The antiferromagnetic mixed valence ternary oxide $\alpha \text{−}{\mathrm{CoV}}_3{\mathrm{O}}_8$ displays disorder on the ${\mathrm{Co}}^{2+}$ site that is inherent to the $Ibam$ space group resulting in a local selection rule requiring that one ${\mathrm{Co}}^{2+}$ and one ${\mathrm{V}}^{4+}$ reside next to each other, thus giving rise to an intrinsically disordered magnet without the need for external influences such as chemical dopants or porous media. The zero-field structural and dynamic properties of $\alpha \text{−}{\mathrm{CoV}}_3{\mathrm{O}}_8$ 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 ${\mathrm{Co}}^{2+}$ over the $16k$ metal sites. However, by applying the sum rules of neutron scattering we observe that the collective magnetic excitations are parametrized with an ordered ${\mathrm{Co}}^{2+}$ arrangement and critical scattering consistent with a three-dimensional Ising universality class. The low-energy spectrum is well described by ${\mathrm{Co}}^{2+}$ 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 $T{}_{\mathrm{N}}\sim 19\mathrm{K}$. A crystal field analysis finds two bands of excitations separated in energy at $\hslash \omega \sim 5\mathrm{meV}$ and 25 meV, consistent with a $j{}_{\mathrm{eff}}=\frac{1}{2}$ ground state with little mixing between spin-orbit split levels. 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 $j{}_{\mathrm{eff}}$ manifolds, may play a key role in making the dynamics largely insensitive to disorder.

• Received 8 May 2018
• Revised 29 October 2018

DOI:https://doi.org/10.1103/PhysRevB.98.224410