The volume pinning forces, Fp, of several high-Tc superconductors are compared to each other using literature data and to those obtained in NdBa2Cu3O7-δ samples. It is shown that the pinning at small normal inclusions which is dominant in most high-Tc systems, leads to a peak at around 0.3 when plotting the normalized volume pinning forces, Fp/Fp,max, versus the reduced field defined by h = H/Hirr. Furthermore, relaxation effects play a significant role in the Fp diagrams, especially in the high h regions thus causing the typical rounded-off 'tail'. In contrast to this, NdBCO shows the peak at ≈0.5, thus indicating a strong pinning at small superconducting regions (ΔK/ΔTc-pinning).
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