Flux Creep associated with Strong Pinning in Isovalently Doped Iron-Based Superconductors

Strong pinning in Iron-Based Superconductors leads to the ubiquitous central peak of the irreversible magnetization. Notably, isovalently doped materials such as BaFe$_2$(As$_{1-x}$P$_x$)$_2$ and Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ offer a paradigm for the study of strong pinning because it is the only contribution to the critical current density $j_c$. We have studied flux creep rates as function of field and temperature in the low- and high field regimes in which $j_c$ is limited by the line tension of a single pinned vortex, and by vortex interactions, respectively. For $T < \frac{1}{2}T_c$, screening currents $j$ are of the order of $10^9$ Am$^{-2}$, in spite of a creep rate $d \ln j / d \ln t \sim 0.02$. Creep is initially Anderson Kim-like, \em i.e. \rm, creep barriers $U$ depend on $j$ as $U \propto (1 - j / j_c )$ over an order of magnitude in $j$, before crossing over to a nonlinear behavior. $j_c$ is easily extracted from the high-current, short-time part of the magnetic relaxation. The results cast doubt on the range of applicability of the often-used ''interpolation formula'' $j \propto [1 + (k_BT/U_c) \ln( t + t_0 / \tau )]^{-1/\mu}$ for weak collective pinning.