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

In order to investigate the dynamic properties of the flux line lattice, th e temperature dependencies of the basic and higher harmonic complex AC susc eptibilities have been analysed. In particular, the first (i.e., fundamenta l) and the third harmonic chi(1.3)(T) = chi'(1.3)(T) + i chi "(1.3)(T), hav e been measured at different frequencies. and the measurements have been co mpared 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 a ffirm that the experimental behaviour of the chi'(3)(T) curves is due only to flux creep and flux flow dynamical processes for each temperature T lowe r than the temperature T* close to the peak temperature of the first harmon ic imaginary part, T-p( chi "(1)) In fact, in this temperature region, the chi'(3)(T) Bean critical state prediction does not agree with the experimen tally detected magnetic response. Moreover, the experimental curves show th at, when the frequency is increased, the flux dynamics always gets more rel evant as compared to the critical state, Finally, the superposition of a DC field, H-DC, much higher than the AC field amplitude, allows to evidence t he contribution which is due only to flux creep events in the harmonic resp onse. (C) 2000 Elsevier Science B.V. All rights reserved.