  Ac susceptibility and static magnetization measurements were performed in the
optimally doped SmFeAsO$_{0.8}$F$_{0.2}$ superconductor. The field -
temperature phase diagram of the superconducting state was drawn and, in
particular, the features of the flux line lattice derived. The dependence of
the intra-grain depinning energy on the magnetic field intensity was derived in
the thermally-activated flux creep framework, enlightening a typical 1/H
dependence in the high-field regime. The intra-grain critical current density
was extrapolated in the zero temperature and zero magnetic field limit, showing
a remarkably high value $J_{c0}(0) \sim 2 \cdot 10^{7}$ A/cm$^{2}$, which
demonstrates that this material is rather interesting for the potential future
technological applications.
