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

Abstract

© 2020 American Physical Society. 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 J(eff) = 1/2 ground state. And, clear dispersive spin waves are observed with a large spin gap of similar to 13 meV. The magnon spectra were fitted using an XXZ-type J(1)-J(2)-J(3) Heisenberg Hamiltonian with single-ion anisotropy assuming no magnetic exchange interaction between the honeycomb layers. The best-fit parameters show ferromagnetic exchange interactions J(1) = -2.08 meV and J(2) = 0.26 meV for the nearest- and second-nearest neighbors and a sizable antiferromagnetic exchange interaction J(3) = 4.21 meV for the third-nearest neighbor with the strong easy-axis anisotropy K = - 2.06 meV. 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 alpha = J(2)/J(x) = 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