$\gamma$-triclinic and $\alpha$-monoclinic polymorphs of ${\mathrm{CoV}}_2{\mathrm{O}}_6$ are two of the few known transition-metal ion-based materials that display stepped $\frac{1}{3}$ magnetization plateaus at low temperatures. Neutron diffraction [M. Markkula et al., Phys. Rev. B 86, 134401 (2012)], x-ray dichroism [N. Hollmann et al., Phys. Rev. B 89, 201101(R) (2014)], and dielectric measurements [J. Singh et al., J. Mater. Chem. 22, 6436 (2012)] have shown a coupling between orbital, magnetic, and structural orders in ${\mathrm{CoV}}_2{\mathrm{O}}_6$. We apply neutron inelastic scattering to investigate this coupling by measuring the spin-orbit transitions in both $\alpha$ and $\gamma$ polymorphs. We find the spin exchange and anisotropy in monoclinic $\alpha -{\mathrm{CoV}}_2{\mathrm{O}}_6$ to be weak in comparison with the spin-orbit coupling $\lambda$ and estimate an upper limit of $|J/\lambda |\sim 0.05$. 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 ${\mathrm{Co}}^{2+}$ ion results in a direct coupling between higher-energy orbital levels, the magnetic ground state, and elastic strain. ${\mathrm{CoV}}_2{\mathrm{O}}_6$ 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.

• Received 1 May 2015
• Revised 12 August 2015

DOI:https://doi.org/10.1103/PhysRevB.92.125116