Spin-orbit transitions in $\alpha$ and $\gamma$-CoV${}_2$O${}_6$

Abstract

$\gamma$-triclinic and $\alpha$-monoclinic polymorphs of CoV${}_2$O${}_6$ 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)], x-ray dichroism [N. Hollmann et al. Phys. Rev. B 89, 201101(R) (2014)], and dielectric measurements [K. Singh et al. J. Mater. Chem. 22, 6436 (2012)] have shown a coupling between orbital, magnetic and structural orders in CoV${}_2$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 $\alpha$-CoV${}_2$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 Co${}^{2+}$ ion results in a direct coupling between higher energy orbital levels, the magnetic ground state, and elastic strain. CoV${}_2$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.

(15 pages, 10 figures)

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