  The temperature dependence of the 1st and 3rd harmonics of the AC magnetic
susceptibility has been measured on melt grown YBCO samples for different
frequencies and amplitudes of the AC magnetic field and intensity of a
contemporaneously applied DC field. With the help of critical state models and
of numerical simulations, we have devised a novel method, based on the combined
analysis of the 1st and the 3rd harmonics that allows to distinguish different
temperature ranges dominated by the different dissipative magnetic flux
regimes. In particular, we identified three principal zones in the temperature
dependence of the real part of the 3rd harmonic: the zone 1, in the temperature
range below the peak of the imaginary part of the 1st harmonic, Tp, and the
zone 2, characterized by negative values in a temperature region of the real
part of the 3rd harmonics, just above Tp, both dominated by the creep regime;
the zone 3, just below Tc, in which we revealed the presence of Thermally
Assisted Flux Flow (TAFF). By the identification of these zones, an estimation
of the value of the pinning potential can be obtained.
