Harmonic susceptibilities and pinning properties of MgB2 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