Spin waves in the two-dimensional honeycomb lattice XXZ-type van der Waals antiferromagnet CoPS3

The magnetic excitations in CoPS3, a two-dimensional van der Waals (vdW) antiferromagnet with Co2+ ion on a honeycomb lattice, have been measured using powder inelastic neutron scattering. The absence of spin-orbit exciton around 30 meV indicates that Co2+ ions in CoPS3 have an S=3/2 state rather than a spin-orbital entangled Jeff=1/2 ground state. And, clear dispersive spin waves are observed with a large spin gap of Gê+13 meV. The magnon spectra were fitted using an XXZ-type J1-J2-J3 Heisenberg Hamiltonian with single-ion anisotropy assuming no magnetic exchange interaction between the honeycomb layers. The best-fit parameters show ferromagnetic exchange interactions J1=-2.08meV and J2=-0.26meV for the nearest- and second-nearest neighbors and a sizable antiferromagnetic exchange interaction J3=4.21meV for the third-nearest neighbor with the strong easy-axis anisotropy K=-2.06meV. The anisotropic XXZ-type Hamiltonian could only achieve a suitable fitting. The exchange interaction for the out-of-plane spin component is smaller than that for the in-plane one by a ratio +¦=Jz/Jx=0.6. Our result directly shows that CoPS3 is an experimental realization of the XXZ model with a honeycomb lattice in two-dimensional vdW magnets.