The Magnetic Hamiltonians for the Layered Transition Metal-PS₃ Antiferromagnets

The TM-PS₃ (TM = Mn, Fe, Co, Ni) family of compounds are layered van der Waals magnets. They have been extensively studied in the past but are now the subject of renewed interest as they are intrinsically magnetic and, like graphite, can be delaminated down to monolayer thicknesses. The compounds are isostructural, with the transition metals forming a honeycomb lattice in the quasi-two-dimensional planes, but they show remarkably different magnetic behaviour. Ordered magnetic structures, magnetic anisotropies and exchange constants, and magnetic critical properties vary considerably with the transition metal, essentially a result of removing electrons one at a time from a half-filled d shell. The differences could be understood if appropriate magnetic Hamiltonians could be determined. Knowledge of representative Hamiltonians is also essential information for the suitability of these compounds in device applications. Neutron inelastic scattering is an essential tool for this study as the neutron cross-section is directly proportional to the dynamic structure factor, S(<b>Q</b>,ω), which can be calculated directly from the Hamiltonian, and is able to probe the magnetic dynamics over the whole Brillouin zone. Experiments to test the magnetic Hamiltonians for the TM-PS₃ compounds will be summarised, with particular emphasis on neutron inelastic scattering, and an opinion on the outstanding problems to be solved in determining appropriate Hamiltonians will be expressed.