Limited Ferromagnetic Interactions in Monolayers of MPS₃ (M = Mn and Ni)

We present a systematic study of the electronic and magnetic properties of two-dimensional ordered alloys, consisting of two representative hosts (MnPS₃ and NiPS₃) of transition metal phosphorus trichalcogenides doped with 3d elements. For both hosts, our DFT + U calculations are able to qualitatively reproduce the ratios and signs of all experimentally observed magnetic couplings. The relative strength of all antiferromagnetic exchange couplings, both in MnPS₃ and in NiPS₃, can successfully be explained using an effective direct exchange model: it reveals that the third-neighbor exchange dominates in NiPS₃ due to the filling of the t_2g subshell, whereas for MnPS₃, the first-neighbor exchange prevails, owing to the presence of the t_2g magnetism. On the other hand, the nearest neighbor ferromagnetic coupling in NiPS₃ can only be explained using a more complex superexchange model and is (also) largely triggered by the absence of the t_2g magnetism. For the doped systems, the DFT + U calculations revealed that magnetic impurities do not affect the magnetic ordering observed in the pure phases, and thus, in general in these systems, ferromagnetism may not be easily induced by such a kind of elemental doping. However, unlike for the hosts, the first and second (dopant-host) exchange couplings are of similar order of magnitude. This leads to frustration in the case of antiferromagnetic coupling and may be one of the reasons of the observed lower magnetic ordering temperature of the doped systems.