Low temperature magnetic properties of Ce₃Pd₂₀Si ₆

Magnetization of a single crystalline Ce₃Pd₂₀Si ₆ was measured at low temperatures down to 90mK in fields up to 11 T for the three principal directions [100], [110], and [111]. For all directions, two transitions were observed in the temperature variation of the magnetization M(T) at 0.2 T; a broad peak at 0.28K associated with a phase-III transition, and an upward bending at ∼0:5K due to a phase-II ordering from the paramagnetic state (phase-I). The latter critical temperature T_I-II is found to be strongly enhanced in magnetic fields B, depending on the field direction, and T_I-II(B) becomes highly anisotropic. These results suggest that phase-II is an antiferroquadrupole state originated from a Γ8 ground state. By contrast, no appreciable anisotropy is found in the phase-III state. At 90 mK, phase-III collapses by applying a field of ∼0:63 T for all directions. Above this field, the magnetization curve M(B) becomes strongly anisotropic. The magnetization value per Ce in the phase-I region at 90mK is apparently smaller than the Γ8 moment, indicating that either 8c or 4a site has a Γ7 ground state.