Scaling of pinning forces in NdBa₂Cu₃O_7-δ superconductors

The scaling of the volume pinning forces, F_p=j_c×B_a, is performed versus the reduced fields b_f = B_a/B_fp (where B_a is the applied field, and B_fp denotes the peak position of F_p.max) and h = H_a/H_irr (where H_irr denotes the irreversibility field) for two different NdBa₂Cu₃O_7-δ superconductors, a single crystal and a melt-processed sample. In both cases, an excellent natural scaling is found to hold. Based on these good scaling results, we extend the scaling analysis towards lower temperatures. There, the scaling parameters are no longer directly accessible within our experimental field range, but can be deduced from the F_p(B_a) curves normalized using fitting values of B_fp and F_p.max so that these overlap with the high temperature curves. It is shown that such a procedure allows to obtain reasonably good estimates of F_p.max and H_irr, which leads in turn to an estimation of the irreversibility line well down to 0 temperature. At low temperatures, deviations from the analytical dependencies appear which are due to interference with the central and intermediate peak in the magnetization loops. The shape of the irreversibility line is discussed.