Detection of the flux creep regime in the AC susceptibility curves by using higher harmonic response

In order to investigate dynamic properties of the flux line lattice, the temperature dependencies of the basic and higher harmonic complex AC susceptibilities have been analyzed. In particular, the first (i.e., fundamental) and the third harmonic χ1,3(T) = χ′1,3(T)+iχ″1,3(T), have been measured at different frequencies, and the measurements have been compared with both analytical and numerical results. In this way, by using a combined analysis of the first and the third harmonics, it is possible to affirm that the experimental behaviour of the χ′3(T) curves is due only to flux creep and flux flow dynamical processes for each temperature T lower than the temperature T* close to the peak temperature of the first harmonic imaginary part, Tp(χ″1). In fact, in this temperature region, the χ′3(T) Bean critical state prediction does not agree with the experimentally detected magnetic response. Moreover, the experimental curves show that, when the frequency is increased, the flux dynamics always gets more relevant as compared to the critical state. Finally, the superposition of a DC field, HDC, much higher than the AC field amplitude, allows to evidence the contribution which is due only to flux creep events in the harmonic response