Nuclear magnetic resonance using DC SQUIDs with APF

Abstract

We are using low- T c multiloop DC SQUIDs with additional positive feedback (APF), operating in a flux-locked loop mode out to several megahertz, to perform nuclear magnetic resonance on low temperature samples. The optimum input configuration depends on the sample under investigation. For systems with short T 2 and low frequencies a broadband input circuit is best, with the NMR pickup coil forming a flux transformer with the input coil of the SQUID. This system has been used to perform NMR on several metals, including UPt 3 in the superconducting state, and on submonolayer 3 He films at low kilohertz frequencies. For systems with narrow lines a tuned input configuration offers improved sensitivity, especially when the pickup coil can be cooled to millikelvin temperatures. Here the NMR pickup coil forms part of a series resonant tank circuit, attached to the input coil of the SQUID. We are presently using such a system, tuned to 880 kHz, to detect signals from thin 3 He films (of thickness 100 nm) adsorbed on a surface area of 1 cm 2 . Cooling these films to below 1 mK will enable the study of superfluidity in 3 He films thinner than the bulk superfluid coherence length.