  We report on a ternary intermetallic compound, "YMn2Zn20", comprising a
pyrochlore lattice made of Mn atoms. A series of In-doped single crystals
undergo no magnetic long-range order down to 0.4 K, in spite of the fact that
the Mn atom carries a local magnetic moment at high temperatures, showing
Curie-Weiss magnetism. However, In-rich crystals exhibit spin-glass transitions
at approximately 10 K due to a disorder arising from the substitution, while,
with decreasing In content, the spin-glass transition temperature is reduced to
1 K. Then, heat capacity divided by temperature approaches a large value of 280
mJ K-2 mol-1, suggesting a significantly large mass enhancement for conduction
electrons. This heavy-fermion-like behavior is not induced by the Kondo effect
as in ordinary f-electron compounds, but by an alternative mechanism related to
the geometrical frustration on the pyrochlore lattice, as in (Y,Sc)Mn2 and
LiV2O4, which may allow spin entropy to survive down to low temperatures and to
couple with conduction electrons.
