Flux line dynamics were investigated in bulk superconductors, obtained by reactive liquid infiltration, by measuring the alternating-current magnetic susceptibility. The fundamental and third harmonics were studied as a function of temperature, direct-current magnetic field, alternating-current field amplitude and frequency. In order to determine the dynamic regimes governing vortex motion, experimental results were compared with susceptibility curves obtained by numerical calculations of the non-linear diffusion equation for the magnetic field. The frequency behavior of the third harmonic response, which could not be explained by frequency-dependent critical state models, was related to the current dependence of the flux creep activation energy in the diffusion coefficient. In this way, it was shown that the measured curves could be correctly interpreted within the framework of a vortex glass description.

Vortex Dynamics and Pinning Properties Analysis of MgB2 Bulk Samples by AC Susceptibility Measurements. C.Senatore, M.Polichetti, D.Zola, T.Di Matteo, G.Giunchi, S.Pace: Superconductor Science and Technology, 2003, 16, 183-7