Anomaly of Magnetization in the Superconducting Mixed State of UPt3

Magnetization curves of UPt 3 along the c-axis were measured at low temperatures down to 80 mK. Below 350 mK a peak of the irreversible magnetization was found just below the superconducting upper critical field H c 2 . However, the peak disappeared above 390 mK, which is much lower than the superconducting critical temperature of 580 mK, suggesting an unconventional mechanism of the flux pinning near H c 2 . No distinct anomaly was observed in the magnetization curves at the phase transition between B and C superconducting phases.