Spin-orbit transitions in $\alpha$ and $\gamma$-CoV$_{2}$O$_{6}$

TL;DR

This study investigates the spin-orbit transitions in the polymorphs of CoV₂O₆ using neutron inelastic scattering, revealing how local distortions and spin-orbit coupling intertwine structural and magnetic properties, and exploring their relation to magnetization plateaus.

Contribution

It provides the first detailed measurement of spin-orbit transitions in CoV₂O₆ polymorphs, highlighting the role of local distortions and spin-orbit coupling in magnetic behavior.

Findings

Weak spin-exchange and anisotropy in monoclinic α-CoV₂O₆ compared to spin-orbit coupling

Larger spin exchange and two-dimensional excitations in triclinic γ-CoV₂O₆

Identification of a magnetic excitation independent of the local crystalline electric field

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.