  The RFe$_2$Zn$_{20}$ series manifests strongly correlated electron behavior
for the non-magnetic R = Y member and remarkably high temperature,
ferromagnetic ordering ($T_C$ = 86 K) for the local moment bearing R = Gd
member (a compound that is less than 5% atomic Gd). In contrast, the
isostructural RCo$_2$Zn$_{20}$ series manifests a more typical ordering
temperature ($T_N$ = 5.7 K for GdCo$_2$Zn$_{20}$) and YCo$_2$Zn$_{20}$ does not
show signs of correlated electron behavior. Studies of
R(Fe$_x$Co$_{1-x}$)$_2$Zn$_{20}$ (R = Gd, Y), combined with bandstructure
calculations for the end members, reveal that YFe$_2$Zn$_{20}$ is a nearly
ferromagnetic Fermi liquid and that the remarkably high $T_C$ associated with
GdFe$_2$Zn$_{20}$ is the result of submerging a large local moment into such a
highly polarizable matrix. These results indicate that the RFe$_2$Zn$_{20}$
series, and more broadly the RT$_2$Zn$_{20}$ (T = Fe, Co, Ni, Mn, Ru, Rh, Os,
Ir, Pt) isostructural family of compounds, offer an exceptionally promising
phase space for the study of the interaction between local moment and
correlated electron effects near the dilute R limit.
