A highly sensitive first-order gradiometer based on double relaxation oscillation SQUID's (DROS's) for multichannel use is presented. The white flux noise level of the bare DROS's is 4.5 μΦ0/√Hz (ε=275 h). With wire-wound first-order gradiometers, having a baseline of 40 mm, the white magnetic field noise equals 4 fT/√Hz. As a result of the high flux-to-voltage transfer of the DROS's of about 1 mV/Φ0, this low noise level could be obtained with simple room-temperature flux locked loop electronics based on direct voltage readout. The relaxation frequency of the present DROS's is approximately 1 GHz. No degradation of the DROS characteristics caused by interference between relaxation oscillations in two adjacent channels has been observed, although the gradiometers are spaced by less than 1 mm. This shows that DROS's can be used in multichannel SQUID magnetometers.
van Duuren, M. J., Lee, Y. H., Adelerhof, D. J., Kawai, J., Kado, H., Flokstra, J., & Rogalla, H. (1996). Multichannel SQUID magnetometry using double relaxation oscillation SQUIDs. IEEE transactions on applied superconductivity, 6(1), 38-44. https://doi.org/10.1109/77.488279