Magnetization study of the heavy fermion superconductor PrOs ₄Sb₁₂

We have carefully measured the DC magnetization of a high-quality single crystal of PrOs₄Sb₁₂ down to 60 mK with a special interest in the superconducting phase. The temperature dependence of the magnetization M(T) at 100 Oe exhibits two distinct anomalies at T_c1 = 1.83 K and T_c2 = 1.65 K, in good 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.