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 T-c 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 U-0 exhibits a common trend for all the samples with a typical crossover field value (2500 Oe less than or similar to H-cr less than or similar to 5000 Oe) separating regions where single and collective depinning processes are at work.