MAGNETISM IN THE LAYERED TRANSITION-METAL THIOPHOSPHATES MPS3 (M = MN, FE, AND NI)

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 anti-ferromagnetically at low temperatures, the Neel 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.