ac susceptibility investigation of vortex dynamics in nearly optimally doped RFeAsO1-xFx superconductors (R = La, Ce, Sm)

Ac susceptibility and static magnetization measurements were performed in the nearly optimally doped LaFeAsO0.9F0.1 and CeFeAsO0.92F0.08 superconductors, complementing earlier results on SmFeAsO0.8F0.2 [ Phys. Rev. B 83 174514 (2011)]. The magnetic field-temperature phase diagram of the mixed superconducting state is drawn for the three materials, displaying a sizable reduction of the liquid phase upon increasing Tc in the range of applied fields (H<5 T). This result indicates that SmFeAsO0.8F0.2 is the most interesting compound among the investigated ones in view of possible applications. The field-dependence of the intragrain depinning energy U0 exhibits a common trend for all the samples with a typical crossover field value (2500 Oe ≲Hcr≲5000 Oe) separating regions where single and collective depinning processes are at work.