AC SUSCEPTIBILITY MEASUREMENTS AND THE IRREVERSIBILITY LINE OF HIGH-TEMPERATURE SUPERCONDUCTORS

The irreversibility line for high-temperature superconductors has been reported in the literature both from dc magnetization measurements and from ac susceptibility measurements. The latter, determined from the temperature of the peak of the absorptive component of the susceptibility, are controversial because the magnetic-field-energy absorption can be interpreted in different ways. It is shown here that the temperature dependence of the absorptive susceptibility peak measured as a function of ac magnetic field alone for single-crystal Bi2Sr2CaCu2O8+y, Tl2Ba2CaCu2O8+y, and YBa2Cu3O7-x is not related to the irreversibility line. However, the same quantity measured using a weak ac field in the presence of a larger dc field gives good agreement with the irreversibility line determined from dc magnetization data for these materials. An explanation is given for why the absorption peaks are different in the two cases, and it is shown where the relationship to the irreversibility line arises. The case of a ceramic sample is considered, and it is demonstrated that ac measurements cannot be used for irreversibility-line measurements in this material because of internal-magnetic-field effects.