Disentangling orbital and spin exchange interactions for Co2+ on a rocksalt lattice

Abstract

Neutron spectroscopy was applied to study the magnetic interactions of orbitally degenerate Co 2+ on a host MgO rocksalt lattice where no long range spin or orbital order exists. The paramagnetic nature of the substituted monoxide Co0.03Mg0.97O allows for the disentanglement of spin-exchange and spin-orbit interactions. By considering the prevalent excitations from Co 2+ spin pairs, we extract 7 exchange constants out to the fourth coordination shell. An antiferromagnetic next nearest neighbor 180 • exchange interaction is dominant, however dual ferromagnetic and antiferromagnetic interactions are observed for pairings with other pathways. These interactions can be understood in terms of a combination of orbital degeneracy in the t2g channel and the Goodenough-Kanamori-Anderson (GKA) rules. Our work suggests that such a hierarchy of exchange interactions exists in transition metal-based oxides with a t2g orbital degeneracy.

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

“…3(a)-(c)) on polycrystalline α-CoV 3 O 8 with an E i = 150, 60 and 15 meV, respectively at 5 K revealed two clear low |Q| excitations at ∼ 5 meV and ∼ 25 meV. To prevent any weak magnetic signal of interest from being masked by strong phonon bands, a scaled inelastic scattering spectrum γ S(|Q|, E) of an approximate isostructural compound α-ZnV 3 O 8 53 collected with identical experimental conditions was subtracted as a background 94 . Neutron inelastic scattering investigations of α-ZnV 3 O 8 on MARI found no evidence of correlated V 4+ moments over the energy range reported here.…”

“…A key difference between MnF 2 and both α-CoV 3 O 8 and FeF 2 is the presence of strong crystal field effects and spin-orbit coupling in the latter two compounds 133,175 . It is also worth noting that unlike the case of pure CoO 81,91,94,[181][182][183][184] where the large and far reaching exchange constants result in a significant and ultimately problematic entanglement of spin-orbit levels 94 , in the case of α-CoV 3 O 8 , the exchange constants are weak and the Weiss temperature is near 0 K. Both observations suggest that the presence of both strong crystal field effects and spin-orbit coupling with well-separated j eff manifolds, as is the case for α-CoV 3 O 8 , may be central to making the dynamics robust against strong disorder.…”

“…Operator Projection Factor α Before proceeding with the projection of the Landé gfactor onto the j eff = 1 2 ground state spin-orbit manifold, it is important to note that such a doublet manifold is a consequence of an approach commonly used 60,75,81,91,94 to address the orbital triplet ground state in Co 2+ . Such an approach first defines an effective total angular momentumĵ eff =l +Ŝ, wherel is a fictitious orbital angular momentum operator, with eigenvalue l =1 to reflect an triplet orbital degeneracy 104 .…”

“…A comparison between Eqs. B17 and B18 suggests that the Landé gfactor -a fundamental proportionality constant that can be derived directly from the Wigner-Eckart theorem 94 (B20) As required, Eq. B20 reduces to Eq.…”

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]. The local crystalline distortion further supports an Ising anisotropy in the spin orientation [51,52], evidenced by a gap in the magnetic dynamics and also the critical scattering discussed below.…”