Frequency dependence of AC susceptibility due to the viscous motion of flux lines

The authors studied the influence of viscous flux flow on the field distribution and hysteresis losses in high-T_c superconductors with field-dependent critical current density J _c. The results are compared with measurements of AC susceptibility on ceramic YBaCuO and BiSrCaCuO as well as on single-crystal YBaCuO samples. Clear evidence for flux flow effects is seen for frequencies above approximately 1 kHz for all samples. The extreme flux flow model indicates that the flux flow should play an important role at much lower frequencies. The deviations from these results could be caused to some extent by the field dependence of J _c, but at lower frequencies the flux creep seems to be dominant. This is confirmed by the current-voltage characteristics of analogous samples. It is shown that some important differences appear in the frequency dependences of the real and imaginary parts of the AC susceptibility for fields of harmonic and triangular shape. These differences, as well as the weaker flux flow effects in BiSrCaCuO as compared with YBaCuO, offer support for the conclusion that the flux flow effects are generally less important than supposed by some theories