'3He films adsorbed on graphite : NMR and heat capacity down to below 1 mK.
3He films adsorbed on graphite were studied down to temperatures below 1
mK using nuclear magnetic resonance (NMR) and heat capacity as probes. This
is the first work that combined heat capacity and NMR in the same cell.
The substrate used was Grafoil, a form of exfoliated graphite. Continuous
wave NMR was measured using conventional Q-meter circuits, whereas heat capacity
was measured using the standard adiabatic method. Thermometry was
based on a melting curve thermometer, which served as the primary thermometer.
A platinum pulsed NMR and a lanthanum diluted cerium magnesium nitrate
(LCMN) thermometers, both calibrated against the melting curve thermometer
were also employed.
The work focused mainly in the study of the nuclear magnetism when the film
is in a solid phase. For pure 3He films, submonolayer and second layer films were
investigated. Here, it was observed that the Heisenberg model is appropriate to
describe both nuclear magnetic susceptibility and heat capacity of this system.
However, the discrepancies in the exchange constants inferred from the different
experimental probes are attributed to multiple spin exchange effects, in line with
what has been observed in bulk solid 3He.
Another system studied in this work was 3He adsorbed on two layers of deuterium
hydride (HD) preplated graphite. In this case, a cusp in the susceptibility
at a temperature of 1 mK, interpreted as indicative of a magnetic phase transition,
was a particularly intriguing observation