Co Doping Effect on the Crystal and Magnetic Phases in the Frustrated Spinel Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$V$_{2}$O$_{4}$

Co doping effect on the MnV$_{2}$O$_{4}$ spinel system has been investigated by the elastic and inelastic neutron scattering techniques. Our data present that a magnetic phase transition exists from collinear to noncollinear ferrimagnetic structure between the Mn$^{2+}$/Co$^{2+}$ and V$^{3+}$ moments and the Co doping decreases the V$^{3+}$ canting angle. The most remarkable finding is that with Co doping, the collinear to noncollinear transition, which coincides with the cubic to tetragonal structural transition related with the orbital ordering of the V$^{3+}$ ions in pure MnV$_{2}$O$_{4}$, occurs independently without the structural transition. Our results indicate that the Co doping changes the orbital nature of the V$^{3+}$ ions and enhances the magnetic coupling between the Mn$^{2+}$/Co$^{2+}$ and V$^{3+}$ moments. We discuss how the orbital and magnetic order are correlated in this system.