  We report a study of the pinning properties of Fe/MgB2 tapes performed by
means of ac magnetic susceptibility measurements. In particular, the third
harmonic response has been measured as a function of the dc magnetic field up
to 9 T, at different temperatures, for various frequencies and amplitudes of
the ac field. The irreversibility line has been determined using the third
harmonic onset criterion for samples made with different ball-milled MgB2
powders. The study of the harmonic response allows us to probe the nature of
the vortex pinning and to analyse its influence on the enhancement of the
irreversibility field in strongly ball-milled samples. We have also
investigated the dynamical regimes governing the vortex motion in the tapes by
comparing the experimental curves with numerical simulations of the non-linear
diffusion equation for the magnetic field.
