Three-dimensional magnetism and the Dzyaloshinskii-Moriya interaction in S = 3/2 kagome staircase Co ₃ V ₂ O ₈

Abstract

Time-of-flight neutron data reveal spin waves in the ferromagnetic ground state of the kagome staircase material Co3V2O8. While previous work has treated this material as quasi–two-dimensional, we find that an inherently three-dimensional description is needed to describe the spin wave spectrum throughout reciprocal space. Moreover, spin wave branches show gaps that point to an unexpectedly large Dzyaloshinskii-Moriya interaction on the nearest-neighbor bond, with D1 ≥ J1/2. A better understanding of the Dzyaloshinskii-Moriya interaction in this material should shed light on the multiferroicity of the related Ni3V2O8. At a higher temperature where Co3V2O8 displays an antiferromagnetic spin density wave structure, there are no well-defined spin wave excitations, with most of the spectral weight observed in broad diffuse scattering centered at the (0, 0.5, 0) antiferromagnetic Bragg peak.

Entropy driven incommensurate structures in the frustrated kagome staircase Co3V2O8

“…Both of these interactions are within the same buckled kagome plane. Given that the spin density wave structures feature a propagation vector parallel to the b-axis [10] and that the spin wave bandwidth along the [0 K 0] direction is comparable to that within the [H 0 L] plane [18] it is clear that there must be significant magnetic coupling between the buckled kagome planes and that it is therefore necessary to consider furtherneighbor interactions. If we expand our scope to Co-O-V-O-Co pathways there are six additional possible interactions to consider; while these can entail fairly large distances there is evidence that a significant amount of magnetic moment may reside on the V 5+ and O 2− ions [11] making such interaction pathways plausible.…”

“…These possible couplings are shown in figure 2. Spin wave measurements [18] were modeled with J 2 = J 5 = 0, so J 5 is excluded from the figure. The minimal Ising model in Chen et al [10] also considered a temperature-dependent effective interaction between the upper and lower spines on the same kagome layer (referred to as J 3 in that work but corresponding to the fifth nearestneighbor Co-Co distance).…”

“…A full accounting of the relevant magnetic interactions in Co 3 V 2 O 8 is a daunting task given the two crystallographically distinct spin sites, the likely presence of significant moment on V 5+ and O 2− ions [11], significant magnetocrystalline anisotropy [19], and the possibility of changing interaction strengths due to Co displacements [8]. Despite these complications we will show that a model consisting of the Hamiltonian previously used to describe the spin waves [18] plus two interplanar antiferromagnetic interactions suggested by a minimal Ising model [10] reproduces many of the observed features of the magnetic phase diagram. In this model the temperature evolution of the propagation vector does not require any change in coupling strengths but is driven by thermal fluctuations related to the entropy of stacking ferromagnetic and antiferromagnetic layers in patterns with nearly equivalent energies.…”

“…and the energy of different ordered structures is independent of δ. Using the Hamiltonian values given in the supplementary information of [18] this will occur so long as J 8 + 2J LU ≈ 0.3 meV. When J 8 + 2J LU is slightly smaller than this value we would expect a ferromagnetic ground state, but at higher temperatures thermal fluctuations could select spin density waves with a range of commensurate or incommensurate propagation vectors.…”

“…Well-defined spin waves were not observed in the δ = 1/2 spin density wave phase, with the inelastic spectral weight found in broad peaks of diffuse scattering centered at antiferromagnetic positions. An analysis of the spin wave excitations in the ferromagnetic ground state required five distinct superexchange interactions, distinct single-ion anisotropies on the two lattice sites, and a significant Dzyaloshinskii-Moriya interaction [18] to fully explain the spectra.…”