Sound velocity measurements of nuclear-ordered solid He-3 along the melting curve

We have measured the temperature dependence of the 10.98 MHz longitudinal s ound velocity of solid He-3 in the nuclear-ordered U2D2 phase and in the pa ramagnetic phase along the melting curve. The temperature dependence of the sound was attributed to the contributions from the nuclear spin system and the molar volume change along the melting curve. The sound velocity increa sed with temperature as T-4 in the U2D2 phase and the sound anisotropy due to the exchange interaction was found to be about 10% among single-domain c rystals investigated. The average value of the Gruneisen constant of the sp in wave velocity in the ordered phase was <(gamma)over bar>(c) = 16 and is compared to the calculated value of the multiple-exchange model.