FLUX-CREEP, FLUX-PINNING AND CRITICAL-CURRENT DENSITY OF Y-BASED, GD-BASED, AND TL-BASED SUPERCONDUCTIVE THIN-FILMS

Magnetic hysteresis loops have been measured for YBa2Cu3O6+x, GdBa2Cu3 O6+x and Tl2Ba2Ca2Cu3O10-y films at different temperatures with a vibr ating sample magnetometer. Based on Bean model, magnetic critical curr ent density J(cm) has been derived approximately. The field and temper ature dependence of J(cm) for all samples can be written as: J(cm) = J (c0)(T) x f(B,T), where f(B,T) = 1 - Aln(B/B0(T)), which is similar to the transport critical density strongly affected by flux creep: J(ct) = J(c0)(B, T)(1 - (k(B)T/U0)ln(BOMEGAd/E(c))). The extraordinary simi larity suggests that J(cm) is determined not only by flux pinning but also by flux creep. In the first formula, f(B, T) may be correlated wi th the effect of flux creep on J(cm), and J(c0)(T) is determined by fl ux pinning. J(c0)(T) is independent of magnetic field and is proportio nal to (T(c) - T). Similar results have also been found for other samp les. It may be the common characteristic of high-T(c) superconductors. Magnetic relaxation of YBa2Cu3O6+x and GdBa2Cu3O6+x films has been me asured at LN2 temperature. Using the equation of Hagen et al., effecti ve activation energy U0 has been deduced, which is about 0.3 eV, one o rder bigger than the value obtained by A = k(B)T/U0. U0 is almost inde pendent of the field in the field range selected.