Excitons in van der Waals magnetic materials

TL;DR

This review surveys how excitons in two-dimensional van der Waals magnets cohabit with and are controlled by magnetic order. It outlines exciton types (Frenkel, Wannier–Mott, and charge-transfer) and the exchange and spin–orbit mechanisms that couple excitons to magnons and magnetic textures, highlighting strong magneto-optical effects and tunable selection rules. The authors catalog representative materials (e.g., CrI3, NiPS3, CrSBr) and discuss phenomena from exciton–magnon coupling and polaritons to moiré-engineered states and optical control of spin textures, culminating in potential magneto-photonic devices and quantum transduction applications. The work emphasizes both fundamental physics and practical opportunities for opto-spintronics and quantum technologies based on coupled light, charge, and spin in 2D magnets.

Abstract

Two-dimensional magnetic semiconductors provide a unique materials platform in which long-range magnetic order coexists with strongly bound excitons. Because excitonic states and magnetic moments originate from the same electronic orbitals and are coupled through intrinsic exchange interactions, optical excitations in these systems exhibit pronounced sensitivity to magnetic order. Recent experiments have revealed unusually strong magneto-optical responses, as well as direct coupling between excitons and magnons, establishing new routes for controlling light-matter interactions with spin degrees of freedom. This Review surveys key developments in the field, focusing on representative material systems, experimental signatures of exciton-magnetism coupling, and the theoretical frameworks used to describe these phenomena. We conclude with perspectives on how this rapidly evolving field could enable next-generation optoelectronic and quantum technologies leveraging the coupled dynamics of light, charge, and spin.

Figures (6)

• Figure 1: Exciton-magnetism coupling and tunability in 2D van der Waals magnets. Reduced dimensionality and dielectric screening lead to enhanced excitonic binding energies in layered magnetic materials. External control can be established via electrostatic gating, strain engineering, or by creating moiré superlattices in van der Waals heterostructures. This platform provides a robust framework for investigating enhanced exciton–spin coupling, exciton-magnon interactions, and the emergence of strong light-matter coupling in magnetic exciton-polariton systems.
• Figure 2: Crystal structures and magnetic orders of representative 2D magnets. Top (left) and side (right) views of the crystal structures of (a)CrI3, (b)NiPS3, and (c)CrSBr. The red arrows indicate orientations of magnetic moments of the metal ions. In bulk CrI3, Cr^3+ spins are ferromagnetically (FM) aligned along the $c$-axis. In the few-layer limit, adjacent layers often exhibit antiferromagnetic (AFM) coupling, while the intralayer coupling remains ferromagnetic, resulting in layer-dependent magnetism. NiPS3 exhibits an AFM ground state featuring ferromagnetically ordered zigzag chains within the basal plane. In CrSBr, Cr^3+ spins align along the $b$-axis within each layer, with antiparallel alignment between adjacent layers characteristic of an A-type AFM order.
• Figure 3: Nature of hybrid excitons in 2D magnetic semiconductors.(a) Schematic of excitonic transitions in chromium trihalides (CrX3, X = Cl, Br, I), originating from symmetry-forbidden ligand-field $d$--$d$ transitions. (b) Relaxation of selection rules through hybridization between Cr $d$-orbitals and halogen $p$-orbitals. This hybridization scales with the halogen’s atomic number and spin-orbit coupling strength (Cl -> Br -> I), resulting in enhanced excitonic brightness and Wannier-like delocalization. (c) Comparison of excitonic compositions across various 2D magnets, categorized by $d$--$p$, inter-site $d$--$d$, and on-site $d$--$d$ contributions. Unlike standard Wannier excitons in transition metal dichalcogenides (e.g., MoS2), magnetic excitons exhibit significant on-site $d$--$d$ character, though the specific contribution varies across materials. Unlike pure Frenkel excitons, they also possess significant inter-site $d$--$d$ and $d$--$p$ contributions. (d, f) Calculated wavefunctions for (d) 1.37 eV and (f) 1.77 eV excitons of CrSBr, demonstrating a transition from localized Frenkel-like character to delocalized Wannier-like character, respectively. (e) Conceptual illustration of the hybrid exciton continuum, spanning from localized $d$--$d$ transitions toward delocalized band-like states.
• Figure 4: Interplay of excitons with magnetic ground states.(a--c) Magnetic circular dichroism in monolayer CrI3. (a) Polarization-resolved PL spectra ($\sigma^+$, red; $\sigma^-$, blue) at 15 K. (b) Circular polarization degree as a function of out-of-plane magnetic field, tracing a ferromagnetic hysteresis loop. (c) Schematic of the circular-polarization selection rules governed by the ferromagnetic spin arrangement. (d--f) Linear dichroism and Néel vector rotation in NiPS3. (d) Linear polarization-resolved PL showing horizontal (H) and vertical (V) components. Inset shows the temperature dependence of the degree of polarization. (e) Degree of linear polarization versus in-plane magnetic field for two emission peaks ($S_1, S_2$). Dashed lines indicate the stoichiometric zigzag chain directions. (f) Diagram illustrating the rotation of the linear polarization axis; the Néel vector ($\mathbf{L}$), electric dipole ($\mathbf{E}$), and magnetic field ($\mathbf{B}$) are shown. (g--i) Magnetism-coupled excitons in CrSBr. (g) PL spectra of CrSBr as a function of layer number ($N$); shaded regions indicate surface (orange) and bulk (red) excitonic states. (h) Energy redshift from a bilayer sample as an external field along the $c$-axis reduces the magnetic canting angle from the $180^{\circ}$ AFM state. (i) Microscopic mechanism showing spin-dependent electron hopping. Interlayer delocalization is forbidden in the AFM state but becomes allowed as spins align, leading to the observed excitonic redshift. Figure adapted from: ref.Seyler2018, Springer Nature Ltd (a,b); ref.Hwangbo2021, Springer Nature Ltd (d); ref.Wang2021, Springer Nature Ltd (e,f); ref.Shao2025, Springer Nature Ltd (g); and ref.Wilson2021, Springer Nature Ltd (h).
• Figure 5: Manifestations of exciton--magnon coupling in CrSBr.(a) Schematic of all-optical excitonic detection of magnons. An external magnetic field cants the sublattice spins to an equilibrium angle $\theta_0$. A femtosecond pump pulse generates a population of excitons and coherent magnons; the resulting spin precession modulates the exciton localization and transition energy. This time-resolved energy shift is detected via transient reflectivity ($\Delta R/R$). (b) Reflectance spectrum of CrSBr normalized to the SiO2 substrate. (c) Transient reflectance $\Delta R/R$ as a function of probe energy and delay time, after background subtraction. Periodic oscillations correspond to coherent magnon modes at 24 and 34 GHz. (d) Magnon dispersion with applied magnetic field in CrSBr extracted via Fourier transform of the time-domain data. (e, f) Magnon-mediated exciton--exciton interactions. (e) Schematic of the coupling mechanism. (f) Measured nonlinear energy shift of exciton at a fixed fluence as a function of the applied magnetic field; the interaction strength peaks at intermediate magnetic fields and vanishes at zero or saturation fields. (g, h) Magnon-drag effect. (g) Representation of an incoherent magnon flux driving exciton transport. (h) Temperature dependence of the excitonic effective diffusion coefficient, exhibiting a distinct peak at the Néel temperature ($T_N$), characteristic of magnon--exciton drag effect and contrasting with classical exciton diffusion. Figure adapted from: ref.Bae2022, Springer Nature Ltd (b,c); ref.Diederich2023, Springer Nature Ltd (d); ref.Datta2025, Springer Nature Ltd (f); and ref.Dirnberger2025, Springer Nature Ltd (g,h).