Direct detection of NMR at low frequencies using a DC SQUID amplifier

We have constructed a broadband (DC to 50 kHz) pulsed NMR spectrometer which uses a DC SQUID to detect precessing magnetization directly. The superconducting receiver coil and the SQUID input coil form a flux transformer, the most sensitive input coupling scheme for NMR detection at frequencies below a few hundred kHz. Using this spectrometer free induction decay signals have been obtained from platinum powder in the temperature range 1.4 to 4.2 K and at frequencies from 5 to 40 kHz. The observed signal to noise corresponds to that calculated, with the noise being dominated by flux noise in the SQUID. Signals from 1021 Pt195 spins have been observed in a single shot at 37 kHz and 1.4 K. In addition free precession signals have been obtained from 1020 He3 gas spins at 4.2 K and at frequencies between 0.2 and 40 kHz.