Harmonic analysis of the AC magnetic response on directionally solidified YBaCuO samples.

The transport properties and the dynamics of the flux lines have been investigated on YBaCuO melt textured by measuring the AC susceptibility harmonics as function of the temperature, at different frequencies and amplitudes of the AC magnetic field. The interpretation of the behavior of the experimental curves has been done by their comparison with the results obtained by critical state pictures and by simulations of the diffusion processes of the magnetic field which determine the AC response in presence of different regimes of flux dynamics. The temperature dependencies of the third harmonics of the AC susceptibility at different frequencies have been used in particular to determine what is the flux dynamic regime governing the magnetic response of melt textured samples in the different experimental conditions. A good qualitative agreement has been found between the experimental curves and the ones simulated by assuming that dissipation processes are a sequence of independent flux creep and flux flow events within a collective pinning framework.