Neutron and X-ray studies in suppressing orbital order in FeV2O4 with Cr doping.

FeV$_2$O$_4$ is a spinel compound with an orbitally active V$^{3+}$ cation on a frustrated pyrochlore sublattice and Jahn-Teller active Fe$^{3+}$ on a diamond sublattice. Previous studies show that this material has three structural and two magnetic transitions, and that orbital order leads to coupling between the spin and lattice degrees-of-freedom. The opposite end of the doping series is the multiferroic, FeCr$_2$O$_4$, which has spin, but no orbital degree of freedom on the Cr$^{3+}$ and only two structural transitions. Although both materials show a higher temperature collinear ferrimagnetic state and a non-collinear phase at lower temperature, the physics must be different since the canting transition in FeV$_2$O$_4$ is associated with the orbital order at the lowest structural transition. In this talk, I will present the results of synchrotron X-ray and neutron powder diffraction studies of the structural and magnetic transitions in the doping series FeV$_{2-x}$Cr$_x$O$_4$. Specifically, I will comment on the doping-temperature phase diagram we extract from these measurements, and the region of co-existence between distinct non-collinear spin orders which exist at finite doping.