Finding new materials with antiferromagnetic (AFM) Kitaev interaction is an urgent issue for quantum magnetism research. We conclude that Na3Co2SbO6 and Na2Co2TeO6 are new honeycomb cobalt-based systems with AFM Kitaev interaction by carrying out inelastic neutron scattering experiments and subsequent analysis. The spin–orbit excitons observed at 20–28 meV in both compounds strongly support the idea that Co2+ ions of both compounds have a spin-orbital entangled J eff = 1/2 state. Furthermore, we found that a generalized Kitaev–Heisenberg Hamiltonian can describe the spin-wave excitations of both compounds with additional 3rd nearest-neighbor interaction. Our best-fit parameters show significant AFM Kitaev terms and off-diagonal symmetric anisotropy terms of a similar magnitude in both compounds. We also found a strong magnon-damping effect at the higher energy part of the spin waves, entirely consistent with observations in other Kitaev magnets. Our work suggests Na3Co2SbO6 and Na2Co2TeO6 as rare examples of the AFM Kitaev magnets based on the systematic studies of the spin waves and analysis.

Mn 3 Sn has recently attracted considerable attention as a magnetic Weyl semimetal exhibiting concomitant transport anomalies at room temperature. The topology of the electronic bands, their relation to the magnetic ground state and their nonzero Berry curvature lie at the heart of the problem. The examination of the full magnetic Hamiltonian reveals otherwise hidden aspects of these unusual physical properties. Here, we report the full spin wave spectra of Mn 3 Sn measured over a wide momentum -energy range by the inelastic neutron scattering technique. Using a linear spin wave theory, we determine a suitable magnetic Hamiltonian which not only explains the experimental results but also stabilizes the low-temperature helical phase, consistent with our DFT calculations. The effect of this helical ordering on topological band structures is further examined using a tight-binding method, which confirms the elimination of Weyl points in the helical phase. Our work provides a rare example of the intimate coupling between the electronic and spin degrees of freedom for a magnetic Weyl semimetal system. 1 arXiv:1811.07549v1 [cond-mat.str-el]

Ferromagnetism in two-dimensional materials presents a promising platform for the development of ultrathin spintronic devices with advanced functionalities. Recently discovered ferromagnetic van der Waals crystals such as CrI3, readily isolated two-dimensional crystals, are highly tunable through external fields or structural modifications. However, there remains a challenge because of material instability under air exposure. Here, we report the observation of an air-stable and layer-dependent ferromagnetic (FM) van der Waals crystal, CrPS4, using magneto-optic Kerr effect microscopy. In contrast to the antiferromagnetic (AFM) bulk, the FM out-of-plane spin orientation is found in the monolayer crystal. Furthermore, alternating AFM and FM properties observed in even and odd layers suggest robust antiferromagnetic exchange interactions between layers. The observed ferromagnetism in these crystals remains resilient even after the air exposure of about a day, providing possibilities for the practical applications of van der Waals spintronics.

The magnetic excitations in CoPS 3 , 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 J 1 -J 2 -J 3 Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb layers. The best-fit parameters show ferromagnetic exchange J 1 =-2.08 meV and J 2 =-0.26 meV for the nearest and second-nearest neighbors and a sizeable antiferromagnetic exchange J 3 =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=J z /J x =0.6. Moreover, the absence of spin-orbit exciton around 30 meV indicates that Co 2+ ions in CoPS 3 have a S=3/2 state rather than a spin-orbital entangled J eff =1/2 ground state. Our result directly shows that CoPS 3 is an experimental realization of the XXZ model with a honeycomb lattice in 2D vdW magnets.