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Scanning SQUID microscopy(SSM)is a powerful technique for imaging magnetic field distribution. The flux mapunderinvestigation is typically measured by a small superconductive pick-up loop coupled to SQUID. Such applications require high spatial resolution SQUID sensors with pick-up loop of a few micrometers in size. Such innovative instruments have been successfully employed in both basic physics experiments and applications such as:imaging of magnetic vortices in superconducting structure, symmetry of the order parameter in the high-Tc superconductor, study of magnetic materials properties, susceptibility of micrometric material particles.A SQUID sensor for magnetic microscopy should have an high spatial resolution, that is the capability to detect magnetic sources as small as possible, a high magnetic flux gain G=Φr/Φp (ratio between the measured magnetic flux Φr and the flux applied to the pick-up loop Φp) and should not be sensitive to any background field.
MicroSQUID in magnetometer configuration
We have developd LTc SQUIDs with integrated micro pick-up coils to employ as magnetic field sensors in high spatial resolution measurements. The circular pick-up coils, having an inner diameter ranging from 5 to 50 µm and a line width of 2 µm, are widely separated (1.5 mm) from the SQUID body to minimize magnetic interferences. The micro pick-up coils are connected to the SQUID by both magnetometer and gradiometer configurations. All SQUID types contain an integrated modulation/feedback coil for flux locked loop operations. Due to an high intrinsic responsivity the sensors have showed at T=4.2 K a white magnetic flux noise spectral density as low as 3µΦ/Hz½ measured in flux locked loop configuration using a direct coupled scheme without additional positive feedback. Such devices have been successfully employed to image the spontaneous zero-field currents in superconducting networks of temperature-controlled pi-junctions with weakly ferromagnetic barriers
Performance
Design
Main References:
Low critical temperature dc-SQUIDs for high spatial resolution applications
C. Granata, A. Vettoliere, R. Vaccarone, M Russo
Applied Superconductivity, IEEE Transactions on 17 (2), 796-799
Imaging spontaneous currents in superconducting arrays of π-junctions
S M Frolov, M JA Stoutimore, T A Crane, D J Van Harlingen, V A Oboznov, V V Ryazanov, A Ruosi, C Granata, M Russo
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