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MAIN
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RESUME
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RESEARCH
|
PUBLICATION
|
FAMILY
|
RESEARCH
My research is to measure the two-dimensional magnetism of 3He on GTA Grafoil
and HOPG ZYX graphite substrate,
a model system for the study of two-dimensional magnetism.
We applied SQUID Nuclear Magnetic Resonance (NMR) technique for the very low frequency NMR
needed in this experiment.
Pulsed and CW NMR are performed on two-dimensional 3He films on both Grafoil and ZYX graphite substrates at
very low applied field (<15 Gauss) and very low temperatures (0.3 mK to 20 mK).
Evidence were provided for a finite temperature ferromagnetic ordering in two-dimensional
3He films on both Grafoil and
ZYX graphite substrate as H --> 0.
Pulsed and CW NMR experiments for
3He film on Grafoil substrate are applied above 20 atoms/nm2 at fields of
0.03 - 0.48 mT, and on ZYX graphite substrate at a wide range of coverage at fields of
0.24 - 1.39 mT. For both experiments, at coverage above 20 atoms/nm2, the second layer
of 3He has a strongly ferromagnetic tendency. With decreasing temperature,
we find a rapid onset of magnetization that becomes independent of the applied field at
temperatures in the vicinity of 1 mK. Both the
dipolar field and the NMR linewidth grow rapidly
as well, which is consistent with a large (order unity)
polarization of the 3He spins. All these results convinced us as finite temperature
ordering for this magnetic system.
It is proved from thin metal films that the ordering temperature
of 2D systems may become finite when they have a weak anisotropy, especially when
the anisotropy is due to a long ranged potential such as dipolar
interactions. 3He films have nuclear dipolar couplings, and although they are
more than three orders of magnitude
smaller than the leading order exchange, they can nevertheless stabilize long-ranged order.
We found that the increase in structural coherence length by switching from Grafoil to
ZYX graphite substrate does not greatly affect the onset temperature of ferromagnetic order,
while a two-dimensional Heisenberg ferromagnet calculation predicts a 25% reduction of the
onset temperature using ZYX substrate. As a conclusion, we eliminated the possibility of
the substrate finite size effect on the 3He films magnetic transition, and proved that
the ordering of 3He films magnetic system is intrinsic due to long ranged dipolar interaction.