Two-dimensional (2D) iron trihalides (FeX₃, X=F, Cl, Br, I) are an emerging family of van der Waals magnets whose fundamental physical properties are not yet fully understood. In this work, we present a systematic first-principles study incorporating hybrid functional (HSE06) calculations and Hubbard U corrections to unravel the spin state, magnetic order, electronic structure, and doping response in monolayer FeX₃. The high-spin (S=5/2) state is unequivocally established as the universal local ground state across the series. A chemical tuned magnetic transition is identified: FeF₃ adopts N\'eel-type antiferromagnetic (AFM) order, while FeCl₃, FeBr₃, and FeI₃ are ferromagnetic (FM) semiconductors with Curie temperatures (T₂) monotonically increasing from 154 K to 238 K. This trend is driven by the competition between direct AFM exchange and halogen-mediated FM superexchange. Electronically, FeCl₃ and FeBr₃ are identified as bipolar magnetic semiconductors, exhibiting a perfect linear scaling of the band gap with halogen electronegativity. An effective tight-binding model derived from maximally Wannier functions reveals a progressive increase in crystal-field splitting (₎₂ₓ) from 3. 36 eV to 4. 22 eV, underpinning the evolving orbital hierarchy across iron trihalides. Crucially, electron doping induces a nonmonotonic magnetic evolution, from FM to geometrically frustrated AFM (zigzag/stripy), culminating in a reentrant FM state, driven by the competition between kinetic energy minimization and orbital-selective electron correlations. Finally, bilayer systems exhibit a universally robust interlayer AFM coupling driven by pₙ orbital-mediated superexchange across the van der Waals gap. Our work provides a complete microscopic picture of the basic magnetic and electronic properties of the 2D FeX₃, establishing them as a highly versatile material platform for tunable magnetism and spintronics.
Lei et al. (2026) studied this question.