  Recent high-throughput computational searches have predicted many novel
ternary nitride compounds providing new opportunities for materials discovery
in under explored phase spaces. Nevertheless, there are hardly any predictions
and/or syntheses that incorporate only transition metals into new ternary
nitrides. Here, we report on the synthesis, structure, and properties of
MnCoN$_2$, a new ternary nitride material comprising only transition metals and
N. We find that crystalline MnCoN$_2$ can be stabilized over its competing
binaries, and over a tendency of this system to become amorphous, by
controlling growth temperature within a narrow window slightly above ambient
condition. We find that single-phase MnCoN$_2$ thin films form in a
cation-disordered rocksalt crystal structure, which is supported by ab-initio
calculations. X-ray photoelectron spectroscopy analysis suggests that MnCoN$_2$
is sensitive to oxygen through various oxides and hydroxides binding to cobalt
on the surface. X-ray absorption spectroscopy is used to verify that Mn$^{3+}$
and Co$^{3+}$ cations exist in an octahedrally-coordinated environment, which
is distinct from a combination of CoN and MnN binaries and in agreement with
the rocksalt-based crystal structure prediction. Magnetic measurements suggest
that MnCoN$_2$ has a canted antiferromagnetic ground state below 10 K. We
extract a Weiss temperature of $\theta$ = -49.7 K, highlighting the
antiferromagnetic correlations in MnCoN$_2$.
