Ordered magnetism in the intrinsically decorated $j\rm{_{eff}}$ = $\frac{1}{2}$ $α$-CoV$_{3}$O$_{8}$

Abstract

The antiferromagnetic mixed valence ternary oxide $α$-CoV$_{3}$O$_{8}$ displays disorder on the Co$^{2+}$ site that is inherent to the $Ibam$ space group. The zero field structural and dynamic properties of $α$-CoV$_{3}$O$_{8}$~have been investigated using a combination of neutron and x-ray diffraction, DC susceptibility, and neutron spectroscopy. The low temperature magnetic and structural properties are consistent with a random macroscopic distribution of Co$^{2+}$ over the 16$k$ metal sites. However, by applying the sum rules of neutron scattering we observe the collective magnetic excitations are parameterized with an ordered Co$^{2+}$ arrangement and critical scattering consistent with a three dimensional Ising universality class. The low energy spectrum is well-described by Co$^{2+}$ cations coupled $via$ a three dimensional network composed of competing ferromagnetic and stronger antiferromagnetic superexchange within the $ab$ plane and along $c$, respectively. While the extrapolated Weiss temperature is near zero, the 3D dimensionality results in long range antiferromagnetic order at $T\rm{_{N}}\sim$ 19 K. A crystal field analysis finds two bands of excitations separated in energy at $\hbar ω$ $\sim$ 5 meV and 25 meV, consistent with a $j\rm{_{eff}}=\frac{1}{2}$ ground state with little mixing between spin-orbit split Kramers doublets. A comparison of our results to the random 3D Ising magnets and other compounds where spin-orbit coupling is present indicate that the presence of an orbital degree of freedom, in combination with strong crystal field effects and well-separated $j\rm{_{eff}}$ manifolds may play a key role in making the dynamics largely insensitive to disorder.