IDENTIFYING THE LOSS OF CRITICAL CURRENT-DENSITY IN YBA2CU3O7 THIN-FILMS

The magnetic and magnetotransport behavior of a YBa2Cu3O7 thin-film sample was studied by a variety of methods that have been used previously to determine the irreversibility line (IRL). From transport measurements it is possible to identify at least two regions separated by a boundary identified as a vortex-glass transition, with the region below the boundary having zero linear resistance and that above having a low-current linear resistivity which seems to be associated with thermally activated flux motion. Evidence for flux pinning above the glass transition suggests that there is a loss of critical current density (J(c)) rather than an onset of reversible magnetic behavior at the glass transition. The complications of using first-harmonic ac susceptibility (chi(ac)) to define the IRL are discussed. In addition, the third-harmonic chi(ac) is shown to measure the same property that the first-harmonic chi(ac) measures: ac magnetic-field penetration rather than the loss of J(c). Results and a theoretical model showing that there are significant differences between field-cooled dc magnetization measurements when the data are collected on warming versus collecting data on cooling are presented.