The magnetic excitations in CoPS3, a two-dimensional van der Waals (vdW) antiferromagnet with spin S=3/2 on a honeycomb lattice, has been measured using powder inelastic neutron scattering. Clear dispersive spin waves are observed with a large spin gap of ~13 meV. The magnon spectra were fitted using an XXZ-type J1−J2−J3 Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb layers. The best-fit parameters show ferromagnetic exchange J1=−2.08 meV and J2=−0.26 meV for the nearest and second-nearest neighbors and a sizeable antiferromagnetic exchange J3=4.21 meV for the third-nearest neighbor with the strong easy-axis anisotropy K=−2.06 meV. The suitable fitting could only be achieved by the anisotropic XXZ-type Hamiltonian, in which the exchange interaction for the out-of-plane component is smaller than that for the in-plane one by a ratio α=Jz/Jx=0.6. Moreover, the absence of spin-orbit exciton around 30 meV indicates that Co2+ ions in CoPS3 have a S=3/2 state rather than a spin-orbital entangled J_\rm{eff}=1/2 ground state. Our result directly shows that CoPS3 is an experimental realization of the XXZ model with a honeycomb lattice in 2D vdW magnets.
@article{arxiv.2007.11791,
title = {Spin-waves in 2D honeycomb lattice $XXZ$-type van der Waals antiferromagnet CoPS$_3$},
author = {Chaebin Kim and Jaehong Jeong and Takatsugu Masuda and Shinichiro Asai and Shinichi Itoh and Heung-Sik Kim and Andrew Wildes and Je-Geun Park},
journal= {arXiv preprint arXiv:2007.11791},
year = {2020}
}