Magnetic exchange parameters and anisotropy of the quasi-two-dimensional antiferromagnet NiPS3

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 neighbor were required. The fits show robustly that NiPS3 has an easy-axis anisotropy with ?=0.3 meV and that the third nearest neighbor 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 indicates that the nearest-neighbor interaction is ferromagnetic with J1=1.9 meV and that the second nearest-neighbor 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. © 2018 American Physical Society.