Theoretical investigations of two-dimensional intrinsic magnets derived from transition-metal borides M₃B₄ (M = Cr, Mn, and Fe)

Two-dimensional (2D) magnetic materials with high critical temperatures (T_C) and robust magnetic anisotropy energies (MAE) hold significant potential for spintronic applications. However, most of 2D magnetic materials are derived from the van der Waals (vdW) layered bulks, which greatly limits the synthesis of 2D magnetic materials. Here, 2D M₃B₄ (M = Cr, Mn, and Fe; B = Boron), derived from hexagonal and orthorhombic M₃AlB₄ phases by selectively etching Al layers, was studied for its structural stability, electronic structure, and magnetic properties. By utilizing ab initio calculations and Monte Carlo simulations, we found that the orthorhombic Cr₃B₄ shows ferromagnetic (FM) metal and possesses an in-plane magnetic easy axis, while the remaining hexagonal and orthorhombic M₃B₄ structures exhibit antiferromagnetic (AFM) metals with a magnetic easy axis which is perpendicular to the two-dimensional plane. The critical temperatures of these 2D M₃B₄ structures are found to be above the 130 K. Notably, the ort-Mn₃B₄ possesses highest T_C (~600 K) and strongest MAE (~220 µeV/atom) among these borides-based 2D magnetic materials. Our findings reveal that the 2D M₃B₄ compounds exhibit much better resistance to deformation compared to M₂B₂ MBenes and other 2D magnetic materials. The combination of high critical temperature, robust MAE, and excellent mechanical properties makes 2D Mn₃B₄ monolayer exhibits a favorable potential for spintronic applications. Our research also sheds light on the magnetic coupling mechanism of 2D M₃B₄, providing valuable insights into its fundamental characteristics.