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

TL;DR

This study uses neutron scattering to measure magnetic excitations in NiPS3, revealing detailed exchange interactions and anisotropy, and suggesting the magnetic structure is near instability.

Contribution

It provides the first detailed determination of magnetic exchange parameters and anisotropy in NiPS3 using neutron scattering data fitted with a Heisenberg model.

Findings

NiPS3 exhibits dispersive magnons with a 7 meV gap.

The nearest-neighbor exchange is ferromagnetic with J1=1.9 meV.

The third nearest-neighbor exchange is strongly antiferromagnetic with J3=-6.90 meV.

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.