Ferromagnetism and Crystalline Electric Field Effects in Cubic UX$_2$Zn$_{20}$ (X=Co, Rh, Ir)

The properties of a new class of cubic UX$_2$Zn$_{20}$ (X=Co, Rh, Ir) heavy fermion compounds have been investigated by means of magnetic susceptibility, specific heat, electrical resistivity, and x-ray absorption spectroscopy. Both UCo$_2$Zn$_{20}$ and URh$_2$Zn$_{20}$ show peaks in $C(T)$ and $\chi(T)$ at $\sim$5-10 K suggesting the presence of crystalline electric field (CEF) effects in these materials, i.e., a localized $5f^2 $ configuration of uranium. In addition, measurements in high magnetic fields up to 40 T are consistent with a CEF model of a nonmagnetic ground state and a magnetic first excited state separated by $\sim$ 20 K. In contrast, UIr$_2$Zn$_{20}$ exhibits a first-order ferromagnetic transition at T$_c$=2.75 K with a saturation moment $\mu_{sat}$=0.5 $\mu_B$ in the ferromagnetic state. All compounds in this series are heavy fermion materials with enhanced electronic specific heat coefficients $\gamma \sim$ 150-300 mJ/molK$^2$. The physical properties of UX$_2$Zn$_{20}$ (X=Co, Rh, Ir) will be discussed.