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.