  The crossover from localized- to itinerant-electron behavior is associated
with many intriguing phenomena in condensed-matter physics. In this paper, we
investigate the crossover from localized to itinerant regimes in the spinel
system Mn$_{1-x}$Co$_x$V$_2$O$_4$. At low Co doping, orbital order (OO) of the
localized electrons on the V3+ ions suppresses magnetic frustration by
triggering a tetragonal distortion. With Co doping, electronic itinerancy melts
the OO and suppresses the structural phase transition while the reduced
spin-lattice coupling produces magnetic frustration. Neutron scattering
measurements and first-principles-guided spin models reveal that the
non-collinear state at high Co doping is produced by weakened local anisotropy
and enhanced Co-V spin interactions.
