SENSITIVE RF-SQUIDS AND MAGNETOMETERS OPERATING AT 77 K

Large 6 X 6 mm(2) and 8 X 8 mm(2) flux-focussing washer structures with inductances L-S between 25 pH and 500 pH were fabricated from epitaxial, c- axis YBa2Cu3O7 films. Double step- edge junctions having a low l/ f noise were incorporated in these washers to form rf- SQUIDS. A high tank circuit frequency near 150 MHz was chosen to reduce noise and improve sensitivity of the SQUIDS. At 77K, a transfer function exceeding 100 mu V Phi(0) and white flux noise of 3 X 10(-5) Phi(0)/root Hz down to below 0.3 Hz, were attained with L-S = 25 pH. A magnetometer with L-S = 190 pH was demonstrated. The best magnetic field and energy resolutions were 170 fT/root Hz and 5X10(-29) J/ Hz respectively over a signal frequency range to below 1 Hz. The magnetometer was used to record human magnetocardiograms, and, for the first time, auditory evoked human brain responses.