  Nodal line semimetals in two-dimensional (2-D) materials have attracted
intense attention currently. From fundamental physics and spintronic
applications points of view, high Curie temperature ferromagnetic (FM) ones
with nodal lines robust against spin-orbit coupling (SOC) are significantly in
desirable. Here, we propose that FM K2N monolayer is such Weyl nodal line
semimetal. We show that K2N monolayer is dynamically stable, and has a FM
ground magnetic state with the out-of-plane [001] magnetization. It shows two
nodal lines in the low-energy band structures. Both nodal lines are robust
against SOC, under the protection of mirror symmetry. We construct an effective
Hamiltonian, which can well characterize the nodal lines in the system.
Remarkably, the nodal line semimetal proposed here is distinct from the
previously studied ones in that K2N monolayer is 2-D d0-type ferromagnet with
the magnetism arising from the partially filled N-p orbitals, which can bring
special advantages in spintronic applications. Besides, the Curie temperature
in K2N monolayer is estimated to be 942K, being significantly higher than
previous FM nodal lines materials. We also find that, specific tensile strains
can transform the nodal line from type-I to a type-II one, making its nodal
line characteristics even more interesting.
