Fabrication of magnetometers with multiple-SQUID arrays

We have designed and fabricated magnetometers with multiple-SQUID arrays to achieve higher signal to noise ratio compared to that for a single SQUID. In our directly coupled scheme, ten SQUIDs are connected in parallel with the same pickup coil composed of four parallel rectangular loops. Each SQUID having inductance of 70 pH and junctions of 1.5 /spl mu/m width was fabricated from an Au/YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film on a SrTiO/sub 3/ bicrystal substrate with 30/spl deg/ misorientation angle. To avoid forming superconducting loops between junctions of adjoining SQUIDs, a Au wiring layer interrupts a current bias line. The obtained critical current of 20I/sub c/ and normal resistance of R/sub n//20 including contact resistance for the Au wiring layer are 350 /spl mu/A and 0.4 /spl Omega/, respectively. The modulation voltage of 25 /spl mu/V for multiple-SQUIDs is the same level as for a single SQUID. On the other hand, the modulation current of 40 /spl mu/A is several times larger than that for a single SQUID, but rather smaller than the expected value when all SQUIDs have equal parameters. A flux noise level of 15 /spl mu//spl Phi//sub 0//Hz/sup 1/2/ at 100 Hz, including preamplifier noise, was observed by using flux-locked loop electronics in a magnetically shielded room.