Isotropic \Gamma_6 Ground State in Caged Compound NdRu2Zn20

The magnetic susceptibility X, magnetization M, and specific heat C were measured for the caged cubic compound NdRu2Zn20 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 = 1.9 K in the temperature dependences of both (X)(T) and C(T). Below T-C, the magnetization curves M(H) 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 0.5 K 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-electric-field-energy, the magnetic exchange interaction, and the Zeeman energy. It was found that these features originate from the isotropic Gamma 6 ground state mixing with the magnetic Gamma((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.