Comparison of different approaches to modelling the fishtail shape in RE-123 bulk superconductors

We analyse and compare properties of different models proposed so far for m odelling the shapes of F(B) and J(B) curves measured in RE-Ba2Cu3O7-delta ( RE-123, RE = rare earth) samples. The formulas following from the discussed models allow for a direct fit-of experimental data in various representati ons. The models divide in two categories, according to the pinning potentia l used, namely the power-law and the logarithmic potential. We show that no ne of the pinning regimes proposed for low-T-c superconductors accounts for the fishtail effect (FE) observed in high-T-c materials. The conventional expressions allow a good fit of high-T-c experimental data, however, the re sulting fitting parameters are much higher than those predicted by the theo ry. The logarithmic pinning potential justified in high-T-c materials by bo th magnetic and transport experiments leads to F(B) and J(B) functions that decay exponentially at high fields. The fit of experiments on a wide range of RE-123 samples, using only one free parameter, is nearly perfect. The r ole of free parameters is discussed in all the models. (C) 2000 Elsevier Sc ience B.V. All rights reserved.