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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.

We report experimental and theoretical studies on the magnetoelastic interactions in MnPS 3 . Raman scattering response measured as a function of temperature shows a blue shift of the Raman active modes at 120.2 and 155.1 cm −1 , when the temperature is raised across the antiferromagnetic-paramagnetic transition. Density functional theory (DFT) calculations have been performed to estimate the effective exchange interactions and calculate the Raman active phonon modes. The calculations lead to the conclusion that the peculiar behavior with temperature of the two low energy phonon modes can be explained by the symmetry of their corresponding normal coordinates which involve the virtual modification of the super-exchange angles associated with the leading antiferromagnetic (AFM) interactions.

FePS 3 is a van der Waals compound with a honeycomb lattice that is a good example of a two-dimensional antiferromagnet with Ising-like anisotropy. Neutron spectroscopy data from FePS 3 were previously analyzed using a straightforward Heisenberg Hamiltonian with a single-ion anisotropy. The analysis captured most of the elements of the data; however, some significant discrepancies remained. The discrepancies were most obvious at the Brillouin zone boundaries. The data are subsequently reanalyzed, allowing for unequal exchange between nominally equivalent nearest-neighbors, which resolves the discrepancies. The source of the unequal exchange is attributed to a biquadratic exchange term in the Hamiltonian, which most probably arises from a strong magnetolattice coupling. The new parameters show that there are features consistent with Dirac magnon nodal lines along certain Brillouin zone boundaries.

Neutron specular reflectometry (SR) and off-specular scattering (OSS) are nondestructive techniques which, through deuteration, give a high contrast even among chemically identical species and are therefore highly suitable for investigations of soft-matter thin films. Through a combination of these two techniques, the former yielding a density profile in the direction normal to the sample surface and the latter yielding a depth-resolved in-plane lateral structure, one can obtain quite detailed information on buried morphology on length scales ranging from the order of ångströms to ∼10 µm. This is illustrated via quantitative evaluation of data on SR and OSS collected in time-of-flight (ToF) measurements of a set of films composed of immiscible polymer layers, protonated poly(methyl methacrylate) and deuterated polystyrene, undergoing a decomposition process upon annealing. Joint SR and OSS data analysis was performed by the use of a quick and robust originally developed algorithm including a common absolute-scale normalization of both types of scattering, which are intricately linked, constraining the model to a high degree. This, particularly, makes it possible to distinguish readily between different dewetting scenarios driven either by the nucleation and growth of defects (holes, protrusions etc.) or by thermal fluctuations in the buried interface between layers. Finally, the 2D OSS maps of particular cases are presented in different spaces and qualitative differences are explained, allowing also the qualitative differentiation of the in-plane structure of long-range order, the correlated roughness and bulk defects by a simple inspection of the scattering maps prior to quantitative fits.