Strong Pinning and Nonlinear Creep Barriers in Iron-Pnictide Superconductors

The irreversible magnetization of Iron-Based Superconductors is characterized by the presence of an ubiquitous peak of the critical current density $j_c$, centered around zero field. Closer examination shows that the field-dependence of $j_c$ corresponds, in all cases, to a low-field plateau, followed by a power-law decrease, $j_c \propto B^{-\alpha}$ (with $\alpha \sim \frac{5}{8}$) above a cross-over field $B^{*}$. This strongly suggests that vortex pinning at low magnetic field is due to strong pinning by nanometer-scale defects. In isovalently doped materials such as BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, strong pinning is the only contribution to the critical current. The analysis of $j_c$ allows one to extract, without a priori assumptions, the elementary pinning force and the defect density. In BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, the latter quantity is in qualitative agreement with that obtained by H. Shishido \em et al.\rm [Phys. Rev. Lett. {\bf 104}, 057008 (2010)]. The temperature dependence of the screening current above $B^{*}$ is strongly affected by flux creep. The current decays as $j \sim [ (k_BT/U) \ln( t + t_0 / \tau )]^{-1/\mu}$, with $\mu \sim 1.6$, showing that nonlinear creep barriers are not an exclusive feature of weak collective pinning.