We investigated the magnetic phase diagram of the first Pr-based heavy fermion superconductor PrOs4Sb12 by means of high-resolution dc magnetization measurements in low temperatures down to 0.06K. The temperature dependence of the magnetization M(T) at 0.1kOe exhibits two distinct anomalies at Tc1=1.83K and Tc2=1.65K, in 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. The presence of the field-induced ordered phase (called phase A here) is confirmed for three principal directions above 40kOe, with the anisotropic A-phase transition temperature TA: TA[100] > TA[111] >TA[110]. The present results are discussed on the basis of crystalline-electrical-field level schemes with a non-magnetic ground state, with emphasis on a Gamma1 singlet as the possible ground state of Pr3+ in PrOs4Sb12.
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