Strong competition between orbital ordering and itinerancy in a frustrated spinel vanadate

The crossover from localized to itinerant electron regimes in the geometrically frustrated spinel system Mn1-xCoxV2O4 is explored by neutron-scattering measurements, first-principles calculations, and spin models. At low Co doping, the orbital ordering (OO) of the localized V3+ spins suppresses magnetic frustration by triggering a tetragonal distortion. At high Co doping levels, however, electronic itinerancy melts the OO and lessens the structural and magnetic anisotropies, thus increasing the amount of geometric frustration for the V-site pyrochlore lattice. Contrary to the predicted paramagentism induced by chemical pressure, the measured noncollinear spin states in the Co-rich region of the phase diagram provide a unique platform where localized spins and electronic itinerancy compete in a geometrically frustrated spinel.