Multichannel SQUID magnetometry using double relaxation oscillation SQUID's

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 /spl mu//spl Phi//sub 0///spl radic/Hz (/spl epsi/=275 h). With wire-wound first-order gradiometers, having a baseline of 40 mm, the white magnetic field noise equals 4 fT//spl radic/Hz. As a result of the high flux-to-voltage transfer of the DROS's of about 1 mV//spl Phi//sub 0/, 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.