Harmonic Susceptibilities and Pinning Properties of MgB<SUB>2</SUB> Bulk Superconductors

The fundamental and third harmonics of the ac magnetic susceptibility have been studied on MgB2 bulk superconductors, obtained by reactive liquid infiltration. In particular, measurements performed as a function of temperature, dc magnetic field, ac field amplitude and frequency have been compared with susceptibility curves obtained by means of numerical simulations of the non-linear diffusion equation for the magnetic field. The experimental frequency dependence of the χ1″(T) and |χ3|(T) peak amplitude cannot be explained by frequency dependent critical state models. On the contrary, we have shown that the measured curves are correctly derived using a diffusion coefficient dominated by a creep process within the framework of the vortex glass approach