Flux creep, flux pinning and critical current density of Y-, Gd-, and Tl-based superconductive thin films

Magnetic hysteresis loops have been measured for YBa2Cu3O6+x, GdBa2Cu3O6+x and Tl2Ba2Ca2Cu3O10-y films at different temperatures with a vibrating sample magnetometer. Based on Bean model, magnetic critical current density Jcm has been derived approximately. The field and temperature dependence of Jcm for all samples can be written as: Jcm = Jc0(T) × 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: Jct = Jc0(B,T)(1 - (kBT/U0)ln(BΩd/Ec)). The extraordinary similarity suggests that Jcm is determined not only by flux pinning but also by flux creep. In the first formula, f(B,T) may be correlated with the effect of flux creep on Jcm, and Jc0(T) is determined by flux pinning. Jc0(T) is independent of magnetic field and is proportional to (Tc - T). Similar results have also been found for other samples. It may be the common characteristic of high-Tc superconductors. Magnetic relaxation of YBa2Cu3O6+x and GdBa2Cu3O6+x films has been measured at LN2 temperature. Using the equation of Hagen et al., effective activation energy U0 has been deduced, which is about 0.3 eV, one order bigger than the value obtained by A = kBT/U0. U0 is almost independent of the field in the field range selected.