SOUND-VELOCITY OF SOLID HE-3 IN THE NUCLEAR-ORDERED U2D2 PHASE

Sound velocity of solid He-3 with an almost single magnetic domain was measured in the nuclear-ordered U2D2 phase and the paramagnetic phase along the melting curve using 10.98MHz longitudinal ultra-sound The s ound velocity increased with temperatures as T-4 in the U2D2 phase and jumped about 0.1% at the nuclear-ordered temperature T-N. Above T-N, it increased gradually, going through a maximum around 4mK, and then d ecreased as temperature increased. The temperature dependence of the s ound velocity a's attributed to two parts, the change an density of th e solid along the melting curve and the temperature dependent internal energy of the nuclear spin. The nuclear spin contribution to the temp erature dependent sound velocity agreed qualitatively with measured th ermodynamical Quantities. Attenuation of the sound at 10.98MHz was sma ller than 0.2(cm(-1)).