Bipolar ferrimagnetic semiconductor and doping concentration induced carrier spin flip in monolayer NiMnBr6

Two-dimensional magnetic materials with tunable electronic properties have great potential application in spintronic devices. Here, based on first-principles calculations, we systematically study the electronic structures and magnetic properties of monolayer NiMnBr6. The magnetic ground state of monolayer NiMnBr6 is Néel ferrimagnetic (FIM-Néel) with a critical temperature (Tc) of 45 K. The magnetic properties of monolayer NiMnBr6 can be tuned effectively by strain. The magnetic phase transition from the FIM-Néel state to the ferromagnetic (FM) state can be triggered by applying a compressive strain greater than 4.5%. The Tc of the FIM-Néel state and FM state can be increased to 67 K and 95 K by applying 8.0% tensile and compressive strain, respectively. Monolayer NiMnBr6 in both the FIM-Néel state and FM state has large magnetic anisotropy energy. Remarkably, the monolayer NiMnBr6 in the FIM-Néel state acts as a bipolar ferrimagnetic semiconductor (BFIMS), while the compressive strained monolayer NiMnBr6 in the FM state acts as a half FM semiconductor (HFMS). The magnetic configuration of monolayer NiMnBr6 can also be tuned by carrier doping. Interestingly, for monolayer NiMnBr6 with the HFMS phase, the magnetic phase transition from the FM state to FIM-Néel state can be achieved with the increase of the hole doping concentration, which leads to the achievement of a doping concentration induced carrier spin flip. Our results show that monolayer NiMnBr6 is a promising candidate for exploring two-dimensional magnetism and spintronic devices.