Strain-tunable bipolarized Weyl semimetal and valley Hall effect in two-dimensional d-wave altermagnetic Fe₂I₂

The discovery of altermagnetism, a novel magnetic order characterized by real-space antiferromagnetic alignment and reciprocal-space anisotropic spin polarization, has opened new avenues in spintronics and topological quantum materials. In this paper, we employ first-principles calculations to investigate the electronic structure, magnetic properties, and topological behavior of monolayer Fe₂I₂. We predict that this material hosts a stable d-wave altermagnetic ground state with an out-of-plane magnetic orientation. Our analysis reveals the presence of Weyl points with opposite Chern numbers near the Fermi level, establishing monolayer Fe₂I₂ as a bipolarized Weyl semimetal. Upon incorporating spin-orbit coupling (SOC), the crystal symmetry is reduced, leading to the opening of a finite bandgap (∼58.9 meV) at the Weyl points and inducing a quantum crystal valley Hall effect. Furthermore, we demonstrate that uniaxial strain along the a-axis serves as an effective knob for tuning the topological states: tensile strain induces a migration of Weyl points from the Γ-X to the X-M direction, while compressive strain significantly modulates the valley bandgap. With a predicted high Nel temperature of 630 K, monolayer Fe₂I₂ emerges as a promising candidate for ultrafast, low-power topological spintronic devices.