Intrinsic magnetic properties of NdFeAsO${}_{0.9}$F${}_{0.1}$ superconductor from local and global measurements

Abstract

Magneto-optical imaging was used to study the local magnetization in polycrystalline NdFeAsO${}_{0.9}$F${}_{0.1}$ (NFAOF). Individual crystallites up to $\sim 200\times 100\times 30$ $\mu m^3$ in size could be mapped at various temperatures. The in-grain, persistent current density is about $j\sim 10^5$ A/cm${}^2$ and the magnetic relaxation rate in a remanent state peaks at about $T_m\sim 38$ 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 $3D\to 2D$ crossover in the vortex structure. The 2D Ginzburg parameter is about $G{i}^{2D}$ implying electromagnetic anisotropy as large as $\epsilon \sim 1/30$. Below $T_m$, the static and dynamic behaviors are consistent with collective pinning and creep.

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