Frequency dependence of the local ac magnetic response in type-II superconductors

The local ac magnetic response in type-II superconductors is analyzed on the basis of the critical state model, taking into account magnetic relaxation effects. The results show that the frequency must be introduced as an independent parameter to the model, in addition to the shielding current which itself is a function of frequency. Using a simplified model for the relaxation law, the calculated frequency dependence of the third harmonic response compares well with experimental data obtained in a Y-Ba-Cu-O crystal. Application of this analysis to ac measurements of magnetic relaxation in the short time limit is discussed.