IRREVERSIBILITY LINE IN YBA2CU3O7 THIN-FILMS - CORRELATION OF TRANSPORT AND MAGNETIC-BEHAVIOR

Using magnetic and magnetotransport measurements, we show that a YBa2CU3O7 thin-film sample has a single irreversibility line (IRL), which is the same as the vortex-glass-transition phase boundary. Results suggest that the effects of pinning persist above the glass-transition temperature T(g), and that it is the loss of critical-current density J(c) rather than an onset of reversible magnetic behavior that characterizes the vortex-glass transition. Measurements of dc magnetization are shown to confirm a theoretical model that explains why the determination of T(g) requires that field-cooled data be collected on cooling (FCC) rather than on warming (FCW), as is frequently done. The use of the ac susceptibility response for measuring T(g) is shown to be a valid measure only at low frequency. This is because both the fundamental-frequency and third-harmonic ac susceptibilities measure the onset of ac flux penetration rather than the onset of irreversibility. The frequency dependence of the onset temperature of ac flux penetration (T(on)) is shown to follow the vortex-glass-model dependence T(on) = C(2pif){1/[(z-1)v]}+T(g), with values of z and v consistent with those determined from scaling of the magnetotransport data.