Ferromagnetism of 2D Solid 3He Investigated by SQUID NMR

Pulsed NMR measurements have been performed on 3He films adsorbed on an exfoliated graphite sample of area 2 m2. The spectrometer is constructed using a DC SQUID with additional positive feedback, operating in flux locked loop mode with a bandwidth of 3.4 MHz. The input circuit, is a superconducting flux transformer, and so is intrinsically broadband. This spectrometer can therefore operate from typical frequencies used in conventional NMR down to otherwise inaccessible low kHz frequencies. With this system studies at the “ferromagnetic anomaly” have been undertaken at frequencies from 2 kHz to 100 kHz (0.06 to 3.09 mT), with the static magnetic field oriented normal to the nominal direction of the graphite platelets. At 50 and 100 kHz, measurements of the susceptibility are fit by high temperature series expansions between 100 mK and 5 mK, to determine an exchange constant J=1.86 mK. Below 1.3 mK the dipolar frequency shift increases linearly with temperature down to 0.3 mK, extrapolating to 10 kHz, close to our calculated value of 9.6 kHz, for fully polarised spins with an assumed lattice spacing of 0.392 nm for the second layer solid. Below 1.5 mK, a significant field dependence to the magnetisation and dipolar frequency shift appears at fields comparable to the characteristic dipolar field, below which the equilibrium alignment of the magnetization is field dependent. Assuming the frequency shift, in applied fields of 1.54 and 3.09 mT, to be proportional to the sample polarisation we find that it is not possible to fit the observed temperature dependence by 2D ferromagnetic spin-wave theory, with a consistent set of parameters for both fields. The theory is applied in the range 0.15>T/J>0.5, takes into account the Zeeman gap and finite system size, and includes the k=0 spin wave term. At present, the data provide no unambiguous evidence for a finite ordering temperature induced by the anisotropic dipolar interaction.