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

TL;DR

This study investigates the magnetic properties and superfluid density of aligned NdFeAsO$_{0.86}$F$_{0.14}$ powder, revealing strong flux pinning and a quadratic temperature dependence of the penetration depth, indicating potential pair-breaking effects.

Contribution

First measurement of critical current and superfluid density in aligned NdFeAsO$_{0.86}$F$_{0.14}$ powder with insights into flux pinning and gap structure.

Findings

Strong flux pinning observed at high fields.

Quadratic temperature dependence of penetration depth.

No evidence of the fishtail effect.

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 $\lambda_{ab,\mathrm{L}}$, 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.