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Abstract

The antiferromagnetic mixed valence ternary oxide alpha-CoV3O8 displays disorder on the Co(2+ )site that is inherent to the Ibam space group resulting in a local selection rule requiring that one Co2+ and one V4+ 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-CoV3O8 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 Co2+ 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 Co2+ arrangement and critical scattering consistent with a three-dimensional Ising universality class. The low-energy spectrum is well described by Co2+ 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-N similar to 19 K. A crystal field analysis finds two bands of excitations separated in energy at (h) over bar omega similar to 5 meV and 25 meV, consistent with a j(eff) = 1/2 aground 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(eff) manifolds, may play a key role in making the dynamics largely insensitive to disorder.