Single Vortex Pinning and Penetration Depth in Superconducting NdFeAsO1-xFx

Abstract

We use a magnetic force microscope (MFM) to investigate single vortex pinning and penetration depth in NdFeAsO1−xFx, one of the highest-Tc 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, Fdepin ' 4.5 pN, corresponding to a critical current up to Jc ' 7×105 A/cm2. Furthermore, our MFM measurements allow the first local and absolute determination of the superconducting in-plane penetration depth in NdFeAsO1−xFx, λab = 320 ± 60 nm, which is larger than previous bulk measurements.