Home >  About ISSP >  Publications > Activity Report 2018 > Xxxx Xxxx

Spin Waves in the Two-Dimensional Honeycomb Lattice XXZ-Type van der Waals Antiferromagnet CoPS3

C. Kim, J. G. Park and T. Masuda

The recent introduction of magnetic van der Waals (vdW) materials has opened new and novel opportunities to examine the low-dimensional magnetism in real materials. In particular, TMPS3 (TM = Mn, Fe, Co, Ni) family has attracted special interests in the community as a class of antiferromagnetic 2D vdW materials. The crystal structure of TMPS3 is a monoclinic structure with a C 2/m space group, where a weak vdW force couples honeycomb layers with edge-shared TMS6 octahedra on the ab-plane along the c-axis. Since magnetic structure and exchange interactions in TMPS3 depend on the TM elements, they provide an excellent playground to validate spin dynamics theory in low dimensions experimentally.

CoPS3 has been less studied among them due to the difficulty in synthesizing high-purity samples [1]. Therefore, the magnetic Hamiltonian of CoPS3 has been unknown so far and needs investigation. CoPS3 has antiferromagnetic phase transition at TN = 122 K with zig-zag magnetic order. The spins are aligned along the a axis with a small canting to the c axis. The magnetic susceptibility shows a difference between H // ab and H // c in the paramagnetic state, which implies XY-like anisotropy [1].

Moreover, Co2+ magnetic systems have recently drawn significant attention because Co compounds may host Kitaev interactions [2]. A spin-orbital entangled state is an essential ingredient to realize the Kitaev interactions, which can be verified by measuring spin excitations. Typically, a spin-orbit exciton, the crystal field excitation between Jeff = 1/2 to 3/2 state, is strong evidence of a spin-orbital entangled state. Most cobalt compounds have such excitation at 20~30meV, proportional to the strength of spin-orbit coupling of cobalt [3].

In this study [4], we examined such a possibility using inelastic neutron scattering at the time-of-flight spectrometer of HRC at J-PARC. Our temperature-dependent measurements show that CoPS3 does not show the spin-orbit exciton expected near 30 meV at high temperature in stark contrast with initial expectations. Figure 1 shows the temperature dependence of magnetic excitations. Above TN = 122 K, there is no sign of excitations near 30 meV. This can only be interpreted as evidence for the ground state of CoPS3 being an S = 3/2 state, not the spin-orbital entangled Jeff = 1/2 state. Figure 2 shows the spin-wave spectrum of CoPS3 with incident neutron energy Ei = 71 meV. The measured spin-wave spectrum shows two magnon branches over 40 meV with a massive 13 meV spin gap. Also, the spin-wave spectrum shows extra gap-like features at 24~27 meV. Based on the former analysis about temperature dependence of magnetic excitations, we use magnetic Hamiltonian with S = 3/2 system to explain the spin-waves of CoPS3. This magnon was well-fitted using an anisotropic Heisenberg (XXZ) model with a reasonable anisotropy coefficient α ≡ Jz/Jx = 0.6 and strong easy-axis single-ion anisotropy K = 2.4 meV along the a axis. The magnetic structure from our magnetic Hamiltonian also consistent with the reported magnetic structure. In summary, our experiment and theoretical analysis suggest CoPS3 as another exciting platform to study anisotropic XXZ-type spin Hamiltonian in the honeycomb antiferromagnet. Moreover, it provides an excellent playground for future investigation of low-dimensional magnetism with magnetic van der Waals materials.

  • [1] A. R. Wildes et al., J. Phys.: Condens. Matter 29, 455801 (2017).
  • [2] H. Liu et al., Phys. Rev. B 97, 014407 (2018).
  • [3] K. Tomiyasu et al., Phys. Rev. B 84, 054405 (2011).
  • [4] C. Kim et al., Phys. Rev. B 102, 184429 (2020).
  • C. Kima, J. Jeonga, P. Parka, T. Masuda, S. Asai, S. Itohb, H. S. Kimc, A. Wildesd, and J. G. Parka
  • aSeoul National University
  • bKEK
  • cKangwon National University
  • dInstitut Laue-Langevin