Two-Dimensional Altermagnetic Iron Oxyhalides: Real Chern topology and Valley-Spin-Lattice coupling

TL;DR

The study identifies monolayer Fe2X2O as a new class of 2D altermagnetic real Chern insulators with semiconducting gaps and spin-polarized corner modes, arising from equal nontrivial real Chern numbers in both spin channels. Through first-principles calculations, it reveals a strong coupling among altermagnetism, valleys, spins, and lattice, enabling valley-selective optical excitation and strain-tunable valley polarization. It also demonstrates strain- and multiferroic-driven control of Néel vectors in Fe2Cl2O, linking ferroelasticity to magnetic topology. Collectively, these findings establish a versatile platform for spintronics and valleytronics based on magnetic topological states in 2D materials.

Abstract

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$_2X_2$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 switche the Néel vector. These findings position 2D iron oxyhalides as a promising platform for exploring altermagnetism and magnetic topological states for spintronics and valleytronics.

Figures (8)

• Figure 1: (a) Side view and (b) top view of the crystal structure of monolayer Fe$_2X_2$O ($X$ = Cl, Br, I). (c) Brillouin zone, with high-symmetry points indicated. (d) Calculated phonon spectra of Fe$_2$Br$_2$O. (e) Result of ab-initio molecular dynamics simulations for Fe$_2$Br$_2$O.
• Figure 2: Panels (a)–(c) illustrate the FM and two AFM configurations of the Fe$_2X_2$O ($X$ = Cl, Br, I) monolayer. Panels (d) show the normalized magnetic moments of monolayer Fe$_2$Br$_2$O as functions of temperature, obtained from Monte Carlo simulations.
• Figure 3: Panels (a)–(c) show the band structures and projected density of states (PDOS) of monolayer Fe$_2$Cl$_2$O, Fe$_2$Br$_2$O, and Fe$_2$I$_2$O, respectively. SOC is neglected in the calculation. Red and blue colors denote spin-up and spin-down states, respectively. Panel (d) illustrates the two conduction band valleys of these materials. Panel (e) depicts the four valence band valleys of monolayer Fe$_2$Br$_2$O. Red and blue denote spin-up and spin-down polarizations, respectively.
• Figure 4: (a) The nanodisk geometry used to calculate the corner states. (b) Energy spectrum of the Fe$_2$Br$_2$O nanodisk, showing a group of four isolated corner modes within the bulk gap. Insets show the charge density distributions of the two groups of corner modes. (c) and (d) show the energy spectra for the spin-up and spin-down channels, separately. (e) and (f) demonstrate that the spin polarization of the corner modes is tied to the AFM Néel vector. The spin polarization is reversed upon flipping the Néel vector.
• Figure 5: (a,b) Calculated linear dichroism $\eta(\bm{k})$ near the (a) $X$ and (b) $Y$ points of monolayer Fe$_2$Br$_2$O. The lines indicate the equi-energy contours of the local band gap. (c) Schematic illustration of the optical transition selection rules for the two valleys. (d) Band structures around the X point of monolayer Fe$_2$Br$_2$O and Fe$_2$I$_2$O, where the irreducible representations of the top two valence bands and the lowest conduction band are indicated.