  Localized spins and itinerant electrons rarely coexist in
geometrically-frustrated spinel lattices. We show that the spinel CoV2O4 stands
at the crossover from insulating to itinerant behavior and exhibits a complex
interplay between localized spins and itinerant electrons. In contrast to the
expected paramagnetism, localized spins supported by enhanced exchange
couplings are frustrated by the effects of delocalized electrons. This
frustration produces a non-collinear spin state and may be responsible for
macroscopic spin-glass behavior. Competing phases can be uncovered by external
perturbations such as pressure or magnetic field, which enhance the
frustration.
