FREQUENCY-DEPENDENCE OF THE MAGNETIC IRREVERSIBILITY LINE IN HIGH-TEMPERATURE SUPERCONDUCTORS

The magnetic irreversibility line (IL) has been measured as a function of magnetic field and frequency using an ac inductance technique in single crystals of YBaCuO and BiSrCaCuO and in consolidated YBaCuO fibers. The IL is defined as the temperature of the peak in X] as a function of magnetic field. Above the IL magnetic flux is mobile in the superconductor and the critical current J c is zero; below the IL flux is pinned and a Jc can exist. Very small (<1 K) shifts in the temperature of the peak in X] in YBaCuO crystals are seen in fields up to 400 Oe, and for frequencies up to 20 kHz. In contrast, crystals of BiSrCaCuO show shifts downward of the X] peak temperature of more than 40 K at 100 Hz, and in fields of 400 Oe. Intermediate behavior is seen for YBaCuO fibers. These data and particularly the frequency dependence of the IL are discussed in terms of flux creep/flux flow in these materials.