Topological semimetal and insulator features in ferromagnetic MPX₃ (M = Mn, Fe, Co, Ni; X= S, Se) monolayers

Two-dimensional layered materials (2DMs) continue to attract interest as building blocks for novel devices as their properties can be tuned by controlling such parameters as e.g. layer-stacking order or external pressure. Moreover, the 2DM family has recently grown to include robust magnetic materials allowing for an additional degree of freedom via control of the magnetic-ordering states. Here, the electronic and topological properties of MPX3 (M = Mn, Fe, Co, Ni, and X= S, Se) monolayers in their ferromagnetic state were studied using density functional theory. Monolayer MnPSe3 exhibits topological semimetal features that transform into topological insulator features when spin-orbit-coupling is introduced. While monolayer MnPS3 is a trivial insulator, topological features are found under a small pressure of 2%. These findings are further investigated using symmetry indicator approaches where we identify the origins of the non-trivial topology in these materials and suggest routes for achieving it experimentally. A thorough discussion on the choice of the Hubbard-U parameter is also presented. Our findings provide a potential avenue for uncovering new topological phases in ferromagnetic non-groundstates after the application of an external magnetic field.