  The recent discovery of two-dimensional (2D) magnetic materials has brought
magnetism to the flatland. This has opened up exciting opportunities for the
exploration of fundamental physics as well as for novel device applications.
Here, we predict a new thermodynamically stable 2D magnetic material,
K$_2$CoS$_2$, which retains its in-plane anti-ferromagnetic order down to the
monolayer and bilayer limits. We find that the magnetic moments ($2.5
\mu_B/$Co) are aligned along the intra-Co chains, from monolayer to bulk. The
non-magnetic electronic spectrum of both the monolayer and bilayer films is
found to host flat bands and van-Hove singularities, which in instrumental in
giving rise to the the magnetic ground state. Based on classical Monte-Carlo
simulations, we estimate the Neel temperature for the antiferromagnetic
monolayer to be $\approx 15$K. Our study thus establishes that K$_2$CoS$_2$
hosts a robust antiferromagnetic state which persists from the monolayer limit
to the bulk material.
