The antiferromagnetic phase of a wurtzite nickel sulfide monolayer

Two-dimensional intrinsic long-range magnetic monolayers with high transition temperatures have attracted great interest in both fundamental studies and practical applications. In this study, we use a combination of first-principles calculations based on density functional theory (DFT), and unitary transformation of the effective Heisenberg model to investigate the electronic structure and magnetic properties of a [NiS]₂ monolayer. The phonon calculations reveal that the [NiS]₂ monolayer is dynamically stable in the wurtzite phase. This material is an out-of-plane easy-axis antiferromagnetically ordered monolayer with the Néel temperature close to room temperature. The intrinsic AFM ground state arises from the presence of top and bottom FM sublattices coupled together via AFM coupling, in which the net magnetic moment of each Ni atom is evaluated as 0.5μ_B. The spectrum of the spin-wave of [NiS]₂ is investigated within the spin-wave theory of antiferromagnets in terms of the first-order Holstein-Primakoff approximation of the anisotropic Heisenberg model combined with the Bogoliubov diagonalization transformation. For the long wavelength limit, the magnon dispersion shows linear behavior with the wave vector, which is expected for conventional antiferromagnetism. The magnon velocity of approximately ∼600 m s^-1 is predicted for the [NiS]₂ monolayer by calculating the slope of the magnon spectrum. Due to strong spin-orbit coupling, the [NiS]₂ monolayer has relatively large magnetic anisotropy energy, causing the existence of the 12 meV gap at the Γ point in the magnon spectrum. The magnon energy gap limits the number of thermally excited states, which is essential for maintaining intrinsic long-range antiferromagnetic order in two dimensions.