Negligible Kitaev interaction in the van der Waals antiferromagnet NiBr$_2$ revealed by neutron spectroscopy

Abstract

Using time-of-flight neutron spectroscopy, we have mapped out the magnetic excitations in the van der Waals antiferromagnet NiBr$_2$ over a broad range of energies and momenta. The observed well-defined magnon dispersions are quantitatively reproduced by linear spin-wave theory based on a minimal bilinear Hamiltonian that includes Heisenberg exchange interactions up to third-nearest neighbors, interlayer couplings, and a single-ion anisotropy term, with no need to invoke a Kitaev interaction. This finding is consistent with the fact that bromine is lighter than iodine, leading to weaker ligand-mediated spin-orbit coupling in NiBr2 than in NiI2, and consequently a reduced Kitaev interaction. Our results establish NiBr$_2$ as a Heisenberg-dominated van der Waals antiferromagnet and support the view that the ligand spin-orbit coupling governs the strength of the bond-dependent interactions within the NiX$_2$ (X = halogen) family.