Aligned crystallite powder of NdFeAsO$_{0.86}$F$_{0.14}$: magnetic hysteresis and penetration depth

Abstract

We report the basal-plane critical current and superfluid density of magnetically aligned NdFeAsO$_{0.86}$F$_{0.14}$ powder. This sample has individual crystallite grains permanently oriented with their c axis along the external field. Magnetic irreversibilities at high field suggest strong flux pinning of basal-plane critical currents, with monotonic field dependence and no evidence of the "fishtail" effect. The small particles provide a sensitive indicator of \textit{dc} flux penetration, and allow analysis of the temperature dependence of $ab-$plane London penetration depth $λ_{ab,\mathrm{L}}$, which is quadratic at low $T$. This feature may not necessarily be due to the nodes in the gap, but may be rather a sign of a strong pair-breaking. A quantitative determination of the absolute magnitude of $λ_{ab,\mathrm{L}}$ is hindered by the need for accurate knowledge of the particle size distribution.