  The magnetic and structural properties of single crystal
Fe$_{1-x}$Co$_x$V$_2$O$_{4}$ samples have been investigated by performing
specific heat, susceptibility, neutron diffraction, and X-ray diffraction
measurements. As the orbital-active Fe$^{2+}$ ions with larger ionic size are
gradually substituted by the orbital-inactive Co$^{2+}$ ions with smaller ionic
size, the system approaches the itinerant electron limit with decreasing V-V
distance. Then, various factors such as the Jahn-Teller distortion and the
spin-orbital coupling of the Fe$^{2+}$ ions on the A sites and the orbital
ordering and electronic itinerancy of the V$^{3+}$ ions on the B sites compete
with each other to produce a complex magnetic and structural phase diagram.
This phase diagram is compared to those of Fe$_{1-x}$Mn$_x$V$_2$O$_{4}$ and
Mn$_{1-x}$Co$_x$V$_2$O$_{4}$ to emphasize several distinct features.

Materials Science