Tunable anomalous valley Hall effect and magnetic phase transition in MHfN2Cl2 (M = V, Cr) bimetallic nitrogen halide monolayers.

Transition metal nitrogen halides (TMNHs) are environmentally friendly and widely studied van der Waals two-dimension (2D) materials. However, there are still few 2D TMNHs with magnetic properties that have been synthesized, which greatly limits the further applications of them in spintronics and valleytronics. Here, we substitute Hf atoms by magnetic elements V and Cr in β-type TMNHs Hf₂N₂Cl₂, which evolved into ferromagnetic half-metals and ferrovalley semiconductors MHfN₂Cl₂ (M = V, Cr). Meanwhile, the valley splitting (VS) properties and magnetic properties can be effectively regulated by modulating the strong correlation effect of transition metals and applying biaxial strain. VS in VHfN₂Cl₂ increases linearly with the increasing strain and the U value; however, VS is robust to the strain and decreases linearly with the increasing U value in CrHfN₂Cl₂, and the maximum VS reaches to 175 and 62 meV for the two monolayers. CrHfN₂Cl₂ remains robustly ferromagnetic under different strain and U values, while there is magnetic phase transition in VHfN₂Cl₂ from ferromagnetic to antiferromagnetic, accompanied by a half-metal-to-semiconductor transition. The Curie temperatures of VHfN₂Cl₂ and CrHfN₂Cl₂ can be increased by a maximum of 2.5 and 2.36 times under modulation. This opens different ideas for the study of bimetallic TMNHs in spintronics and valleytronics. [ABSTRACT FROM AUTHOR]