Abstract: Due to the complicated interplays of spin-frustration, orbital-ordering, and electron-itinerancy, the spinel vanadate, AV2O4, exhibits the interesting physical phenomena and attracts a lot of attention in both scientific and industrial areas. With the method of adjusting the A-site ions, we studied the mechanisms of those specific physical properties, such as the magnetic-structural phase transition, the effects of the local and itinerant electrons. Applying the techniques of magnetic susceptibility, specific heat, variable temperature X-ray diffraction and neutron scattering, and first-principles calculations, we studied the systems of Mn1??xCoxV2O4 and Fe1??xCoxV2O4, and found that: (i) For the low Co-doping compounds, the system was subject to local V3+ and A-site Fe2+. The geometrical frustration from the V-tetrahedra was weak, and the orbital sequence action tended to accompany the structural phase transitions with the magnetic ordering. Hence, the spin-lattice coupling was strong. (ii) For the high Co-doping compounds, the strong electronic itinerance decreased the orbital effects of the V-ion, and the exchange energy, JAB, enhanced, which induced the system isotropic. Therefore, the magnetic phase transition temperature increased, while the structural phase transition temperature shifted to a lower temperature or even finally disappears.

Key words: Spin-orbital coupling; magnetic frustration; spinel vanadate; Neutron diffraction; Inelastic neutron scattering