Investigation of multiple SQUID arrangements in single layer high Tc magnetometers

Single layer high T-c de SQUID sensors on bicrystal substrates were investigated. Special attention was paid to achieve a large flux-to-voltage transfer function in order to ensure stable operation of the SQUID electronics and 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 degrees bicrystals. The steep edges achieved by sub-mu m patterning increased the resistance of the superconducting structures against large external fields. The design for large flux-to-voltage transfer functions focused on multiple de SQUID arrangements. For magnetometers with a directly coupled pickup-loop three junction SQUIDs show no advantage compared to standard two junction SQUIDs, but the series connection of two ordinary de SQUIDs does. Although for magnetometers with inductively coupled pickup-loop twenty SQUIDs in series were used the effect on the flux-to-voltage transfer coefficient was poor, due to differences in the critical current and inductance of individual 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.