Microscopic investigation of rf vortex nucleation in Nb 3 Sn films using a near-field magnetic microwave microscope

Abstract We use a near-field magnetic microwave microscope to investigate and compare rf vortex nucleation in two superconducting radio-frequency-quality Nb 3 Sn films fabricated by different methods: a conventional vapor-diffused film and an electrochemically plated film followed by thermal annealing, both of which are deposited on Nb substrates. The microscope applies a localized rf magnetic field to the sample surface and measures the resulting third-harmonic response P 3 f , which is particularly sensitive to rf vortex nucleation triggered by surface defects. Both Nb 3 Sn films exhibit nontrivial P 3 f ( T ) structures below 7 K that display the key signatures associated with rf vortex nucleation at local defects. The electrochemical film additionally shows multiple P 3 f ( T ) structures between 14 K and 16 K that are absent in the vapor-diffused sample. Our results highlight the influence of fabrication method on rf vortex penetration properties and demonstrate the utility of third-harmonic response as a local diagnostic tool for surface defects in Nb 3 Sn films.