Magnetism in the layered transition-metal thiophosphates MPS₃(M=Mn, Fe, and Ni)

Abstract

Anisotropic magnetic susceptibilities of single crystals of the layered transition-metal thiophosphates MnPS₃, FePS₃, and NiPS₃ have been measured as a function of temperature. The materials order antiferromagnetically at low temperatures, the Néel temperatures being 78, 123, and 155 K, respectively. In the ordered state, the magnetization axis lies perpendicular to the layers for MnPS₃ and FePS₃, while for NiPS₃ it lies in the layer. In the paramagnetic regime, the anisotropies of these compounds are different; while the susceptibility for MnPS₃ is isotropic and that for NiPS₃ shows only a weak ansiotropy, FePS₃ exhibits highly anisotropic susceptibility. The anisotropic susceptibilities have been analyzed to obtain information on the state of the magnetic ions and the nature of magnetic interactions between them. The results show that MnPS₃, FePS₃, and NiPS₃ form a unique class of compounds. Although all three compounds are isostructural with the magnetic lattice being the two-dimensional honeycomb, the spin dimensionalities for the three are different. While MnPS₃ is best described by the isotropic Heisenberg Hamiltonian, FePS₃ is most effectively treated by the Ising model and NiPS₃ by the anisotropic Heisenberg Hamiltonian. The origin of the anisotropy in these compounds has been discussed, and it is shown how it arises from a combination of spin-orbit coupling and the trigonal distortion of the MS₆ octahedra. The magnetic exchange constant, J and the zero-field splitting energies of the ground state of the transition-metal ion have been evaluated from the anisotropic paramagnetic susceptibilities.