ULTRASOUND MEASUREMENTS OF THE B-T DIAGRAM OF THE HEAVY-FERMION MATERIAL UPT3 IN VERY HIGH MAGNETIC-FIELDS

We present elastic constant measurements of the heavy-fermion supercon ductor UPt3 at temperatures down to 50 mK and in static magnetic field s up to 30 T. These experiments yield information about the metamagnet ic (MM) transition itself as a function of field as well as about the accompanying transitions as a function of temperature both above and b elow the critical field B = 20.5 T. A comparison is made with analogo us measurements in CeRu2Si2. Moreover, we present new results on the r egion of the antiferromagnetic spin density wave (AF-SDW) in the B-T p lane and its anisotropy with respect to the direction of the magnetic field. Here a comparison with the anisotropy of the superconducting cr itical fields gives clues about the interaction of superconductivity a nd magnetism in UPt3.