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. 
G. Prando, P. Carretta, R. De Renzi, S. Sanna, A. Palenzona, M. Putti,   M. Tropeano
cond-mat.supr-con