Ferromagnetism of 3He Films in Very Low Applied Fields

Abstract

³He films on graphite provide a model system for 2D magnetism. At low temperatures, the thermodynamic properties of these films are dominated by exchange interactions. However more subtle interactions such as dipolar coupling and other anisotropies may eventually play a substantial role, especially in small magnetic fields. Our recent experiments use a SQUID system for both pulsed and continuous wave NMR at very low applied fields (B<1mT). The spin polarization can be estimated from both the amplitude and frequency shift. We find that at coverages above 20 atoms/nm² there is a field independent (zero field) magnetization at finite temperatures. The temperature where this magnetization develops is significantly below J/k where J is the effective exchange energy. Although the apparent ordering occurs over a broad temperature range, there is a trend towards a “narrowing” of the transition region down to the lowest applied fields. Contrary to recent estimates of exchange parameters which assume a single phase, we find that below 24 atoms/nm², only a fraction of the spins contribute to the ferromagnetic behavior. This is more consistent with a mixed phase system in that coverage region.