DISSIPATION BEHAVIOR OF YBA2CU3OX SINGLE-CRYSTALS STUDIED THROUGH MAGNETIZATION AND MAGNETIZATION DECAY MEASUREMENTS

Hysteresis loop and magnetization decay measurements performed on a YBCO single crystal in the low temperature range (T less-than-or-equal-to 44 K) are presented. The hysteresis loops show the typical behaviour observed in such kind of sample with the so called 'fishtail effect'. The magnetization decays are analysed in terms of the collective creep theory and, in the explored temperature range, they indicate a very different behaviour of the sample at two relatively near magnetic field values (mu(o) H = 1.6 T and mu(o) H = 3.4 T) where the measurements were performed. The results support the hypothesis of a structural variation of the density and kind of pinning centres induced by field and temperature. This hypothesis could also explain the behaviour of the electric field E versus the current density J measured by 'flux creep annealing'.