Ac susceptibility and static magnetization measurements were performed in the nearly-optimally doped LaFeAsO$_{ 0.9}$F$_{ 0.1}$ and CeFeAsO$_{ 0.92}$F$_{ 0.08}$ superconductors, complementing earlier results on SmFeAsO$_{ 0.8}$F$_{ 0.2}$ [Phys. Rev. { \bf B 83}, 174514 (2011)]. The magnetic field -- temperature phase diagram of the mixed superconducting state is drawn for the three materials, displaying a sizeable reduction of the liquid phase upon increasing $T_{ c}$ in the range of applied fields ($H \leq 5$ T). This result indicates that SmFeAsO$_{ 0.8}$F$_{ 0.2}$ is the most interesting compound among the investigated ones in view of possible applications. The field-dependence of the intra-grain depinning energy $U_{ 0}$ exhibits a common trend for all the samples with a typical crossover field value (2500 Oe $\lesssim H_{ cr} \lesssim 5000$ Oe) separating regions where single and collective depinning processes are at work. Analysis of the data in terms of a simple two-fluid picture for slightly anisotropic materials allows to estimate the zero-temperature penetration depth $\lambda_{ ab}(0)$ and the anisotropy parameter $\gamma$ for the three materials. Finally, a sizeable suppression of the superfluid density is deduced in a $s^{ \pm}$ two-gap scenario.
Categories: cond-mat.supr-con