Flux dynamics and critical state in YBCO by AC harmonic susceptibility

The temperature dependences of the third harmonics of the AC susceptibility , chi'(3)(T) and chi "(3)(T), have been used to study the vortex dynamics i n Melt Textured YBCO. The experimental curves measured at different AC fiel d amplitudes and frequencies, have been compared with the simulations of ch i'(3)(T) and chi "(3)(T) obtained by considering the diffusion of the magne tic flux in the presence of different dynamic regimes. The comparison made as function of the AC field frequency shows that, at low frequencies, the m agnetic response is mainly described in terms of a Bean critical state mode l, As the frequency is increased, the contributions coming from the flux dy namic regimes become predominant. Nevertheless, at the highest frequencies, contrary to what was expected, the curves appear again very similar to the ones simulated in terms of a pure Bean critical state model. (C) 2000 Else vier Science B.V. All rights reserved.