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Open access peer-reviewed chapter

Novel Light-Matter Interactions in 2D Magnets

Written By

Tingting Yin

Submitted: 01 June 2023 Reviewed: 12 June 2023 Published: 22 September 2023

DOI: 10.5772/intechopen.112163

Modern Permanent Magnets IntechOpen
Modern Permanent Magnets Fundamentals and Applications Edited by Dipti Ranjan Sahu

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Modern Permanent Magnets - Fundamentals and Applications

Edited by Dipti Ranjan Sahu

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Abstract

Since the discovery of intrinsic long-range magnetic order in two-dimensional (2D) layered magnets, e.g., Cr2Gr2Te6 and CrI3 in 2017, it has attracted intensive studies of new physical phenomena in these systems down to a few atomic layers, especially, their magnetism ground states at finite temperatures. Recently, the light-matter interactions in 2D magnets, including light absorption, emission, scattering, et al., have gradually drawn researchers’ attention and are current active research directions. The mechanism of light-matter interactions in 2D magnets challenges the knowledge of materials physics, which drives the rapid development of materials synthesis and device applications. In this chapter, an overview of crystal structures, magnetic properties, and electronic band structures is presented. More importantly, the current status of light-matter interactions in 2D magnets will be discussed, which provides a solid basis for understanding novel physical phenomena in 2D magnets and proves the importance of tuning the magnetic, electronic, and vibrational degrees of freedom for designing novel 2D magnet-based device applications.

Keywords

  • 2D magnets
  • light-matter interaction
  • magnetism
  • light emission
  • light scattering

1. Introduction

Materials with reduced dimensionality have become particularly important, owing to their unexpected physical and chemical properties. Since the discovery of graphene, fabricated via the Scotch tape exfoliation method from graphite in 2014 [1, 2], scientists have made intensive efforts to search two-dimensional (2D) van der Waals (vdW) materials. This family includes metals, e.g., NbSe2, semiconductors, e.g., transition-metal dichalcogenides (TMDs), insulators, e.g., hexagonal boron nitride (hBN), and other elemental 2D vdW materials [3, 4, 5]. In particular, exfoliated ultrathin 2D vdW materials down to atomic thickness (one or several unit layers) display unique and fascinating electronic and optical properties. For example, optical excitation on the monolayer TMDs will generate a strongly bound electron-hole pair called an exciton, instead of fee electrons and holes as in traditional bulk semiconductors [6]. Moreover, their extraordinary mechanical property makes these atomic thin materials with breakthrough of integration technologies accelerate the on-chip device applications [7].

Magnetism in two dimensions has been long pursued by scientists. 2D magnets are at the heart of numerous theoretical [8, 9], experimental [10, 11] and technological advances [11], which are considered as one of the most promising solutions for next-generation spintronic devices in a revolution in semiconductor technology [12, 13, 14]. However, 2D vdW materials with intrinsic magnetism had been missing until quite recently. Based on the theoretical point of view, the transition of magnetism from bulk to 2D, the fluctuations of any kind will increase and can easily destroy long-range magnetic order at any finite temperature [15]. In addition, the superexchange will also be modified with the dimension reduction of the system due to the breaking of the inversion symmetry or the modification of the electronic/lattice structure [16]. Thus, the magnetic properties of 2D vdW materials are far less experimentally investigated for a long time.

With the first discovery of intrinsic magnetic order in the monolayer/few-layer limit in 2D ferromagnets, Cr2Ge2Te6 and CrI3 in 2017 [17, 18], this trend may be poised to change. Subsequently, various 2D vdW magnets have been rapidly discovered and studied [19]. In contrast to the conventional magnetic materials, the magnetic order of 2D magnets can persist down to the monolayer limit, thus possessing a vast reservoir of properties. For example, (i) 2D magnets have strong quantum confinement and mechanical flexibility [20]; (ii) 2D magnets possess good sensitivity to external fluctuations (doping, strain, electric field, et al.) [21]; (iii) 2D magnets can form various artificial heterostructures by a simple stacking process [22]; (iv) 2D magnets show thickness-dependent tuning properties [23]. Thus, 2D magnets have been demonstrated promising building blocks for next-generation information devices. Currently, intense experimental and theoretical investigations are almost devoted to explore the intrinsic magnetism, increase Curie temperatures, and design/fabricate 2D magnet-based devices with advanced functionalities. However, the investigation of light-matter interactions in these 2D magnets is still at the primary stage, there have already been reported unique optical properties in some ferromagnetic (FM) or antiferromagnetic (AFM) ones, exhibiting quite different light emission/scattering as compared with conventional nonmagnetic 2D materials. The mechanism of light-matter interactions in 2D magnets challenges the knowledge of materials physics, which drives the rapid development of materials synthesis and device applications.

In this chapter, the special topic of light-matter interactions in atomically thin 2D magnets is mainly discussed. We begin with a brief review of the recent remarkable progress of achieved 2D vdW magnets, mainly on CrX3 (X = Cl, Br, I), MPS3 (M = Fe, Ni, Mn), CrSBr and CrPS4, with special emphasis on the magnetic properties and electronic band structures by changing the number of layers. Then, we mainly discuss light absorption, emission and scattering behaviors in 2D magnets. Additionally, the mechanism of light-matter interactions in 2D magnets determined by complex interactions between magnetic, electronic and vibrational degrees of freedom will be elaborated in detail, which plays a key role in the state-of-the-art of opto-electronic device applications.

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2. Recently developed 2D magnets

In recent years, a great deal of new 2D magnets have been experimentally discovered and theoretically predicted. Here, we classified these emerging 2D intrinsic magnets into binary transition metal halogenides: CrX3 (X = Cl, Br, I) and ternary transition metal compounds, MPS3 (M = Fe, Ni, Mn), CrSBr and CrPS4. Their crystal structures, magnetic properties, and electronic band structures will be comprehensively described below.

2.1 Crystal structures

CrX3(X = Cl, Br, I). Chromium trihalides are substances in which vdW interfaces between layers X-Cr-X are dominant [28]. Within each layer, the chromium trihalides are isostructural and consist of a honeycomb structure of edge-sharing octahedral, with central chromium coordinated to six monovalent halide anions at the corner, as shown in Figure 1a [24, 29, 30]. Besides, these layers stack in monoclinic phase at a high temperature with space group C2/m: CrCl3 (above 240 K) [31, 32, 33], CrBr3 (above 420 K) [33] and CrI3 (above 220 K) [30]. On the other hand, these layers stack in rhombohedral phase with space group R3¯ at a low temperature, as shown in Figure 1b. Thus, bulk CrX3 crystals are in the low-temperature rhombohedral crystallographic phase at room temperature [25]. MPS3(M = Fe, Ni, Mn). The transition metal phosphorous trisulfides share a defining common structural feature such that (P2S6)4− anion sublattice appears within each layer crystal. The honeycomb arrangement of the transition metal ions is distributed around (P2S6)4− bipyramids, as show in Figure 2a [36, 37, 38, 39]. CrSBr shows a layered crystal structure with vdW interactions, in which each layer is made of two buckled planes of CrS sandwiched between Br sheets and magnetic Cr3+ ions form a rectangular lattice, as shown in Figure 3a [40, 43, 44]. The crystal symmetry remains unchanged between 15 and 300 K [45]. CrPS4 possesses a monoclinic symmetry with C2/m space group, in which puckered layers of S atoms are in hexagonal close packing parallel to the a-axis and the Cr atoms form a square lattice [46, 47]. In each layer, the Cr atom is surrounded by six S atoms, resulting in this distorted CrS6 octahedron. The P atom is coordinated in the center of three CrS6 octahedra. The weak vdW force is between sulfur layers, as shown in Figure 4a [49, 50, 51].

Figure 1.

a, Crystalline structure of CrX3 in top view showing the honeycomb arrangement of the Cr atoms [24] Copyright 2022, American Physical Society. b, Top view of the CrX3 layers in the monoclinic (left) and rhombohedral (right) phases. Two CrX3 layers without the X− ions to show the relative stacking order of the bottom (dark blue) and top (blue) Cr3+ layers in two phases [25] Copyright 2021, American Chemical Society. c, Interlayer exchange of CrX3: interlayer Cr nearest-neighbor (J1⊥, in blue) and the second-neighbor (J2⊥, in red) in AB-stacking (left) and the nearest-neighbor (J’1⊥, in green) in AB’-stacking (right) [26] Copyright 2018, American Chemical Society. d, Illustrations showing the magnetic ground states of bilayer CrI3 (left), CrBr3 (middle), and CrCl3 (right). e, Molecular orbital energy diagram for CrBr3. The d-d LF transitions arise from the ground and excited configurations of the d3 electrons in the t2g* and eg* orbitals. The dashed orange box shows d-d transitions and the blue arrows show possible LMCT transitions. f, Exciton energy levels of monolayer CrBr3 calculated using the first-principles GW-BSE method (left). Top view of excitonic wave function in real space for selected states that are responsible for the exciton bands of CT1, exciton1 (EX1) and exciton2 (EX2) (right) [27] Copyright 2022, American Chemical Society.

Figure 2.

a, Crystal structure of MPS3 viewed along the c and b axis [34] Copyright 2020, American Physical Society. b, The magnetic ground states of bilayer MnPS3, FePS3 and NiPS3. Arrows denote spin orientation. Star denotes an inversion center of the AFM structure [34] Copyright 2020, American Physical Society. c, Calculated electronic band structure of bulk NiPS3 with the red and blue circles corresponding to projected bands from Ni d-orbitals and S atoms. [35] Copyright 2022, AAAS.

Figure 3.

a, Side and top views of the crystal structure of CrSBr [40] Copyright 2022, Wiley-VCH GmbH. b, The magnetic order of CrSBr, FM in-plane but layered in alternating directions for a bulk antiferromagnetism [41] Copyright 2022, Wiley-VCH GmbH. c, Splitting of d orbitals of the Cr atom under the octahedral crystal field of the CrSBr monolayer and the spin-polarized band structure of FM CrSBr [42] Copyright 2018, American Chemical Society.

Figure 4.

a, Crystal structure of CrPS4 with the C2/m space group [46] Copyright 2020, Springer Nature. b, Magnetic structure of CrPS4, the back and red arrows point to the direction of magnetic moments [48] Copyright 2020, WILEY-VCH Verlag GmbH and Co. KGaA. c, Molecular orbital energy diagram in the optical transitions of CrPS4 (top) and configuration diagram of Cr3+ for the d-d transitions (bottom) [46] Copyright 2020, Springer Nature. d, Spin-dependent band structures for bilayer CrPS4 in the AFM and FM states [49] Copyright 2021, American Physical Society.

2.2 Magnetic properties

CrX3(X = Cl, Br, I) exhibit multiple magnetic phases. The superexchange coupling occurs between two adjacent Cr3+ sites and is mediated through the halide ions, which serves as the predominant exchange pathways in the chromium trihalides, as shown in Figure 1c. The interlayer exchange interaction between Cr atoms in different layers. Figure 1d presents the details of the magnetic order of atomically thin CrX3. Within a single layer, the chromium moments of CrX3 are ferromagnetically coupled as intuited by the Goodenough-Kanamori-Anderson rules [29, 52, 53]. In the case of monolayer CrI3 and CrBr3, the magnetic moments aligned perpendicular to the crystal plane [30, 54], while they aligned parallel to the crystal plane for monolayer CrCl3 [33, 55, 56, 57]. In the bulk, two adjacent layers of CrI3 and CrBr3 show FM interlayer exchange [30, 54], and AFM interlayer exchange in CrCl3 [33, 55, 56, 57]. However, for bilayer CrI3, two adjacent layers show AFM interlayer exchange [17]. Moreover, the corresponding magnetic ordering temperatures are 61 K for bulk CrI3 [30], 37 K for bulk CrBr3 [58], and 17 K for bulk CrCl3 [33]. MPS3(M = Fe, Ni, Mn) are a class of vdW stacking AFM insulator, which possess different AFM ordering with the different transition metal due to their different spin dimensionalities. The magnetic ground states of MPS3 are summarized in Figure 2b. All of the compounds are AFM, but only MnPS3 acquires the Néel state where each magnetic site is anti-aligned with all nearest neighbors [59, 60]. In contrast, the ground states of FePS3 and NiPS3 exhibit Zigzag-type ordering, where each transition metal atom is aligned with two nearest neighbors and anti-aligned with one nearest neighbor [61, 62]. Based on the Mermin-Wagner theorem, FePS3 is predicted to have an easy axis, whereas NiPS3 and MnPS3 both have easy planes coinciding with the atomic planes [62]. The Néel temperatures are 118 K for bulk FePS3 [63, 64, 65], 155 K for bulk NiPS3 [66], and 78 K for bulk MnPS3 [61]. CrSBr is an AFM semiconductor with the A-type antiferromagnetism, which is ascribed to the halogen-mediated (Cr-Br-Cr) and chalcogen-mediated (Cr-S-Cr) strong superexchange interactions and weak interlayer coupling [40, 67]. As a result, each rectangular layer exhibits in-plane anisotropic FM order, and these FM layers couple antiferromagnetically along the stacking direction, as shown in Figure 3b [41, 43, 44]. The AFM Néel temperature is 132 K for bulk CrSBr and FM Curie temperature is 180 K [40, 43]. CrPS4 is a promising vdW AFM semiconductor with the A-type antiferromagnetism, consisting of out-of-plane FM monolayers coupled antiferromagnetically, as shown in Figure 4b [48, 68]. The Néel temperature is ~36 K for bulk CrPS4, below which the spin-flip transition from AFM to FM orders occurs [69, 70].

2.3 Electronic band structures

CrX3 (X = Cl, Br, I). In this crystal field geometry, the Cr d splits into a t2g* triplet and an eg* doublet. Cr3+ has a valence of three electrons, which fill the t2g majority-spin band according to Hund’s first rule, leaving all other d bands empty. Since all three CrX3 compounds have FM order down to the monolayer below Curie temperatures, we use CrBr3 as an example to present the molecular orbital energy diagram based on ligand-field (LF) theory, as shown in Figure 1e. In the dashed orange box allows d-d transitions and the blue arrows show possible ligand-to-metal charge transfer (LMCT) transitions [71]. However, an accurate first-principles calculation of the electronic structure of CrX3 should account for both the dielectric polarization from the ligand groups and the on-site Coulomb interactions among the localized spin-polarized electrons in 2D limit [24, 27, 72]. Thus, the recent work has achieved the accurate electronic band structures of CrBr3 monolayer with first-principles GW-BSE calculations including the excitonic effect, as shown in Figure 1f [27]. The calculation yields a series of bright exciton energy levels with energies of 1.68, 2.14, and 2.72 eV, which coincides with the measured absorption spectrum. The real-space exciton wave functions corresponding to LMCT, EX1 and EX2 states are shown in Figure 1f [27]. The electron and hole are localized separately on the anion and cation for LMCT state, which confirms the CT characteristics of higher energy absorptions in CrBr3. While the excitonic wave functions predominantly occupy the Cr atoms for EX1 and EX2 states, confirming the d-d origin. MPS3 (M = Fe, Ni, Mn). Here, we mainly discuss the electronic band structure of NiPS3, since the band-edge excitons have been observed in few-layer NiPS3 with interesting optical phenomena. NiPS3 exhibits a unique electronic structure with a highly localized electronic band composed by d orbitals. A recent work calculated the electronic band structure and density of states for bulk NiPS3 using first-principles density functional theory + U method, as shown in Figure 2c [35, 39, 73, 74]. The electronic bands near the conduction band minimum are predominantly contributed by Ni d orbitals, the small dispersion of these bands is due to the localization of the Ni d orbitals. In contrast, the valence bands are mostly from S p orbitals. The calculation indicates an indirect band gap of ~1.6 eV, consistent with the previous results [75, 76]. However, the most recently published work demonstrates that Zhang-Rice triplet (ZRT) state and Zhang-Rice singlet (ZRS) state are formed from the spin-orbital coupling between Ni p orbital and S p orbitals in APM NiPS3. The transition from ZRT to ZRS states will host a spin-orbit-entangled exciton state [77]. CrSBr combines a direct electronic band gap with layered A-type AFM order. Initially, due to the symmetry breaking under the octahedral crystal field, five degenerate d orbitals of the Cr atom are split into eg levels (dz2 and dxy) with higher energy and t2g levels (dx2-y2, dyz, and dxz) with lower energy, as shown in Figure 2c (left) [42]. In the monolayer CrSBr structure, distortion of octahedron leads to further splitting of eg and t2g levels, consequently, the five d orbitals are no longer degenerate. With the coupling with atomic orbitals of S atoms, the spin-polarized band structures of CrSBr are formed, as shown in Figure 2c (right) [42]. Furthermore, the more accurate electronic structure calculations of monolayer CrSBr in its FM ground state within the GW approximation demonstrate a semiconducting band gap of ~1.8 eV and highly anisotropic band dispersion [78]. First-principles GW-BSE calculations unveil the different exciton wave functions for AFM and FM CrBrS bilayers. In the AFM bilayer, the electron is localized in the same layer as the hole. In the FM bilayer, the electron wave function is delocalized across both layers for a hole fixed in the bottom layer [78]. Thus, the optical transitions dominant by these excitonic transitions will be tuned during the AFM to FM phase transition. According to the crystal structure of CrPS4, each Cr atom is located in a distorted octahedral interstice formed by six S atoms, thus inducing an octahedral ligand perturbation field that splits that 3d orbitals of Cr3+ into t2g and eg orbitals. The ground state (4A2g) and three lowest excited states (2Eg, 4T2g, 4T1g) emerge from the ground term (4F) of d3, as shown in Figure 4c [46, 79]. The optical transitions should be from the spin-allowed d-d transition of the Cr3+ ion. For more realistic scenarios, spin is considered in the electronic band structure calculations, as shown in Figure 4d, the band gap for bilayer CrPS4 is 0.83 eV (AFM state), 0.72 eV (spin up in the FM state) and 1.43 eV (spin down in the FM state), respectively [49]. However, for more accurate electronic band structures of CrPS4, the excitonic effect should be considered.

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3. Light-mater interactions in 2D magnets

vdWs layered materials offer fascinating opportunities for studying light-matter interactions in the 2D limit. So far, many interesting optical phenomena have been observed in monolayer semiconducting TMDs and other non-metallic 2D materials. However, none of these 2D materials possesses long-range magnetic order. The discovery of 2D system hosting intrinsic magnetic order represents a seminal addition to the rich landscape of vdW materials. More importantly, the interdependence of structure and magnetism, along with strong light-matter interactions, provides a new platform to explore the optical control of magnetic, electronic, and vibrational degrees of freedom at the 2D limit. Here, we briefly summarize the optical properties of several 2D magnets that have been studied in recent years. The correlation between emitted photons, phonons and spins (magnetic orders) in layered magnets provides routes for investigating magneto-optics in 2D materials, and hence opens a path for developing opto-spintronic devices and AFM/FM-based quantum information technologies.

3.1 Light absorption and emission of 2D magnets

The excitonic absorption and emission properties have been widely studied in CrX3(X = Cl, Br, I), which can be tuned by changing their magnetic ordering. Figure 5a presents the optical absorption and photoluminescence (PL) properties of bulk CrBr3 in FM state (10 K) [27]. It is clear that two excitonic absorption peak X1 and X2 are located in the visible region, and PL is in the near-infrared region. As explained in this work PL originates from the d-d transitions modified by polaronic effect in CrBr3, resulting in a ultra long emission lifetime of around 4.3 μs [27, 80]. Besides, the d-d transition in the out-of-plane magnetic field shows circularly selective PL [71]. Figure 5b shows the PL from monolayer CrBr3 at 2.5 K in −0.5 and 0.5 T with σ+σ+ and σ−σ− excitation-collection configurations, where σ+(σ−) represents the left (right) circularly polarized light. The PL at ±0.5 T shows opposite helicity, indicating that the spin of electrons in CrBr3 is coupled to the circularly polarized light [80]. NiPS3 exhibits strongly correlated electrons, where the excitons’ unique coupling to spin and orbital degrees of freedom in this AFM 2D magnet. The non-equilibrium driven of such dressed quasiparticles offers a promising platform for realizing unconventional many-body phenomena and phased beyond thermo-dynamic equilibrium. Recently, spin-orbital-entangled excitons that arise from Zhang-Rice state have been achieved and observed in this vdW correlated insulator NiPS3 under photo excitation. As shown in Figure 5c (left), NiPS3 has a charge-transfer (CT) gap of ~1.8 eV at 20 K (AFM state) [81]. Below the charge excitation lies a rich spectrum of sub-gap absorption resonances, including on-site d-d transitions at around 1.1 and 1.7 eV [81]. More importantly, there is a complex of spin-orbit-entangled excitons at around 1.5 eV, which has been assigned to the transition from a ZRT to ZRS state [77]. Besides, PL spectrum of a few-layer NiPS3 sample with a thickness of about 10 nm shows two broadened emission peaks at E3d ~1.23 eV and at B ~1.825 eV at 300 K, as shown in Figure 5c (right). A detailed analysis of the PL line-shape fitting of the PL spectrum reveals that the A1 peak (~1.366 eV) is still involved in the broadened E3d peak [82]. Further thickness-dependent PL intensity results indicate E3d peak may be from the Ni 3d eg* band to the Ev transition assisted by phonons at 300 K. The inset of Figure 5c (right) shows the probable band scheme of the band edge for PL band-edge emissions. The E3d peak is an indirect-like emission caused by the intermediate band of Ni 3d eg* existed in NiPS3. The A1 and B emissions are originated from the direct Ec bottom to the Ev top and to the spin-split-off band below Ev, respectively [82]. CrPS4 is a promising ternary AFM semiconductor with PL in the near-infrared wavelength region. Recently, a Fano resonance, arising from quantum interference between a discrete optical transition and a continuous background, is observed in PL from CrPS4 flakes with decreasing temperature below the Néel temperature. Figure 5d (left) shows the plot of (αhν)2 versus the photon energy hν, which can determine the absorption edge is ~2.00 eV ascribed to the CT transitions from the 3p of the S band to unoccupied 3d band of the Cr atom. In the energy below the absorption edge, two weak peaks can be observed at 1.54 and 1.65 eV, as show in the inset of Figure 5d (left) [51]. At the low temperature 4 K, PL emission in the near-infrared region is observed, where PL shows several narrow peaks with two asymmetric profiles located at 1.333 and 1.367 eV emerging [79]. Such an asymmetric line shape is a typical feature of the Fano resonance, which can be well described by a Fano formula (the fitted red solid line). This work explains that the continuous background responsible for the Fano resonances is attributed to the d-d transition of the Cr3+ Center, predominantly the spin-forbidden 2Eg to 4A2g transition with contributions of the broad-band 4T2g to 4A2g transition [51]. Since this Fano resonance is only observed with the temperature below the Néel temperature, it indicates that AFM order of CrPS4 is of great importance to the occurrence of this quantum inference. However, the applied out-of-plane magnetic field shows no obvious influence on the PL spectra, as demonstrated by the inset of Figure 5d (right), which may be due to the saturation of the discrete state transition at 10 K.

Figure 5.

a, Optical absorption and overlaid PL spectrum of CrBr3 thick samples measured at 10 K (left). The d-d (X1 and X2) and CT transitions are marked and illustrated in a simplified energy diagram in the inset. Time-resolved PL decay curve obtained at 4 K by integrating the PL intensity over the spectral range of 1.25–1.4 eV (right). The fitting (solid line) yields an averaged lifetime of 4.3 μs. Inset: schematic of light emission from Cr3+ metal ion in the octahedral crystal field [27] Copyright 2022, American Chemical Society. b, Spontaneous circularly polarized PL spectra for σ+σ+ (red) and σ−σ− (blue) circular polarization components from monolayer CrBr3 at 2.7 K under the applied out-plane magnetic field of - 0.5 T (left) and 0.5 T (right) [80] Copyright 2019, American Chemical Society. c, Optical absorption (α) of NiPS3 below the CT gap at 20 K (left). The features around 1.5 eV at the spin-orbital-entangled exciton transitions. The broad structures around 1.1 and 1.7 eV are on-site d-d transitions [81] Copyright 2021, Springer Nature. PL spectra of the band-edge emissions of a few-layer NiPS3 sample at 300 K (right) [82]. Copyright 2021, Springer Nature. The fitting curves of the PL peaks are also included. The left inset shows a microscopic image of the few-layer sample, and the right inset shows the possible band-edge scheme for the observed PL emissions. d, Differential reflectance spectrum of a few-layer CrPS4 flake (black dots), and the liner fitting (red line) reveals the absorption edge at 2.00 eV (left). Inset: the zoomed-in spectrum. PL spectrum of CrPS4 at 4 K fitted by the Fano formula (right). The continuum band is composed of four Gaussian peaks and one Lorentzian peak. Inset: PL spectra obtained at 10 K with applied field of 0 T and 7 T [51] Copyright 2020, American Chemical Society.

3.2 Light scattering of 2D magnets

Raman scattering measures light in elastically scattered from collective quasiparticle excitations. In particular, Raman scattering from spin-phonon coupling, electron-phonon coupling, phase transitions, and magnetic excitations has yielded incisive information on recently developed 2D magnets. CrI3 has been demonstrated that hosts various interesting light scattering properties: (i) the tuning of in elastically scattered light through symmetry control in atomically thin CrI3, as shown in Figure 6a [83, 87]. Raman spectra taken at 5 K in the cross-linear polarization channel (xy) shows a forbidden Raman activity of Ag series mode, as shown in Figure 6a (left) [83]. In contrast, there are two modes appearing in the cross-linearly polarized Raman scattering at 77 and 126 cm−1, labeled as P1 and P2 in Figure 6a (right) [83]. These modes have been previously attributed to one-magnon excitations since they appear in the magnetically ordered state and have their largest intensity in xy configuration, as shown in the inset of Figure 6a (right). Besides, in bilayer CrI3, the A1g phonon mode becomes Davydov-split into two modes of opposite parity, which exhibit divergent selection rules that depend on inversion symmetry and the underlying magnetic order. At zero magnetic field (AFM state), there are two peaks distinct in energy appeared at 126.7 and 128.8 cm−1 in the cross- and co-linearly polarized channels, respectively (top right corner of Figure 6a) [84]. When the field was above the spin-flip transition (−1 T) to fully align the spins into FM states, the 126.7 cm−1 peak presented in the AFM states is abruptly suppressed and only 128.8 cm−1 peak is observed in both polarized channels (bottom right corner of Figure 6a) [84]. These findings shed light on exploring the emergent magneto-optical effects in 2D magnets. (ii) the 2D magnons has been directly observed in atomically thin CrI3 [88]. The ultra-low frequency magneto-Raman spectroscopy probed the effects of symmetry on magnetic excitations in monolayer CrI3, a low-frequency feature emerges on both Stokes and anti-Stokes sides when a magnetic field is applied at −4 T. Interestingly, these Stokes and anti-Stokes modes obey opposite optical selection rules: for a given magnetization orientation, Stokes mode only appears when excited by on helicity of light, while anti-Stokes mode emerges with excitation of the opposite helicity. Further magnetic-field and temperature dependence measurements demonstrate that these Stokes and anti-Stokes modes are the signature of an acoustic magnon. Besides, strong coupling between magnons and phonons are directly observed in a 2D AFM semiconductor FePS3, vis magneto-Raman spectroscopy at magnetic fields up to 30 T [85]. The temperature-dependent Raman spectra on FePS3 shows a Raman active magnon peak at 121 cm−1. And the broad Raman band evolves into three sharp Lorentz peaks, P1, P2 and P3 as temperature decreased to below TN, as shown in Figure 6b (left). The evolution of the Raman spectrum of FePS3 in an out-of-plane magnetic field shows that P1, P2 and P3 peaks have no energy shift with low magnetic field, therefore verified as phonons, as shown in Figure 6b (middle). In contrast, the peak M exhibits a Zeeman splitting, confirming its magnonic character. These two split peak sections are labeled as M↓ and M↑, which have opposite spins. The M↓ and M↑ peak positions of show linear dependence on a magnetic field with similar slopes (0.99 and −0.94 cm−1/T), which is the effective g factor of the magnon, as shown in Figure 6b (right). Until 22.5 T, the shift of P1 and P2 is negligible, while the field-driven anticrossing of M↑ and P3 with a 6.1 cm−1 energy gap signals a repulsive interaction between magnon M↑ and P3 modes when their energies are in resonances, which is evidence that two modes are strongly coupled [85]. Due to the correlated-electron system of NiPS3, the scattering of incident photons with d electrons in Ni2+ ions has been observed at ~1.0 eV [35]. As shown in Figure 6c (left), the measured distinct optical spectra as the excitation wavelength varying from 454 nm (2.73 eV) to 531 nm (2.34 eV), show a peak at the same position ~1.39 eV assigned to the phonon sideband of the coherent ZR exciton. In contrast, there are two other peaks, R1 and R2 exhibiting an obvious redshift with decreasing of the excitation energy, and, finally, disappear when the excitation energy is below ~2.35 eV. These two peaks present a fixed energy difference between the collected signal and the laser excitation, suggesting that their origin is Raman scattering rather than luminescence. The intensity of electronic Raman peaks of R1 and R2 exhibit a clear dependence on the excitation energy, indicating a resonant effect, which is related to an external electronic state as an intermediate state, as shown in Figure 6c (right). Fitting results demonstrate that the intensities of R1 and R2 reach the maximum at ~2.61 and ~2.73 eV, respectively. The different resonant energies of these two modes indicate a scattered light resonance. After calculation, they obtained the intermediate state energy of ~1.70 eV, which matches the energy of the second excited states (3T1g) from the ground states. The electronic Raman scattering process is proposed in between the ground state and first excited triplet state for Ni2+ ion in a trigonally distorted octahedral environment, as shown in Figure 6c (right). Phonon chirality and spin-phonon coupling have been observed in 2D CrBr3[86]. The helicity-resolved Raman scattering measurement at 10 K can be used to resolve the chirality of phonon modes in CrBr3 (FM phase), since chiral phonon modes originate from the circular vibration of sublattices. As shown in Figure 6d (left), the Raman peaks corresponding to Eg modes appear only in the cross-circularly polarized configuration (σ+σ−), which means that Raman scattering involved Eg modes reverses the helicity of the incident photons. In contrast, Raman scattering involving the two Ag modes does not change the helicity of the incident photons. The corresponding polar plot of Eg and Ag modes clearly presents their distinct polarization properties, where Eg modes have maximum Raman intensity in the cross-circularly polarized configuration (σ+σ−) rather than the almost extinct Ag modes (inset in the left side of Figure 6d). Since Raman scattering involving Eg modes switches the polarization of circularly polarized light, the Eg modes in CrBr3 must have a non-zero pseudo-angular momentum (PAM), based on the conservation of PAM in the Raman scattering process. Besides, single-phonon Raman scattering process can only induce zone-center phonons due to the conservation of crystal momentum. However, the zone-center phonons have real eigenvectors which have zero phonon circular polarization by definition. Thus, the only zone-center Raman active phonon modes that can reverse the helicity of incoming light are complex superposition of the degenerate Eg modes, as demonstrated by the top right corner of Figure 6d. Furthermore, it is nature to explore the magnetic effect on the phonon properties of CrBr3 upon cooling, given the existence of a 2D long-range FM order below Curie temperature in CrBr3. As shown in Figure 6d (the bottom right corner), the Raman peak shift and linewidth change of Ag mode with decreasing temperature exhibit an anomalous behavior when the temperature is near to the Curie temperature. For example, in the temperature range between 290 and 50 K, a conventional hardening of phonon with decreasing temperature is observed due to the suppression of the anharmonic phonon-phonon interactions. While such temperature dependence becomes much stronger with the onset of magnetic ordering below ~50 K, as demonstrated by the additional anomalous phonon hardening due to the short-range local ordering of magnetic moments.

Figure 6.

a, The left: Raman spectra of few-layer CrI3 at 5 K (0 T) for xx (black) and xy (red) polarization configurations. Spectra in xx were divided by two for clarity. Peaks that appear below Tc are highlighted with blue asterisks. Inset shows intensity as a function of collection polarization angle for P2 and Ag6 in a polar plot [83] Copyright 2020, Springer Nature. The right: co- and cross-linearly polarized Raman spectra taken in an AFM state at 0 T (top) and the fully spin-up polarized state at −1.0 T of Ag Raman mode from bilayer CrI3 [84] Copyright 2020, American Chemical Society. b, Temperature-dependent Raman spectra of FePS3 single crystal (left). A single broad peak at 120 K splits into four sharp peaks at 4.2 K. Raman spectra of FePS3 in magnetic field 0–30 T (middle). Raman spectra have been vertically shifted for clarity, in steps of 2.5 T. Zeeman splitting of the M branch identifies it as magnon mode. The two branches are denoted as M↑ (red) and M↓ (blue). Magnetic-field-dependent peak position of P1, P2, P3, M↑ and M↓, which are denoted as black squares, black circles, green diamonds, red up-triangles, and blue down-triangles (right). The solid lines are fitted to the data [85] Copyright 2021, American Physical Society. c, Optical spectra of NiPS3 plotted in terms of Raman shift, i.e., energy difference from excitation energy, excited by different lasers with energies from 2.73 to 2.34 eV (left). Integrated intensity of two electronic Raman (ER) peaks as a function of excitation energy (middle). The dashed lines are the fitting curves. Schematic illustration of ER scattering in between the ground state and first excited triplet state for Ni2+ ion in a trigonally distorted octahedral environment (D3h) (right). Oh represents the crystal field splitting in an octahedral field; hν0 and hν1 (hν2) are the energy of incident and scattered photons of R1 (R2) mode [35] Copyright 2022, AAAS. d, Helicity-resolved Raman spectra of bulk CrBr3 at 10 K (left). Inset: polar plot of the Raman intensities of these four modes versus the rotation of the quarter-wave plate. The 0° and 90°correspond to the (σ+σ+) and (σ+σ−) configuration, respectively. Right: superposition of two orthogonal linear vibrations of Eg1 (143.0 cm−1) results in right-handed or left-handed circular motions at the Γ-point, i.e., chiral phonons with PAM of l = ±1 [86] Copyright 2021, Wiley-VCH GmbH.

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4. Conclusion

In summary, the recently developed 2D magnets and their novel photo-physical phenomena have been briefly summarized in this chapter. Firstly, we introduced the crystal structures, magnetic properties, and electronic band structures of several hot 2D magnets like CrX3 (X = Cl, Br, I), MPS3 (M = Fe, Ni, Mn), CrSBr and CrPS4. Then, according to these features, they exhibited novel light absorption, emission and scattering properties. More importantly, 2D magnets provide us a platform to tune the magnetic, electronic, and vibrational degrees of freedom for designing novel 2D magnet-based device applications. Although great processes have been made in this booming field in the past few years, the research on exploring light-matter interactions in 2D magnets is still in its infancy. There are many aspects that need researchers to further explore: growth of new 2D magnets, fabrication 2D magnetic heterostructures, development of advanced optical characterization techniques, and applying various state-of-the-art external perturbations for effectively engineering, et al. In essence, 2D magnets offer us an entirely new opportunity to explore novel photo-physical phenomena, therein looking promising for many more exciting outcomes shortly.

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Acknowledgments

T. Yin gratefully acknowledges strong support from the Presidential Postdoctoral Fellowship of Nanyang Technological University.

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Conflict of interest

The authors declare no conflict of interest.

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Written By

Tingting Yin

Submitted: 01 June 2023 Reviewed: 12 June 2023 Published: 22 September 2023

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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