Application of low frequency SQUID NMR to the ultra-low temperature study of atomically layered 3He films adsorbed on graphite

Low frequency pulsed NMR is a powerful technique for the investigation of the nuclear susceptibility and spin-dynamics of strongly correlated systems down to ultra-low temperatures. We describe a versatile broadband pulsed NMR spectrometer using a two- stage Superconducting Quantum Interference Device (SQUID). This instrument has enabled the investigation of the spin-dynamics of ³He films adsorbed on exfoliated graphite into the microkelvin temperature range. In prior work we reported a SQUID NMR study of twodimensional ferromagnetism on a triangular lattice, where ³He films adsorbed on graphite provide an ideal model system. Here we describe the detection of the much weaker signals from strongly correlated fluid films in the second layer of ³He on graphite, using a spectrometer with improved sensitivity. We have measured the low frequency spin-spin relaxation and spin-lattice relaxation of two-dimensional ³He into the microkelvin range. These show an unusual time and frequency dependence. We also describe the use of the ¹³C-signal from the exfoliated graphite for thermometry, and the unusual properties of the spin-lattice relaxation of that system.