In solid 3He, the exchange interaction responsible for the antiferromagnetic
ordering of the nuclear spins arises from the tunneling of 3He atoms between
neighboring sites due to zero-point motion. Therefore, the strength of the interaction
is an extremely strong function of molar volume. Consequently, as thermodynamics
dictates, the transition near Hc2 produces a very clear signature in pressure,
which can be detected by a sensitive Straty-Adams capacitive strain gauge during
a field sweep.
We have employed adiabatic sweeping of the field to study the critical fields
Hc1 and Hc2 up to 2.5 T in previous work. (The lower critical field Hc1 of solid 3He
is for transition between two antiferomagnetic phases of different symmetries and
cannot be described by the Bose-Einstein condensation hypothesis.) The magnetic
field of this experiment has been limited to 2.5 T available with a small homemade
magnet that sits inside vacuum space above the copper demagnetization stage.
The larger molar volume for which Hc2 < 2.5 T is v ~ 21.8 cm3/mole, with Tc
well below 1 mK, resulting in long time constants between the 3He and the thermometer.
Consequently, our current 2.5 T magnet is unsuitable for investigating the field
dependence of Tc, or the temperature dependence of Hc2. Currently, new measurements
in an 8 T magnet, for which Hc2 and Tc are significantly higher, are underway.
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"Volume Dependence of HC1, HC2, and ΔP(HC2) in Magnetically Ordered bcc 3He",
N.F. Omelaenko, S. Abe, E.D. Adams, and Y. Takano, To be published.
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