Two-Dimensional Altermagnetic Iron Oxyhalides: Real Chern Topology and Valley–Spin–Lattice Coupling

Altermagnets, a novel class of collinear magnetic materials, exhibit unique spin-split band structures, yet topological insulating states in intrinsic altermagnetic systems are rare. Here, we identify monolayer Fe 2 X 2 O (X = Cl, Br, I) as a new family of 2D altermagnetic real Chern insulators. These materials display robust d -wave altermagnetic ordering, semiconducting band gaps, and nontrivial real Chern numbers per spin channel, yielding spin-polarized topological corner modes. They also feature spin-polarized valleys with strong altermagnetism–valley–spin–lattice coupling, enabling valley-selective excitation via linear dichroism and strain-induced valley polarization. In multiferroic Fe 2 Cl 2 O, magnetism coexists with ferroelasticity, and an applied strain can switch the Néel vector. These findings position 2D iron oxyhalides as a promising platform for exploring the altermagnetism and magnetic topological states for spintronics and valleytronics.