  Topological spin structures, such as magnetic skyrmions, hold great promises
for data storage applications, thanks to their inherent stability. In most
cases, skyrmions are stabilized by magnetic fields in non-centrosymmetric
systems displaying the chiral Dzyaloshinskii-Moriya exchange interaction, while
spontaneous skyrmion lattices have been reported in centrosymmetric itinerant
magnets with long-range interactions. Here, a spontaneous anti-biskyrmion
lattice with unique topology and chirality is predicted in the monolayer of a
semiconducting and centrosymmetric metal halide, NiI$_2$. Our first-principles
and Monte Carlo simulations reveal that the anisotropies of the short-range
symmetric exchange, when combined with magnetic frustration, can lead to an
emergent chiral interaction that is responsible for the predicted topological
spin structures. The proposed mechanism finds a prototypical manifestation in
two-dimensional magnets, thus broadening the class of materials that can host
spontaneous skyrmionic states.
