Aligned crystallite powder of NdFeAsO0.86 F0.14: Magnetic hysteresis and penetration depth

We report the basal-plane critical current and superfluid density of magnetically aligned NdFeAsO0.86 F0.14 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 λab,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 λab,L is hindered by the need for accurate knowledge of the particle size distribution.