Highly sensitive miniature SQUID magnetometer fabricated with cross-type Josephson tunnel junctions

We report on a new method suitable for the fabrication of highly sensitive SQUID magnetometers exhibiting very small effective areas. In contrast to commonly used approaches, which mostly make use of constrictions in a superconducting material to form the Josephson junctions, we have been able to fabricate such sensors with SIS cross-type Josephson tunnel junctions in an Nb/AlOx/Nb trilayer process. The small junction size and vanishing idle region of our cross-type junctions and a therefore very low capacitance lead to a high sensitivity of the resulting SQUIDs. First results on fabricated miniature SQUID magnetometer within this technology are shown. We found, that even with SQUID loop dimensions of about 10 × 10 μm2 very low flux noise levels could be achieved, which clearly demonstrate the high potential of our approach. Furthermore we estimate the required SQUID parameter which may allow the detection of single electron spin-flips with such devices in the future.

► A new method for the fabrication of SQUIDs with very small pickup loops is presented. ► These sensors make use of Nb/AlOx/Nb cross-type Josephson tunnel junctions. ► The small junction capacitance leads to highly sensitive SQUIDs. ► This approach could possibly lead to sensors for the detection of single spin flips. ► Moreover the use of SQUID-arrays could be beneficial in e.g. magnetic microscopy.