Magnetism of CuX2 frustrated chains (X = F, Cl, Br): the role of covalency

TL;DR

This study uses advanced DFT calculations to explore how covalency influences magnetic interactions in CuX2 chain compounds, revealing unexpected ferromagnetic coupling due to covalent effects, with implications for modeling complex magnetic systems.

Contribution

It demonstrates that covalency significantly alters magnetic exchange interactions in CuX2 chains, challenging traditional ionic models and showcasing the effectiveness of combined periodic and cluster DFT approaches.

Findings

CuF2 exhibits ferromagnetic J1 for angles < 100° and antiferromagnetic for larger angles.

CuCl2 and CuBr2 show ferromagnetic J1 across studied angles due to covalency effects.

Cluster calculations align well with experimental data, highlighting their utility in magnetic studies.

Abstract

Periodic and cluster density-functional theory (DFT) calculations, including DFT+U and hybrid functionals, are applied to study magnetostructural correlations in spin-1/2 frustrated chain compounds CuX2: CuCl2, CuBr2, and a fictitious chain structure of CuF2. The nearest-neighbor and second-neighbor exchange integrals, J1 and J2, are evaluated as a function of the Cu-X-Cu bridging angle, theta, in the physically relevant range 80-110deg. In the ionic CuF2, J1 is ferromagnetic for theta smaller 100deg. For larger angles, the antiferromagnetic superexchange contribution becomes dominant, in accord with the Goodenough-Kanamori-Anderson rules. However, both CuCl2 and CuBr2 feature ferromagnetic J1 in the whole angular range studied. This surprising behavior is ascribed to the increased covalency in the Cl and Br compounds, which amplifies the contribution from Hund's exchange on the ligand atoms and renders J1 ferromagnetic. At the same time, the larger spatial extent of X orbitals enhances the antiferromagnetic J2, which is realized via the long-range Cu-X-X-Cu paths. Both, periodic and cluster approaches supply a consistent description of the magnetic behavior which is in good agreement with the experimental data for CuCl2 and CuBr2. Thus, owing to their simplicity, cluster calculations have excellent potential to study magnetic correlations in more involved spin lattices and facilitate application of quantum-chemical methods.