Signal enhancement techniques for rf SQUID based magnetic imaging systems

Abstract

We have investigated the rf SQUID (radio-frequency superconducting quantum interference device) and its coupling to tank circuit configurations to achieve an optimal front-end assembly for sensitive and high spatial resolution magnetic imaging systems. The investigation of the YBCO rf SQUID coupling to the conventional LC tank circuits revealed that coupling from the back of the SQUID substrate enhances the SQUID signal while facilitating the front-end assembly configuration. The optimal thickness of the substrate material between the SQUID and the tank circuit is 0.4 mm for LaAlO 3 resulting in an increase of the SQUID flux-voltage transfer function signal, V spp , of 1.5 times, and 0.5 mm for SrTiO 3 with an increase of V spp of 1.62 times compared to that for direct face to face couplings. For rf coupling with a coplanar resonator, it has been found that the best configuration, in which a resonator is sandwiched between the SQUID substrate and the resonator substrate, provides a V spp about 3.4 times higher than that for the worse case where the resonator and the SQUID are coupled back to back. The use of a resonator leads to a limitation of the achievable spatial resolution due to its flux focusing characteristics. This resulted in a favouring of the use of the conventional tank circuits when considering the desired high spatial resolution. The effect of the YBCO flip chip magnetic shielding of the SQUIDs in the back-coupling with the LC tank circuit configuration has also been investigated, with a view to reducing the SQUID effective area to increase the spatial resolution and also for studying the effect of the coupling of various kinds of transformers to the SQUIDs. It is revealed that there is no very considerable change in the flux-voltage transfer function signal level with respect to the effective shield area, while the lowest working temperature of the SQUIDs was slightly shifted higher by a couple of degrees, depending on the shield area.

Low noise SQUID based NDE with non-magnetic scanning system in unshielded environment

“…Our experimental system consists of a RF-SQUID gradiometer and its relevant electronic system, a x-y scanning stage, a dewar with isolating stages, a lock-in amplifier and a function generator as Figure 1 shows the block diagram of the system setup. To eliminate the background magnetic field and noise, A 3 mm wide step-edge junction, high-Tc RF-SQUID, 1 st order gradiometer with 1.5 mm base-line and 1.5 mm diameter washer area was used in combination with conventional LC resonators at 750MHz [3], [4]. We also used a 3 Cm diameter double-D excitation coil to induce eddy currents in the sample.…”

Application of silicon micro hall sensors in variable temperature scanning hall probe microscopy (SHPM) using multiple feedback techniques

“…Superconducting Quantum Interference Devices (SQUID) has the highest level of magnetic sensitivity, 5×10 −18 T, among the local probing techniques (Oral et al, 1996;Akram et al, 2006). However, SQUID can only operate under cryogenic temperatures, which is the major drawback for their applications.…”