Topologically non-trivial spin-textures and multiferroicity in 2D nickel dihalides.

There is currently an increasing enthusiasm towards long-range magnetic order and multiferroicity in two-dimensional materials (2D) both from the fundamental and applicative point of view. In this respect, by means of first-principles calculations and Monte Carlo simulations, we investigate the nickel dihalides (NiX₂, with X= Cl, Br, I), which belong to the promising class of van der Waals materials, with a special focus on the exotic NiI₂ case. NiBr₂ and NiI₂ monolayer exhibit, in fact, anisotropic exchange couplings related to the relevant spin-orbit coupling of the heavy ligands, driving formation of spontaneous non-collinear magnetic configurations, supported by competing ferromagnetic and antiferromagnetic isotropic exchange. In particular, we predict intrinsic chiral topological spin-textures in NiI₂, among other closely lying competing phases; the specific topology of the skyrmionic lattices is determined by the, so far overlooked, two-ion anisotropy, which acts as an emerging chiral interaction. In addition, we also report interesting low-dimensional magnetoelectric and, eventual, multiferroic properties.