  Magnetic frustration in metals is scarce and hard to pinpoint, but exciting
due to the possibility of the emergence of fascinating novel phases. The cubic
intermetallic compound HoInCu$_4$ with all holmium atoms on an fcc lattice,
exhibits partial magnetic frustration, yielding a ground state where half of
the Ho moments remain without long-range order, as evidenced by our neutron
scattering experiments. The substitution of In with Cd results in HoCdCu$_4$ in
a full breakdown of magnetic frustration. Consequently we found a fully ordered
magnetic structure in our neutron diffraction experiments. These findings are
in agreement with the local energy scales and crystal electric field
excitations, which we determined from specific heat and inelastic neutron
scattering data. The electronic density of states for the itinerant bands acts
as tuning parameter for the ratio between nearest-neighbor and
next-nearest-neighbor interactions and thus for magnetic frustration.
