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-T-c iron-based superconductors. In fields up to 20 G, 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-depin similar or equal to 4.5 pN, corresponding to a critical current up to J(c) similar or equal to 7 x 10(5) A/cm(2). Furthermore, our MFM measurements allow the first local and absolute determination of the superconducting in-plane penetration depth in NdFeAsO1-x F-x, lambda(ab) = 320 +/- 60 nm, which is larger than previous bulk measurements.