-Density functional theory (DFT) calculations are used to investigate the electronic and magnetic structures of a two-dimensional (2D) monolayer Li2N. It is shown that bulk Li3N is a non-magnetic semiconductor. The non-spin-polarized DFT calculations show that p electrons of N in 2D Li2N form a narrow band at the Fermi energy EF due to a low coordination number, and the density of states at the Fermi energy (g(EF)) is increased as compared with bulk Li3N. The large g(EF) shows instability towards magnetism in Stoner's mean-field model. The spinpolarized calculations reveal that 2D Li2N is magnetic without intrinsic or impurity defects. The magnetic moment of 1.0 μB in 2D Li2N is mainly contributed by the pz electrons of N, and the band structure shows half-metallic behavior. Dynamic instability in planar Li2N monolayer is observed, but a buckled Li2N monolayer is found to be dynamically stable. The ferromagnetic (FM) and antiferromagnetic (AFM) coupling between the N atoms is also investigated to access the exchange field strength. We found that planar (buckled) 2D Li2N is a ferromagnetic material with Curie temperature Tc of 161 (572) K. editor's choice Copyright c EPLA, 2017Introduction. -Currently, huge research efforts are undertaken to explore new 2D materials for a wide range of applications [1][2][3][4][5]. It is well known from semiconductor physics that reduced dimensionality not only affects the electronic structure [6] and the electronic density of states (DOS) near the Fermi energy, but also the electronic dispersion relations [7]. This effect of the dimensionality is particularly strong in 2D materials which, by definition, consist of a sheet only few atomic layers in thickness. Due to quantum confinement, the electronic bands in the remaining two in-plane directions can display metallic, insulating, or semiconducting behavior even at variance with the behavior in the 3D bulk compound from which they were derived [8][9][10][11][12][13][14][15]. In some cases, 2D materials have very unique electronic properties, i.e., Dirac cones in graphene, silicene, etc. [8,16]. There are also 2D magnetic materials, e.g., transition metal nitrides, where magnetism is due to d electrons [17,18]. Recently, magnetism has been predicted [19] and confirmed experimentally in a 2D compound of chromium [20]. In contrast, our present theoretical work predicts the possibility of 2D magnetism even in the absence of d electrons.

Ab-initio calculations based on density functional theory with local spin density approximation are used to study defects-driven magnetism in bulk α-Li3N. Our calculations show that bulk Li3N is a non-magnetic semiconductor. Two types of Li vacancies (Li-I and Li-II) are considered, and Livacancies (either Li-I or Li-II type) can induce magnetism in Li3N with a total magnetic moment of 1.0 µB which arises mainly due to partially occupied N-p-orbitals around the Li vacancies. The defect formation energies dictate that Li-II vacancy, which is in the Li2N plane, is thermodynamically more stable as compared with Li-I vacancy. The electronic structures of Li-vacancies show half-metallic behavior. On the other hand N-vacancy does not induce magnetism and has a larger formation energy than Li-vacancies. N vacancy derived bands at the Fermi energy are mainly contributed by the Li atoms. Carbon is also doped at Li-I and Li-II sites, and it is expected that doping C at Li-I site is thermodynamically more stable as compared with Li-II site. Carbon can induce metallicity with zero magnetic moment when doped at Li-I site, whereas magnetism is observed when Li-II site is occupied by the C impurity atom and C-driven magnetism is spread over the N atoms as well. Carbon can also induce half-metallic magnetism when doped at N site in Li3N, and has a smaller defect formation energy as compared with Li-II site doping. The ferromagnetic (FM) and antiferromagnetic (AFM) coupling between the C atoms is also investigated, and we conclude that FM state is more stable than the AFM state.