The magnetic exchange parameters and anisotropy of the quasi-two dimensional antiferromagnet NiPS${}_3$

Abstract

Neutron inelastic scattering has been used to measure the magnetic excitations in powdered NiPS${}_3$, a quasi-two dimensional antiferromagnet with spin $S=1$ on a honeycomb lattice. The spectra show clear, dispersive magnons with a $\sim 7$ 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 NiPS${}_3$ has an easy axis anisotropy with $\Delta =0.3$ meV and that the third nearest-neighbour has a strong antiferromagnetic exchange of $J_3=-6.90$ meV. The data can be fitted reasonably well with either $J_1<0$ or $J_1>0$, however the best quantitative agreement with high-resolution data indicate that the nearest-neighbour interaction is ferromagnetic with $J_1=1.9$ meV and that the second nearest-neighbour exchange is small and antiferromagnetic with $J_2=-0.1$ 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 NiPS${}_3$ is close to being unstable.

21 pages, 7 figures, 33 references