Aligned crystallite powder ofNdFeAsO0.86F0.14: Magnetic hysteresis and penetration depth

Abstract

We report the basal-plane critical current and superfluid density of magnetically aligned ${\text{NdFeAsO}}_{0.86}{\text{F}}_{0.14}$ powder. This sample has individual crystallite grains permanently oriented with their $c$ axis along the external measuring 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 dc flux penetration and allow analysis of the temperature dependence of $ab$-plane London penetration depth ${\ensuremath{\lambda}}_{ab,\text{L}}$, 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 ${\ensuremath{\lambda}}_{ab,\text{L}}$ is hindered by the need for accurate knowledge of the particle size distribution.