ANOMALY OF MAGNETIZATION IN THE SUPERCONDUCTING MIXED-STATE OF UPT3

Magnetization curves of UPt3 along the c-axis were measured at low tem peratures down to 80 mK. Below 350 mK a peak of the irreversible magne tization was found just below the superconducting upper critical field H-c2. However, the peak disappeared above 390 mK, which is much lower than the superconducting critical temperature of 580 mK, suggesting a n unconventional mechanism of the flux pinning near H-c2. No distinct anomaly was observed in the magnetization curves at the phase transiti on between B and C superconducting phases.