Spin-orbit transitions in α - and γ -CoV2O6

γ -triclinic and α -monoclinic polymorphs of CoV₂O₆ are two of the few known transition-metal ion-based materials that display stepped 1/3 magnetization plateaus at low temperatures. Neutron diffraction [M. Markkula et al., Phys. Rev. B 86, 134401 (2012)], 10.1103/PhysRevB.86.134401, x-ray dichroism [N. Hollmann et al., Phys. Rev. B 89, 201101(R) (2014)], 10.1103/PhysRevB.89.201101, and dielectric measurements [J. Singh et al., J. Mater. Chem. 22, 6436 (2012)], 10.1039/c2jm16290c have shown a coupling between orbital, magnetic, and structural orders in CoV₂O₆ . We apply neutron inelastic scattering to investigate this coupling by measuring the spin-orbit transitions in both α and γ polymorphs. We find the spin exchange and anisotropy in monoclinic α -CoV₂O₆ to be weak in comparison with the spin-orbit coupling λ and estimate an upper limit of |J /λ |∼ 0.05 . However, the spin exchange is larger in the triclinic polymorph and we suggest the excitations are predominately two dimensional. The local compression of the octahedra surrounding the Co^{2 +} ion results in a direct coupling between higher-energy orbital levels, the magnetic ground state, and elastic strain. CoV₂O₆ is therefore an example where the local distortion along with the spin-orbit coupling provides a means of intertwining structural and magnetic properties. We finish the paper by investigating the low-energy magnetic fluctuations within the ground-state doublet and report a magnetic excitation that is independent of the local crystalline electric field. We characterize the temperature and momentum dependence of these excitations and discuss possible connections to the magnetization plateaus.