Ordered magnetism in the intrinsically decorated jeff=12α−CoV3O8

Abstract

The antiferromagnetic mixed valence ternary oxide α-CoV3O8 displays disorder on the Co 2+ site that is inherent to the Ibam space group resulting in a local selection rule requiring one Co 2+ and one V 4+ reside next to each other, thus giving rise to an intrinsically disordered magnet without the need for any external influences such as chemical dopants or porous media. The zero field structural and dynamic properties of α-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 Co 2+ over the 16k 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 N ∼ 19 K. A crystal field analysis finds two bands of excitations separated in energy at ω ∼ 5 meV and 25 meV, consistent with a j eff = 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 eff manifolds may play a key role in making the dynamics largely insensitive to disorder.

Kitaev interactions in the Co honeycomb antiferromagnets Na3Co2SbO6 and Na2Co2Songvilay1, Robert2, Petit3 et al. 2020Phys. Rev. BSelf Cite998561View full textAdd to dashboardCite

“…3(d) and 3(e)] and a fit to Gaussian functions yields energy values of 27.1(2) meV for the Sb compound and 21.6(1) meV for the Te compound. These energy scales are similar to those reported in other Cobased materials, where the spin-orbit transitions were found between 16 and 34 meV [42][43][44]49].…”

“…In these cobaltate systems, the 3d 7 Co 2+ ions in an octahedral environment have a predominantly t 5 2g e 2 g configuration with S = 3/2 and effective l = 1 moments, and form a j eff = 1/2 Kramers doublet ground state [Fig. 1(b)] [41][42][43][44]. Owing to the electronic configuration, anisotropic interactions are mediated through the oxygen atoms in a 90 • bonding geometry, including exchange processes via t 2g -e g and e g -e g channels.…”

Metastable and localized Ising magnetism in α−CoV2O6 magnetization plateaus

“…Considering the local octahedral field and spin-orbit coupling, the ground state of Co 2+ can be projected onto a j eff = 1 2 [46,47] with the 3 2 spin-orbit levels well separated in energy [45]. α-CoV 2 O 6 differs from rocksalt CoO where a large exchange constant induces strong mixing between the j eff spin-orbit levels [48][49][50].…”

Van Vleck excitons in Ca2RuO4

“…In the case where |J| → |λ| (or equivalently |H MF |/|α λ| → 1), the corresponding increase in splitting induced byĤ M F results in the significant entanglement between individual j eff levels. 69,70 The presence of such a strong admixture renders the modeling of the these systems difficult using psuedo-bosonic (Holstein-Primakoff) approaches that are based on conventional linear spin-wave theory, ultimately making it necessary to employ alternative methods such as the multilevel spin-orbit exciton model discussed here.…”