Non-linear magnetic response of MgB2 bulk superconductors

In a type-II superconductor the pinning properties and, in particular, the current dependence of the thermal activation energy U(J) determine the shape of the current-voltage (I-V) curve. The generation of higher harmonics in the magnetic response to an alternating magnetic field is a consequence of the non-linearity in the I-V characteristic. In this work we report our analysis of fundamental (?1) and third (?3) harmonics of the ac magnetic susceptibility on MgB2 bulk superconductors. In particular, measurements have been performed as a function of the temperature and the dc magnetic field, up to 9 T, for various frequencies and amplitudes of the ac magnetic field. By comparing the experimental susceptibility curves with numerical simulations of the magnetic diffusion in a type-II superconductor, we show that the frequency dependence of the third harmonic response can provide clear information about the characteristics of the non-linearity in the I-V curve and, therefore, the dynamical regimes governing the vortex motion in the analyzed samples.