Spin and Light in 2D Magnets

Author: Denis Avetisyan

A new frontier in materials science combines magnetism and light to unlock novel optoelectronic and quantum phenomena.

This review explores the strong coupling of excitons, magnons, and light in van der Waals magnetic materials and its potential for advanced technologies.

Conventional semiconductors struggle to simultaneously host strong magnetic order and robust excitonic effects, limiting control over spin-light interactions. This Review surveys the rapidly developing field of ‘Excitons in van der Waals magnetic materials’, where the interplay between excitons, magnons, and light offers unprecedented opportunities. Strong coupling within these two-dimensional heterostructures reveals pronounced magneto-optical responses and pathways for manipulating spin via optical excitation. Could harnessing these coupled dynamics pave the way for next-generation optoelectronic devices and quantum technologies leveraging coherent spin-light interactions?

The Inevitable Bottleneck: Light and the Limits of Matter

Many established materials struggle to efficiently couple with light, a critical bottleneck for realizing next-generation quantum technologies. This weak interaction limits the ability to control and manipulate quantum states using photonic signals, hindering progress in areas like quantum computing and communication. The fundamental issue stems from the material’s electronic structure and how it responds to electromagnetic radiation; often, the energy transfer is inefficient or the resulting quantum effects are too weak to be harnessed. Consequently, researchers are actively seeking materials with enhanced light-matter coupling, where photons can more readily create and control excitations within the material – a crucial step towards building robust and scalable quantum devices.

Conventional materials often struggle to efficiently interact with light, hindering progress in fields like quantum computing and advanced optics. However, two-dimensional materials, especially those belonging to the Van der Waals family, present a compelling alternative. These materials, often just a single atom thick, exhibit extraordinary electronic and optical properties stemming from quantum confinement and unique interlayer interactions. The reduced dimensionality enhances light-matter coupling, allowing for stronger and more controllable interactions. This is because electrons in these materials are highly confined, leading to enhanced excitonic effects-bound electron-hole pairs that strongly respond to light. Furthermore, the weak Van der Waals forces between layers allow for the creation of heterostructures with tailored properties, opening possibilities for designing materials with specific optical and electronic functionalities previously unattainable.

Van der Waals materials are emerging as platforms to revolutionize spin and light control due to their robust excitonic behavior and potential for magnetic ordering. Unlike conventional semiconductors where electron-hole pairs rapidly dissociate, these layered materials exhibit strongly bound excitons – quasiparticles where the electron and hole remain tightly connected. This strong binding enhances light-matter interaction, allowing for efficient manipulation of optical properties. Furthermore, the ability to engineer magnetic order within these materials-through layer stacking or chemical modification-opens doors to controlling spin states with unprecedented precision. This synergistic combination of strong exciton binding and magnetic order promises a new paradigm for developing advanced optoelectronic devices and exploring fundamental phenomena at the intersection of light, magnetism, and quantum mechanics, potentially leading to breakthroughs in areas like quantum computing and low-energy spintronics.

A Dance of Quasiparticles: Excitons, Magnons, and the Promise of Coupling

Excitons are bound electron-hole pairs, formed when an electron is excited to a higher energy level, leaving behind a positively charged hole; these quasiparticles behave as neutral entities with a finite lifetime and contribute to optical properties of materials. Magnons, conversely, represent quantized spin waves arising from collective excitations of magnetic moments within a material; they are spin-carrying quasiparticles with no net charge and are crucial to understanding magnetic phenomena. The key distinction lies in their origin and properties: excitons are related to electron-hole interactions and optical transitions, while magnons are collective spin excitations influencing magnetic behavior. Both excitons and magnons possess a finite lifetime, dictated by various decay mechanisms specific to the material and excitation conditions, and their energies are typically within the $meV$ to $eV$ range.

Van der Waals (vdW) materials, characterized by weak interlayer interactions, facilitate strong coupling between excitons and magnons due to the reduced screening and enhanced spatial overlap of their wavefunctions. Unlike traditional bulk materials where dielectric screening hinders such interactions, the 2D nature of vdW materials minimizes this effect, allowing for larger coupling strengths. Specifically, the confinement of both excitons and magnons within the 2D plane promotes efficient energy exchange, leading to the formation of hybrid exciton-magnon polaritons. This strong coupling is further enhanced by the ability to heterostructure different vdW materials, tailoring the electronic and magnetic properties to optimize the exciton-magnon interaction and achieve control over both optical and spin-related phenomena.

Exciton-magnon coupling facilitates the coherent control of both optical and spin-related properties within a material. Specifically, manipulating the exciton – a bound electron-hole pair exhibiting optical absorption – influences the magnon, a quantized spin wave governing magnetic behavior, and vice-versa. This interaction enables the potential development of devices where light can control magnetism, and magnetism can influence optical responses. Potential functionalities include optically controlled magnetic storage, spin-based optoelectronics, and novel sensor technologies leveraging the interplay between photonic and magnetic signals. The ability to manipulate these degrees of freedom independently or in concert offers pathways to devices beyond the scope of conventional semiconductor or magnetic technologies.

Material Manifestations: Where Theory Meets Two Dimensions

Chromium triiodide (CrI3), nickel phosphosulfide (NiPS3), and chromium sulfur bromide (CrSBr) are representative examples of two-dimensional Van der Waals magnetic materials that exhibit strong exciton-magnon coupling. This coupling arises from the interaction between photoexcited electron-hole pairs (excitons) and collective magnetic excitations (magnons) within the layered structures of these materials. The Van der Waals bonding allows for the creation of atomically thin crystals, enhancing this interaction. These materials are characterized by their magnetic ordering and optical properties influenced by the reduced dimensionality, leading to observable effects like modifications of exciton energies and linewidths, and the potential for manipulating magnetic states with light.

The layered structures of Van der Waals materials, coupled with their intrinsic magnetic order, directly influence their optical anisotropy and exciton characteristics. The reduced dimensionality and weak interlayer coupling result in highly anisotropic dielectric functions, leading to polarization-dependent optical absorption and reflection. Furthermore, the magnetic order-ferromagnetic, antiferromagnetic, or otherwise-introduces spin-dependent selection rules for optical transitions, modifying exciton energies and lifetimes. This interplay between structural anisotropy and magnetic order manifests in unique exciton behavior, including modifications to exciton binding energies, the emergence of dark excitons, and the potential for spin-valley locking, all of which deviate from the behavior observed in conventional bulk semiconductors.

Chromium sulfide bromide (CrSBr) is a layered Van der Waals material exhibiting strong light-matter interaction characterized by Rabi splitting energies that surpass 200 meV. This substantial Rabi splitting indicates a significant coupling strength between excitons and optical modes within the material. The magnitude of this coupling is directly related to the efficiency of energy transfer between light and matter, and values exceeding 200 meV represent a considerable enhancement compared to many other two-dimensional materials. This strong coupling regime facilitates enhanced control over excitonic states and opens possibilities for developing novel optoelectronic devices and exploring fundamental quantum phenomena.

Nickel phosphorus trisulfide (NiPS3) is a layered Van der Waals material characterized by exceptionally narrow exciton linewidths, measured as low as 0.35 meV. This reduced linewidth indicates a prolonged exciton coherence time, a critical factor for applications requiring coherent control of excitonic states. Longer coherence times minimize decoherence effects, enabling more precise and efficient manipulation of excitons for potential use in advanced optoelectronic devices and quantum technologies. The narrow linewidth also suggests a high degree of crystalline perfection and reduced scattering within the material, contributing to the preservation of exciton coherence.

Probing the Interaction: Mapping the Landscape of Light and Spin

Characterizing the magnetic behavior of these novel materials relies heavily on magneto-optical techniques, notably the Magneto-Optical Kerr Effect (MOKE), Circular Dichroism (CD), and Linear Dichroism (LD). MOKE sensitively detects changes in magnetization at a surface by measuring alterations in polarized light reflection, providing insights into magnetic ordering and domain structures. Complementing this, both CD and LD analyze the differential absorption of left- and right-circularly polarized light, or light polarized along different linear axes, respectively, revealing information about the electronic and magnetic transitions within the material. These spectroscopic methods aren’t merely diagnostic tools; they allow researchers to map magnetic landscapes with high spatial resolution and understand how light interacts with the magnetic order, which is fundamental for tailoring materials with specific magneto-optical properties.

Spectroscopic techniques, including Magneto-Optical Kerr Effect, Circular Dichroism, and Linear Dichroism, provide a powerful means of investigating the interplay between excitons and magnons within these materials. Researchers utilize these methods to discern how the coupling between these quasiparticles alters the established magnetic order-the arrangement of magnetic moments-and simultaneously affects the material’s response to light. By analyzing changes in optical spectra under varying magnetic fields, scientists can map the energy levels and interactions governing exciton-magnon coupling, revealing how this interaction modifies both the magnetic properties and the way the material absorbs and emits light. This detailed understanding is crucial not only for fundamental materials science, but also for tailoring these materials for advanced applications where control over both optical and magnetic properties is paramount.

The materials under investigation exhibit significantly amplified exciton oscillator strengths, a characteristic that directly translates into remarkably strong magneto-optical effects. This enhancement arises from the unique interplay between light and matter within these compounds, allowing for substantial modification of optical properties through the application of magnetic fields. Consequently, these materials are prime candidates for the development of advanced opto-spintronic devices – technologies that merge optics and spintronics to manipulate information using both light and electron spin. The strength of the magneto-optical response enables efficient control of spin polarization with light, paving the way for faster, more energy-efficient data storage, processing, and communication technologies, as well as novel sensor designs.

The precise manipulation of exciton-magnon interactions opens avenues for groundbreaking technologies, extending far beyond conventional spintronics. Researchers envision novel devices where information is encoded and processed not just with electron spin, but with the collective excitations of these coupled quantum phenomena. This coherent control allows for the potential creation of ultra-fast, low-energy consumption spintronic components, and even lays the groundwork for advancements in quantum information processing. By harnessing the long-range coherence of excitons and magnons, it may become possible to build robust quantum bits – qubits – and develop entirely new paradigms for quantum computing and communication, potentially exceeding the limitations of current silicon-based technologies.

Towards Quantum Horizons: Transduction and the Future of Control

Quantum transduction represents a pivotal advancement in the quest to build practical quantum technologies, specifically by facilitating the conversion of quantum information between distinct physical systems – excitons and magnons. Excitons, bound states of an electron and a hole in a material, excel at processing information via light, while magnons are quantized spin waves ideally suited for storing information within magnetic materials. The ability to efficiently translate quantum states between these quasiparticles unlocks possibilities for novel quantum sensors, capable of unprecedented sensitivity, and secure communication devices leveraging the unique properties of both light and magnetism. This interconversion isn’t merely a transfer of data; it’s a bridging of fundamentally different quantum realms, potentially allowing for the creation of hybrid quantum systems that combine the strengths of each component and overcome individual limitations in coherence or scalability.

The robust interaction between excitons and magnons-quasiparticles representing electron-hole pairs and magnetic vibrations, respectively-creates a fertile ground for generating polaritons. These hybrid quasiparticles, born from the strong coupling of light and matter excitations, exhibit unique properties distinct from their progenitors. Unlike traditional quasiparticles, polaritons can possess a substantial light component, allowing for faster propagation and enhanced nonlinearities. This behavior opens possibilities for creating novel quantum states with tailored characteristics, potentially leading to breakthroughs in areas like low-threshold lasers, enhanced optical devices, and ultimately, more efficient quantum information processing. The ability to engineer these polariton states through precise control of material properties and external stimuli represents a significant step toward realizing advanced quantum technologies.

Continued advancements in quantum technologies hinge on meticulous refinement of the materials used to host and manipulate quantum states. Current research prioritizes tailoring material properties – such as crystal structure and composition – to maximize the interaction, or coupling strength, between light and matter at the quantum level. Beyond material science, a significant focus lies in engineering scalable device architectures; moving beyond proof-of-concept demonstrations requires fabricating complex systems with many interacting quantum units while maintaining coherence and control. These efforts aren’t simply about incremental improvements, but represent a fundamental shift towards building robust, practical quantum devices capable of revolutionizing sensing, communication, and computation.

The pursuit of strong coupling in van der Waals materials feels less like construction and more like tending a garden. This review of exciton-magnon interactions demonstrates that predicting system behavior with absolute certainty is a fool’s errand. Each layered material, each carefully induced interaction, is a hypothesis exposed to the unpredictable climate of quantum mechanics. As John Stuart Mill observed, “It is better to be a dissatisfied Socrates than a satisfied fool.” The drive to control spin and light isn’t about building a device; it’s about cultivating conditions where novel phenomena – like exciton-polaritons – can emerge, even if the precise outcome remains delightfully uncertain. The architecture inevitably propagates its own limitations, and only through embracing that uncertainty can true innovation blossom.

What Lies Beyond?

The pursuit of exciton-magnon coupling in van der Waals heterostructures reveals, as all such pursuits do, not a destination but a shifting border. Each layered material, carefully assembled to coax light and spin into conversation, is a temporary truce in the inevitable thermodynamic war. The very order achieved-the strong coupling, the polariton formation-defines the precise nature of the future failure. One imagines a cascade of defects, a creeping disorder that transforms a coherent hybrid state into a dissonant chorus of uncoupled excitations.

The field now faces a choice. It can chase ever more elaborate designs-perfectly matched interfaces, novel materials combinations-believing that control is attainable through complexity. Or, it can embrace the inherent fragility of these systems, focusing instead on understanding the modes of failure. The latter path, though less glamorous, acknowledges a fundamental truth: systems are not built, they are grown, and growth always carries the seed of its own dissolution.

Ultimately, the promise of advanced optoelectronics and quantum technologies rests not on achieving perfect control, but on learning to navigate the controlled chaos. The true innovation may not be a new device, but a new tolerance for imperfection-a recognition that order is merely a transient state, a fleeting arrangement before the universe reclaims its preferred state of gentle disorder.

Original article: https://arxiv.org/pdf/2602.10409.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/