AC inductance measurements of high temperature superconductors

Abstract

AC inductance measurements are frequently used to determine the properties of superconductive materials. Both the real and the imaginary (absorptive) components of the sample susceptibility can be measured by these techniques. For high temperature superconductors the absorptive component typically shows one or more peaks versus temperature, with the peak temperature depending upon the AC frequency and AC field strength and whether a DC field is present. The dependence of this peak temperature 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 line from irreversible regions below the line. This behavior has been discussed in terms of magnetic glass transitions, flux lattice melting, and thermally activated flux flow theories. The IL is important because theory suggests that a critical current JC cannot exist in the reversible region, so that this line defines the magnetic field temperature conditions in which a JC can exist. In this work data will be presented determining the IL for bulk and particularly for thin film HTSC 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.