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 (T_c) 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 T_c 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.