Fabrication and characterization of an integrated 9-channel Superconducting quantum interference device magnetometer array

We have fabricated and characterized nine Superconducting Quantum Interference Device (SQUID) magnetometers, which were integrated on a single chip. The magnetometers are based on a directly-coupled multiloop SQUID with Nb/Al-AlO_x/Nb junctions, and are arranged in a 3 × 3 matrix in the area of 10 mm × 10 mm. The diameter of the circular pick-up loop of each magnetometer is 2.5 mm and the distance between neighboring magnetometers is 2.75 mm. In the operation with a flux locked loop (FLL) with direct voltage readout, typical flux-locked noise level of less than 3×10^-6 Φ₀/√Hz in the white region were obtained with the magnetometer in a superconductive shield. From the calibration of the sensitivity, typical field noise was measured to be 7 fT/√Hz. In combination with a small cryostat, we expect that this magnetometer will be useful for multi-channel measurement of magnetic fields requiring highly spatial resolution in various applications.