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.

High-transition-temperature superconducting quantum interference devices

“…This magnetometer exhibited a white noise of 60 fT Hz Ϫ1/2 at frequencies down to about 5 Hz; the additional 1/f noise was due to flux noise in the concentrator. Subsequently, Zhang et al (1994) fabricated single-layer rf SQUIDs with several directly coupled pickup loops, improving the noise to 120 fT Hz Ϫ1/2 . They also fabricated a device in which the SQUID, in the form of a slit, was coupled to a 9 ϫ9 mm 2 pickup loop, and achieved 90 fT Hz Ϫ1/2 down to about 4 Hz.…”

Local magnetic probes of superconductors

“…The current predominant use of niobium SQUIDs clearly limits the working temperature range to less than 9.2 K. It may, however, become possible to use high-Tc SQUIDs to perform similar measurements at higher temperatures and in several cases their 77 K noise ® gures have been shown to be comparable with low-Tc SQUIDs at 4.2 K (Zhang et al 1994, Lee et al 1995. Simple high-Tc scanning systems already exist but seem unlikely to achieve a spatial resolution comparable with existing low-Tc systems for some years.…”

Squid Fundamentals

“…The best magnetic field sensitivity reported was 170rr Hz-l/2 at 1 Hz. Subsequently, Zhang et al [144] coupled an rf SQUID washer with inner and outer dimensions of 0.2mm and 8mm to a flux transformer with a 40x40mm 2 pickup loop and achieved a magnetic field noise of 24ftHz-l/2 at frequencies down to 0.5Hz. The rf bias frequency was 150MHz.…”