Ultrasonic measurements on the heavy fermion superconductor UPt3 indicate evidence of the possibility of two or more superconducting phases in a magnetic field. At low fields there is a large hysteresis in the attenuation which was interpreted by Muller et al. as a metastable phase. We report on similar measurements, where in addition to a hysteresis in the attenuation there is an associated hysteretic temperature change. We believe the hysteresis in attenuation to be the result of a temperature increase due to flux jumps. A lower critical field of the order of 10 mT is estimated.
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