Vortex dynamics and irreversibility line in optimally doped SmFeAsO${}_{0.8}$F${}_{0.2}$ from ac susceptibility and magnetization measurements

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.