Abstract
The spinel vanadates have become a model family for exploring orbital order on the frustrated pyrochlore lattice, and recent debate has focused on the symmetry of local crystal fields at the cation sites. Here, we present neutron scattering measurements of the magnetic excitation spectrum in FeV2O4, a recent example of a ferrimagnetic spinel vanadate which is available in single-crystal form.We report the existence of two emergent magnon modes at low temperatures, which draw strong parallels with the closely related material, MnV2O4. We were able to reproduce the essential elements of both the magnetic ordering pattern and the dispersion of the inelastic modes with semiclassical spin-wave calculations, using a minimal model that implies a sizable single-ion anisotropy on the vanadium sublattice. Taking into account the direction of ordered spins, we associate this anisotropy with the large trigonal distortion of VO6 octahedra, previously observed via neutron powder diffractionmeasurements. We further report on the spin gap, which is an order of magnitude larger than that observed in MnV2O4. By looking at the overall temperature dependence, we were able to show that the gap magnitude is largely associated with the ferro-orbital order known to exist on the iron sublattice, but the contribution to the gap from the vanadium
sublattice is in fact comparable to what is reported in the Mn compound. This reinforces the conclusion that the spin canting transition is associated with the ordering of vanadium orbitals in this system, and closer analysis indicates closely related physics underlying orbital transitions in FeV2O4 and MnV2O4.
