Intrinsic magnetic properties of NdFeAsO_(0.9)F_(0.1) superconductor from local and global measurements

Magneto-optical imaging was used to study the local magnetization in polycrystalline NdFeAsO$_{0.9}$F$_{0.1}$ (NFAOF). Individual crystallites up to $\sim200\times100\times30$ $\mu m^{3}$ in size could be mapped at various temperatures. The in-grain, persistent current density is about $j\sim10^{5}$ A/cm$^{2}$ and the magnetic relaxation rate in a remanent state peaks at about $T_{m}\sim38$ K. By comparison with with the total magnetization measured in a bar-shaped, dense, polycrystalline sample, we suggest that NdFeAsO$_{0.9}$F$_{0.1}$ is similar to a layered high-$T_{c}$, compound such as Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ and exhibits a $3D\to2D$ crossover in the vortex structure. The 2D Ginzburg parameter is about $Gi^{2D}% \simeq10^{-2}$ implying electromagnetic anisotropy as large as $\epsilon \sim1/30$. Below $T_{m}$, the static and dynamic behaviors are consistent with collective pinning and creep.