Directly-coupled DC-SQUID magnetometers made of Bi-Sr-Ca-Cu oxide films

High-T/sub c/ DC-SQUID magnetometers made of Bi-Sr-Ca-Cu oxide films were designed and fabricated. A directly coupled scheme where a SQUID loop and a pick-up loop are connected directly in parallel was chosen to avoid fabricating the multilayered structure. The flux noise, which was measured in FLL (flux locked loop) operation at 4.2 K, increased in a form of 1/f as the frequency decreased below 20 Hz. The level of white noise at the frequency range above 20 Hz was about 1*10/sup -4/ Phi /sub 0// square root Hz, corresponding to the field resolution of 1.5 pT/ square root Hz at the pickup loop.