The spin wave scattering from the quasi-two dimensional antiferromagnet NiPS3

NiPS3 is a quasi-two-dimensional material with the Ni2+ forming a honeycomb structure in crystallographic planes that are weakly bound by van der Waals forces. It is similar to graphite in this regard although, unlike graphite, it is magnetic, and hence it is of great interest to the graphene community. Recent neutron spectroscopy measurements show a remarkable ungapped Y-shaped dispersion of the spin waves that extend up to 55 meV. The low energy excitations are narrow in Q both in and normal to the planes. This type of excitation is highly unexpected for a two-dimensional material, which should only have very weak dispersion normal to the planes, and is reminiscent of gapped Y-shaped dispersion in Hg-based superconductors. We wish to use MERLIN with rep-rate multiplication to measure simultaneously the low and high energy spin waves, and thus to parameterise the Hamiltonian.