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 beta-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.