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 μm width was fabricated from an Au/YBa₂Cu₃O_7-δ thin film on a SrTiO₃ bicrystal substrate with 30° 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_c and normal resistance of R_n/20 including contact resistance for the Au wiring layer are 350 μA and 0.4 Ω, respectively. The modulation voltage of 25 μV for multiple-SQUIDs is the same level as for a single SQUID. On the other hand, the modulation current of 40 μA 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 μΦ₀/Hz¹2/ at 100 Hz, including preamplifier noise, was observed by using flux-locked loop electronics in a magnetically shielded room.