Magnetic phase diagram of the heavy fermion superconductor PrOs ₄Sb₁₂

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 0.06 K. The temperature dependence of the magnetization M(T) at 0.1 kOe exhibits two distinct anomalies at T_c1 = 1.83 K and T_c2 = 1.65 K, in agreement with the specific heat measurements at zero field. Increasing magnetic field H, both T_C1(H) and T_c2(H) move toward lower temperatures without showing a tendency of intersecting to each other. Above 10 kOe, the transition at T_c2(H) appears to merge into a line of the peak effect which is observed near the upper critical field H_c2 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 40 kOe, with the anisotropic A-phase transition temperature T_A: T_A^[100] > T_A^[111] > T_A^[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 Γ_I singlet as the possible ground state of Pr³⁺ in PrOs₄ Sb₁₂.