Single-vortex pinning and penetration depth in superconducting NdFeAsO[subscript 1−x]F[subscript x]

Abstract

We use a magnetic force microscope (MFM) to investigate single-vortex pinning and penetration depth in NdFeAsO[subscript 1−x]F[subscript x], one of the highest-T[subscript 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[subscript depin] ≃ 4.5 pN, corresponding to a critical current up to J[subscript c] ≃ 7 × 10[superscript 5] A/cm[superscript 2]. Furthermore, our MFM measurements allow the first local and absolute determination of the superconducting in-plane penetration depth in NdFeAsO[subscript 1−x]F[subscript x],λ[subscript ab] = 320 ± 60 nm, which is larger than previous bulk measurements.