Broadband nuclear magnetic resonance using DC SQUID amplifiers

Abstract

Abstract We have constructed two pulsed NMR spectrometers in which the signal is coupled to the input coil of a low Tc DC SQUID using a superconducting flux transformer, yielding broadband response, with bandwidth determined by the SQUID electronics. A 50 kHz bandwidth commercial system has been used to observe free induction decay signals from platinum powder, bulk platinum, 3He gas and surface monolayers of 3He in the temperature range from 1.4 to 4.2 K and at frequencies from 5 to 40 kHz. The observed signal-to-noise ratio is as calculated with the noise dominated by flux noise in the SQUID in all samples but the bulk metal. A second system, which operates in flux-locked loop mode with bandwidth of 3.4 MHz using a SQUID with additional positive feedback, has been used to observe NMR signals from platinum powder at frequencies from 38 to 513 kHz and at a temperature of 4.2 K. The advantage of this technique in the study of systems with short T2 at frequencies below 1 MHz is discussed. In addition we discuss the benefits of both broadband and tuned input circuits for NMR detection and we describe the performance of a spectrometer with a tuned input circuit which has been used to obtain signals at 1 MHz from platinum powder at 4.2 K and from ∼2 layers of 3He absorbed on a surface area of 0.11 m2 at 1.7 K. The amplifier noise temperature is predicted to be 60 mK in the 3He experiment. This demonstrates the potential of the tuned set-up for measurements at low millikelvin temperatures on systems with low spin density and with T2 greater than several hundred microseconds.