Vortex dynamics and irreversibility line in optimally doped SmFeAsO_0.8F_0.2 from ac susceptibility and magnetization measurements

Ac susceptibility and static magnetization measurements were performed in the optimally doped SmFeAsO_0.8F_0.2 superconductor. The field-temperature phase diagram of the superconducting state was drawn, and, in particular, the features of the flux lines were derived. The dependence of the intragrain 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 intragrain critical current density was extrapolated in the zero-temperature and zero-magnetic-field limit, showing a remarkably high value J_c0(0)~2×10⁷ A/cm², which demonstrates that this material is rather interesting for potential future technological applications.