AC INDUCTANCE MEASUREMENTS OF HIGH-TEMPERATURE SUPERCONDUCTORS

AC inductance measurements are frequently used to determine the proper ties of superconductive materials. Both the real and the imaginary (ab sorptive) components of the sample susceptibility can be measured by t hese techniques. For high temperature superconductors the absorptive c omponent typically shows one or more peaks versus temperature, with th e peak temperature depending upon the AC frequency and AC field streng th and whether a DC field is present. The dependence of this peak temp erature upon DC magnetic field is called the irreversibility line IL. The IL is different for different materials and for different forms of the same material; it is considered to represent a critical magnetic field line which separates magnetically reversible regions above the l ine from irreversible regions below the line. This behavior has been d iscussed in terms of magnetic glass transitions, flux lattice melting, and thermally activated flux flow theories. The IL is important becau se theory suggests that a critical current J(c) cannot exist in the re versible region, so that this line defines the magnetic field - temper ature conditions in which a J(c) can exist. In this work data will be presented determining the IL for bulk and particularly for thin film H TSC materials as a function of AC frequency, magnetic field strength, and temperature. Some correlations will be made with critical currents calculated from hysteresis measurements and the critical state model.