Isotropic \Gamma_6 Ground State in Caged Compound NdRu2Zn20

The magnetic susceptibility $\chi$, the magnetization $M$, and the specific heat $C$ were measured for the caged cubic compound NdRu$_2$Zn$_{20}$ at temperatures down to 0.5 K in the magnetic field $H$ along the three crystallographic principal axes, [100], [110], and [111]. The ferromagnetic phase transition was observed at the Curie temperature $T_{\rm C}=1.9$ K in both the temperature dependence of $\chi(T)$ and $C(T)$. Below $T_{\rm C}$, 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 $T_{\rm C}$. The easy direction of magnetization at 0.5 K changes from the [111] direction in 0 T to the [110] diretion in 7 T. We analyzed these characteristic behaviors theoretically taking into account the crystalline-electric-field energy, the magnetic exchange interaction, and the Zeeman energy. It was found that these features are originated from the isotropic $\Gamma_6$ ground state mixing with the magnetic $\Gamma_8^{(1)}$ excited state. The temperature and the magnetic field dependences of $M(T,H)$ and $C(T,H)$ are quantitatively well analyzed by this theoretical calculation.