Neutron scattering from single crystals has been used to determine the magnetic structure and magnon dynamics of FePS 3 , an S = 2 Ising-like quasi-two-dimensional antiferromagnet with a honeycomb lattice. The magnetic structure has been confirmed to have a magnetic propagation vector of k M = [ 01 1 2 ] and the moments are collinear with the normal to the ab planes. The magnon data could be modeled using a Heisenberg Hamiltonian with a single-ion anisotropy. Magnetic interactions up to the third in-plane nearest neighbor needed to be included for a suitable fit. The best fit parameters for the in-plane exchange interactions were J 1 = 1.46, J 2 = −0.04, and J 3 = −0.96 meV. The single-ion anisotropy is large, = 2.66 meV, explaining the Ising-like behavior of the magnetism in the compound. The interlayer exchange is very small, J = −0.0073 meV, proving that FePS 3 is a very good approximation to a two-dimensional magnet.
Neutron inelastic scattering has been used to measure the magnetic excitations in powdered NiPS3, a quasi-two dimensional antiferromagnet with spin S = 1 on a honeycomb lattice. The spectra show clear, dispersive magnons with a ∼ 7 meV gap at the Brillouin zone center. The data were fitted using a Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb planes. Magnetic exchange interactions up to the third intraplanar nearest-neighbour were required. The fits show robustly that NiPS3 has an easy axis anisotropy with ∆ = 0.3 meV and that the third nearest-neighbour has a strong antiferromagnetic exchange of J3 = −6.90 meV. The data can be fitted reasonably well with either J1 < 0 or J1 > 0, however the best quantitative agreement with high-resolution data indicate that the nearestneighbour interaction is ferromagnetic with J1 = 1.9 meV and that the second nearest-neighbour exchange is small and antiferromagnetic with J2 = −0.1 meV. The dispersion has a minimum in the Brillouin zone corner that is slightly larger than that at the Brillouin zone center, indicating that the magnetic structure of NiPS3 is close to being unstable.
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