields. Comparisons of the experimental data with theoretical analysis show satisfactory agreements, from which the variations of the pinning force and critical current densities were quantitatively determined. The difference between field-cooled and zero-field-cooled behavior is discussed in terms of flux trapping and the relevant parameters in the theoretical model.