High-resolution magnetization studies of the heavy-fermion superconductor CeCu₂Si₂ at very low temperatures and in high magnetic fields

We report high-resolution dc-magnetization measurements on high-quality stoichiometric single crystals of CeCu₂Si₂ at very low temperatures down to 50 mK and in high magnetic fields up to 115 kOe, especially focusing on the as yet still unidentified phase A (and B at higher fields) that competes for stability with the heavy-fermion superconductivity. Anomalies at the A/B phase transitions are clearly observed in both the temperature and the field dependence of the magnetization. In isothermal magnetization curves M(T = const, H) at 50 mK, a metamagneticlike first-order transition is observed at 71 kOe and 30 kOe for H∥a and c, respectively. The magnitude of the magnetization jump at the first-order transition is only 2 × 10⁻³μ_B/Ce and 8 × 10⁻⁴μ_B/Ce for H∥a and c, respectively. Detailed magnetic phase diagrams are presented for both field orientations. The present results are consistent with a scenario that both A and B phases are spin-density-wave-type long-range-ordered states, where the ordered moments are of the order of 10⁻³μ_B/Ce.