Single Vortex Pinning and Penetration Depth in Superconducting NdFeAsO$_{1-x}$F$_x$

TL;DR

This study employs magnetic force microscopy to analyze vortex pinning and measure the in-plane penetration depth in NdFeAsO$_{1-x}$F$_x$, revealing strong pinning forces and a larger penetration depth than bulk estimates.

Contribution

It provides the first local measurement of the penetration depth in NdFeAsO$_{1-x}$F$_x$ and characterizes vortex pinning forces in this high-$T_c$ superconductor.

Findings

Vortex arrangement is disordered at fields up to 20 G.

Depinning force for a vortex is approximately 4.5 pN.

In-plane penetration depth is measured as 320 ± 60 nm.

Abstract

We use a magnetic force microscope (MFM) to investigate single vortex pinning and penetration depth in NdFeAsO$_{1-x}$F$_x$, one of the highest-$T_c$ iron-based superconductors. In fields up to 20 Gauss, we observe a disordered vortex arrangement, implying that the pinning forces are stronger than the vortex-vortex interactions. We measure the typical force to depin a single vortex, $F_{\mathrm{depin}} \simeq 4.5$ pN, corresponding to a critical current up to $J_c \simeq 7 \times 10^5$ A/cm$^2$. Furthermore, our MFM measurements allow the first local and absolute determination of the superconducting in-plane penetration depth in NdFeAsO$_{1-x}$F$_x$, $\lambda_{ab}=320 \pm 60$ nm, which is larger than previous bulk measurements.