  Two-dimensional (2D) magnetic materials have attracted tremendous research
interest because of the promising application in the next-generation
microelectronic devices. Here, by the first-principles calculations, we propose
a two-dimensional ferromagnetic material with high Curie temperature, manganese
tetranitride MnN$_4$ monolayer, which is a square-planar lattice made up of
only one layer of atoms. The structure is demonstrated to be stable by the
phonon spectra and the molecular dynamic simulations, and the stability is
ascribed to the $\pi$-d conjugation between $\pi$ orbital of N=N bond and Mn
$d$ orbital. More interestingly, the MnN$_4$ monolayer displays robust 2D
ferromagnetism, which originates from the strong exchange couplings between Mn
atoms due to the $\pi$-d conjugation. The high critical temperature of 247 K is
determined by solving the Heisenberg model with the Monte Carlo method.
