Magnetic dynamics of NiPS3

Neutron spectroscopy measurements have been performed on single crystals of the antiferromagnetic van der Waals compound NiPS3. Linear spin-wave theory using a Heisenberg Hamiltonian with single-ion anisotropies has been applied to determine the magnetic exchange parameters and the nature of the anisotropy. The analysis reveals that NiPS3 is less two-dimensional than its sister compounds, with a relatively large ferromagnetic exchange of J' = -0.3 meV between the layered ab planes. In-plane magnetic exchange interactions up to the third-nearest neighbor were required to fit the data. The nearest-neighbor exchange was ferromagnetic with J1 = -2.6 meV, the second neighbor was antiferromagnetic and small with J2 = 0.2 meV, and the dominant antiferromagnetic third-neighbor exchange was J3 = 13.5 meV. The anisotropy was shown to be largely XY-like with a small uniaxial component, leading to the appearance of two low-energy spin wave modes in the spin-wave spectrum at the Brillouin zone center. The analysis could reproduce the spin-wave energies, however, there are discrepancies with the calculated neutron intensities, hinting at more exotic phenomena.