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

Zuev, Yuri L.
Specht, Eliot D.
Cantoni, Claudia
Christen, David K.
Thompson, James R.
Jin, Rongying
Sefat, Athena S.
Mandrus, David G.
McGuire, Michael A.
Sales, Brian C.
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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 $\lambda_{ab,\mathrm{L}}$, 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 $\lambda_{ab,\mathrm{L}}$ is hindered by the need for accurate knowledge of the particle size distribution.
Comment: 6 pages, 6 figures
Condensed Matter - Superconductivity