ULTRASONIC AND MAGNETIZATION STUDIES AT THE METAMAGNETIC TRANSITION IN UPt3

The velocity and attenuation of longitudinal ultrasound were measured on two single crystals of UPt3 in magnetic fields up to 33 T and temperatures down to 40 mK. For fields applied in the basal plane, a strong dip in the velocity is seen at a field of 20 T in both samples and is accompanied by a simultaneous increase in the dc magnetization. At lower temperatures the dip gets more pronounced and sharper. At a temperature around 0.6 K the velocity dip develops a second feature at a slightly higher field of ~21.6 T.