Sensitive RF-SQUIDs and magnetometers operating at 77 K

Large 6-mm*6-mm and 8-mm*8-mm flux-focusing washer structures with inductances L/sub S/ between 25 pH and 500 pH were fabricated from epitaxial, c-axis YBa/sub 2/Cu/sub 3/O/sub 7/ films. Double step-edge junctions having a low 1/f noise were incorporated in these washers to form RF-SQUIDs (superconducting quantum interference devices). A high tank circuit frequency near 150 MHz was chosen to reduce noise and improve the sensitivity of the SQUIDs. At 77 K, a transfer function exceeding 100 mu V/ Phi /sub 0/ and a white flux noise of 3*10/sup -5/ Phi /sub 0// square root Hz down to below 0.3 Hz were attained with L/sub S/=25 pH. A magnetometer with L/sub S/=190 pH was demonstrated. The best magnetic field and energy resolutions were 170 fT/ square root Hz and 5*10/sup -29/ J/Hz respectively, over a signal frequency range to below 1 Hz. The magnetometer was used to record human magnetocardiograms and auditory evoked human brain responses.<>