Investigation of multiple SQUID arrangements in single layer high T-c magnetometers

Single layer high T-c de SQUID sensors on bicrystal substrates were investi gated. Special attention was paid to achieve a large flux-to-voltage transf er function in order to ensure stable operation of the SQUID electronics an d to get low flux noise even in disturbed environment. Josephson junctions on 30 degrees bicrystal were tested as well as sub-mu m junctions on 24 deg rees bicrystals. The steep edges achieved by sub-mu m patterning increased the resistance of the superconducting structures against large external fie lds. The design for large flux-to-voltage transfer functions focused on multiple de SQUID arrangements. For magnetometers with a directly coupled pickup-lo op three junction SQUIDs show no advantage compared to standard two junctio n SQUIDs, but the series connection of two ordinary de SQUIDs does. Althoug h for magnetometers with inductively coupled pickup-loop twenty SQUIDs in s eries were used the effect on the flux-to-voltage transfer coefficient was poor, due to differences in the critical current and inductance of individu al SQUIDs. The sensitivity of the whole magnetometer achieved with inductively coupled single layer pickup-loops is much smaller than with directly coupled ones. For the latter a pickup-loop formed as a conventional square washer shows better results compared to a slotted square.