Abstract
We have optimized the preparation of submicrometer wide symmetric 20° bicrystal Josephson junctions on the basis of YBa2Cu3O7−x (YBCO) films and integrated them in SQUID magnetometers and gradiometers with multilayer flux transformers. The achieved reduction of the curvature of the grain boundary leads to an improvement of the homogeneity of the current flow in the junction due to the d-wave symmetry of the order parameter in YBCO. The junctions have typical normal resistance Rn above 10 Ω and a IcRn product of about 0.4 mV at 77.4 K. The high resistance of the junctions has required an extensive use of low pass filters, rf-shields and microwave absorbers to protect the junctions from increased interference of high frequency electromagnetic fields. Integration of such junctions in SQUID magnetometers with a square 16-mm multilayer flux transformer allowed us for the first time to reach a field resolution of about 3.5 fT/√Hz at frequencies above 10 Hz and about 7 fT/√Hz at 1 Hz at an operation temperature of 77.4 K. SQUID gradiometers with a 10 mm base length of the planar multilayer flux transformer have achieved resolutions of about 15 fT/cm√Hz at frequencies above 10 Hz and about 30 fT/cm√Hz at 1 Hz and temperature 77.4 K.
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