Magnetic Phase Diagram of the Heavy-Fermion Superconductor PrOs4Sb12

We have carefully measured the DC magnetization of a high-quality single crystal of PrOs4Sb12 down to 60mK with a special interest in the superconducting phase. The temperature dependence of the magnetization M(T) at 100Oe exhibits two distinct anomalies at Tc1=1.83K and Tc2=1.65K, in good agreement with the specific heat measurements at zero field. Increasing magnetic field H, both Tc1(H) and Tc2(H) move toward lower temperatures without showing a tendency of intersecting to each other. Above 10kOe, the transition at Tc2(H) appears to merge into a line of the peak effect which is observed near the upper critical field Hc2 in the isothermal M(H) curves, suggesting a common origin for these two phenomena.