Harmonic AC susceptibilities on directionally solidified YBaCuO samples

In order to study the flux lines dynamics on melt textured YBaCuO, the AC magnetic susceptibility harmonics were measured with different frequencies and amplitudes of the AC magnetic field. In particular, we focus attention on the frequency effects on the higher harmonics measured at various AC fields. The analysis of the experimental data was performed by comparing them with the results obtained by a critical state picture and by numerical simulations of the diffusion processes of the magnetic field which determine the AC response in the presence of different regimes of flux dynamics. As the frequency changes, a clear nonuniversal temperature dependence appears in the experimental curves. In qualitative agreement with the numerical results obtained by assuming the existence of dissipation processes as a sequence of independent flux creep and flux flow events within a collective pinning framework.