Density dependence of the exchange energy in the bcc phase of solid helium-3

Abstract

A high-precision Pulse Fourier-Transform Nuclear-Magnetic-Resonance technique was used to measure the magnetic susceptibility of solid helium-3 in the bcc phase. Several molar volume samples ranging from 19.80 ml/mole to 24.40 ml/mole were studied. The helium-4 impurity level was 27 ppm. The measurements were done between 12 mK and 520 mK in a static magnetic field of 17.1 mT. The density dependence of the Curie-Weiss constant could be described by a power law, with a magnetic Gru虉neisen constant equal to 12.8 [plus or minus] 0.3. The Curie-Weiss constant at a molar volume of 24.2 ml/mole was equal to -1.75 mK, which was nearly a factor of two smaller than previous values. This work resolves a long-standing thermodynamic inconsistency between high-magnetic-field pressure measurements and susceptibility measurements. Additional experiments using an independent thermometer provided an upper limit for a possible systematic error in the data to within [plus or minus]0.1 mK. More than a tenfold improvement in the precision of the magnetic susceptibility measurements was achieved. When analysed in terms of a two-parameter model of the exchange interaction, the results imposed more stringent experimental constraints on possible values of exchange integrals at a density near melting.