Nondestructive Evaluation Using a High-Tc SQUID Microscope

We report the application of a scanning high-T
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SQUID (superconducting quantum interference device) microscope with a ferromagnetic flux guide for the nondestructive evaluation of weld seams and wear tracks and scars on austenitic stainless steel plates as well as measurement of magnetic stray fields distribution above patterned by electron lithography 30-nm-thick cobalt films. A soft magnetic amorphous Vitrovac foil was used to guide the flux from the samples, which were held at room temperature, to the liquid-nitrogen-cooled SQUID-sensor and back. The flux guide passes through a hole in the 1 mm × 2.5 mm pick-up loop of the high-T
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SQUID sensor, thereby providing improved coupling of the magnetic flux from the object to the SQUID. In order to avoid the influence of the SQUID biasing the magnetic field on the object under investigation, a modulation and feedback coil was coupled to the pick-up loop of the SQUID directly and beyond the ferromagnetic flux guide. Such decoupling of feedback coil from soft magnetic flux antenna ensures that the high-T
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SQUID microscope does not disturb the sample magnetization during image recording. The SQUID microscope can be used to measure the spatial distribution of the z-component of the stray field above a specimen without mechanical contact to it.