SINGLE-LAYER YBA2CU3O7 RADIO-FREQUENCY SQUID MAGNETOMETERS WITH DIRECT-COUPLED PICKUP COILS AND FLIP-CHIP FLUX TRANSFORMERS

Our aim has been to further improve the magnetic field resolution, B N, of single-layer rf SQUID magnetometers operating in liquid nitrogen. Following the approaches recently introduced in dc SQUIDs, we tested designs with direct-coupled pickup coils having an area Ap=0.6 to 0.8 cm2, and a single-layer thin-film flip-chip flux transformer with Ap=16 cm2. In conditions of still suboptimal coupling between SQUID and the 150 MHz tank circuit, we attained BN≅90 fT/Hz1/2 above 3-4 Hz at Ap=0.8 cm2, and B N≅24 fT/Hz1/2 above 0.5 Hz when Ap=16 cm2. For rf SQUID with lumped-element tank circuit, we project the lower BN limit to be ≤50 fT/Hz1/2 at Ap≤1 cm2. This might be attainable through further coupling optimization, and increase of tank frequency to the highest possible value of ≥500 MHz. © 1994 American Institute of Physics.