Single layer YBa2Cu3O7 radio frequency SQUID magnetometers with direct-coupled pickup coils and flip-chip flux transformers

Abstract

Our aim has been to further improve the magnetic field resolution, BN, 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 BN≂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.