  The flux lines dynamics have been investigated on MgB2 bulk superconductors
obtained by reactive liquid infiltration by measuring the ac magnetic
susceptibility. The fundamental and third harmonics have been studied as a
function of temperature, dc magnetic field, ac field amplitude and frequency.
In order to determine the dynamical regimes governing the vortex motion, the
experimental results have been compared with susceptibility curves obtained by
numerical calculations of the non-linear diffusion equation for the magnetic
field. The frequency behaviour of the third harmonic response, that cannot be
explained by frequency dependent critical state models, has been related to the
current dependence of the flux creep activation energy U(J) in the diffusion
coefficient. In this way we have shown that the measured curves are correctly
interpreted within the framework of a vortex glass description.
