Isotropic Γ₆ ground state in caged cubic compound NdRu₂Zn₂₀

The magnetic susceptibility χ, magnetization M, and specific heat C were measured for the caged cubic compound NdRu₆Zn₂₀ at temperatures down to 0.5K in magnetic fields H along the three principal crystallographic axes, [100], [110], and [111]. A ferromagnetic phase transition was observed at the Curie temperature T_c = 1:9K in the temperature dependences of both χ(T) and C(T). Below T_c, the magnetization curves M{Eth}H{Thorn} show very weak magnetic anisotropy, and the C(T) curves in the fields along the three principal axes show no magnetic anisotropy both below and above T_c. The easy direction of magnetization at 0.5K changes from the [111] direction at 0 T to the [110] direction at 7 T. We analyzed these characteristic behaviors theoretically by taking into account the crystalline-electricfield energy, the magnetic exchange interaction, and the Zeeman energy. It was found that these features originate from the isotropic Γ₆ ground state mixing with the magnetic Γ(1)₈ excited state. The temperature and magnetic field dependences of M(T,H) and C(T,H) are quantitatively in good agreement with this theoretical calculation.