Giant nonreciprocal second-harmonic generation from antiferromagnetic bilayer CrI3

Sun, Zeyuan; Yi, Yangfan; Song, Tiancheng; Clark, Genevieve; Huang, Bevin; Shan, Yongkui; Wu, Shuang; Huang, Di; Gao, Chunlei; Chen, Zhanghai; McGuire, Michael A.; Cao, Ting; Xiao, Di; Liu, Weitao; Wang, Yao; Xu, Xiaodong; Wu, Shiwei

doi:10.1038/s41586-019-1445-3

Cited by 372 publications

(396 citation statements)

References 37 publications

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“…Bulk CrI3 is in a monoclinic phase at room temperature and undergoes a structural phase transition to a rhombohedral phase at around 200 K. 14,25 The main difference between the two phases is different stacking orders of the CrI3 layers. Second harmonic generation 13 , high pressure 17,18 , and Raman 19 experiments demonstrate that exfoliated few-layer CrI3 flakes remain in the monoclinic phase at all temperatures. Given the similarity in magnetic properties between the surface layer group in our samples and few-layer CrI3 flakes, and between the inner layer group and CrI3 bulk crystals, we believe that the surface layers, which is about ~13 nm thick at each surface, should be in the monoclinic phase at all temperatures like few-layer CrI3 flakes, while the inner layers undergo the structural phase transition like bulk CrI3.…”

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confidence: 93%

Coexistence of Magnetic Orders in Two-Dimensional Magnet CrI₃

Niu

Francisco

et al. 2019

Nano Lett.

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The magnetic properties in two-dimensional van der Waals materials depend sensitively on structure. CrI3, as an example, has been recently demonstrated to exhibit distinct magnetic properties depending on the layer thickness and stacking order. Bulk CrI3 is ferromagnetic (FM) with a Curie temperature of 61 K and a rhombohedral layer stacking, while few-layer CrI3 has a layered antiferromagnetic (AFM) phase with a lower ordering temperature of 45 K and a monoclinic stacking. In this work, we use cryogenic magnetic force microscopy to investigate CrI3 flakes in the intermediate thickness range (25 -200 nm) and find that the two types of magnetic orders hence the stacking orders can coexist in the same flake, with a layer of ~13 nm at each surface being in the layered AFM phase similar to few-layer CrI3 and the rest in the bulk FM phase. The switching of the bulk moment proceeds through a remnant state with nearly compensated magnetic moment along the c-axis, indicating formation of c-axis domains allowed by a weak interlayer coupling strength in the rhombohedral phase. Our results provide a comprehensive picture on the magnetism in CrI3 and point to the possibility of engineering magnetic heterostructures within the same material.

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confidence: 93%

Coexistence of Magnetic Orders in Two-Dimensional Magnet CrI₃

Niu

Francisco

et al. 2019

Nano Lett.

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“…It was first synthesized by mechanical exfoliation of bulk CrI 3 crystal, and identified as an Ising ferromagnet with out-of-plane spin orientation [1]. Being strongly sensitive to external electric field, few-layer CrI 3 became the subject of numerous experimental studies [1,[12][13][14][15], particularly aimed to electrical control and manipulation of magnetic states [2][3][4].…”

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confidence: 99%

Orbitally-resolved ferromagnetism of monolayer CrI₃

et al. 2020

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Few-layer CrI 3 is the most known example among two-dimensional (2D) ferromagnets, which have attracted growing interest in recent years. Despite considerable efforts and progress in understanding the properties of 2D magnets both from theory and experiment, the mechanism behind the formation of in-plane magnetic ordering in chromium halides is still under debate. Here, we propose a microscopic orbitally-resolved description of ferromagnetism in monolayer CrI 3 . Starting from first-principles calculations, we construct a low-energy model for the isotropic Heisenberg exchange interactions. We find that there are two competing contributions to the long-range magnetic ordering in CrI 3 : (i) Antiferromagnetic Anderson's superexchange between half-filled t 2g orbitals of Cr atoms; and (ii) Ferromagnetic exchange governed by the Kugel-Khomskii mechanism, involving the transitions between half-filled t 2g and empty e g orbitals. Using numerical calculations, we estimate the exchange interactions in momentum-space, which allows us to restore the spin-wave spectrum, as well as estimate the Curie temperature. Contrary to the nearest-neighbor effective models, our calculations suggest the presence of sharp resonances in the spin-wave spectrum at 5-7 meV, depending on the vertical bias voltage. Our estimation of the Curie temperature in monolayer CrI 3 yields 55-65 K, which is in good agreement with experimental data.

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“…Furthermore, second harmonic generation measurements have revealed the restoration of inversion symmetry when the bilayer is switched from the AFM state to the fully spin-polarized state, highlighting the dependence of symmetry on the magnetic order in CrI3 bilayers 17 . As such, Raman optical selection rules, and hence specific Raman modes, are likely to be controllable when switching between magnetic states.…”

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confidence: 98%

Tuning inelastic light scattering via symmetry control in the two-dimensional magnet CrI3

et al. 2020

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The coupling between spin and charge degrees of freedom in a crystal imparts strong optical signatures on scattered electromagnetic waves. This has led to magneto-optical effects with a host of applications, from the sensitive detection of local magnetic order to optical modulation and data storage technologies. Here, we demonstrate a new magnetooptical effect, namely, the tuning of inelastically scattered light through symmetry control in atomically thin chromium triiodide (CrI3). In monolayers, we found an extraordinarily large magneto-optical Raman effect from an A1g phonon mode due to the emergence of ferromagnetic order. The linearly polarized, inelastically scattered light rotates by ~40⁰, more than two orders of magnitude larger than the rotation from MOKE under the same experimental conditions. In CrI3 bilayers, we show that the same A1g phonon mode becomes Davydov-split into two modes of opposite parity, exhibiting divergent selection rules that depend on inversion symmetry and the underlying magnetic order. By switching between the antiferromagnetic states and the fully spin-polarized states with applied magnetic and electric fields, we demonstrate the magnetoelectrical control over their selection rules. Our work underscores the unique opportunities provided by 2D magnets for controlling the combined time-reversal and inversion symmetries to manipulate Raman optical selection rules and for exploring emergent magneto-optical effects and spin-phonon coupled physics. Main text:Raman scattering measures light inelastically scattered from collective quasiparticle excitations. Since it is highly sensitive to material parameters such as crystal symmetry and local electronic states, Raman spectroscopy has provided a powerful probe of a broad range of condensed matter phenomena, such as charge density waves 1 , superconductivity 2 , ferroelectricity 3 , and topological physics 4 . In particular, Raman scattering from spin-phonon excitations has yielded incisive information on magnetic materials. For instance, in recently developed 2D van der Waals magnets, Raman scattering has been used to reveal magnetic order and phase transitions 5-7 down to a single layer [8][9][10] .Chromium triiodide (CrI3), a van der Waals magnet, was shown to be a layered antiferromagnet in its few-layer form: spins within each layer are ferromagnetically (FM) coupled with strong outof-plane anisotropy, while the interlayer exchange is antiferromagnetic (AFM) 11 . For bilayers, the system undergoes a spin-flip transition upon the application of a moderate magnetic field 11 , switching from a layered AFM state to a fully spin-polarized state. In addition, magneto-optical effects manifest strongly and in distinctly novel ways in CrI3. Examples include the very large magnetic-optical Kerr effect (MOKE) 11,12 and spontaneous helical light emission 13 from ferromagnetic monolayers, and electric-field induced Kerr rotation [14][15][16] and giant second-order nonreciprocal optical effects 17 in antiferromagnetic bilayers. Content:Extended Data...

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Giant nonreciprocal second-harmonic generation from antiferromagnetic bilayer CrI3

Cited by 372 publications

References 37 publications

Coexistence of Magnetic Orders in Two-Dimensional Magnet CrI₃

Coexistence of Magnetic Orders in Two-Dimensional Magnet CrI₃

Orbitally-resolved ferromagnetism of monolayer CrI₃

Tuning inelastic light scattering via symmetry control in the two-dimensional magnet CrI3

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Giant nonreciprocal second-harmonic generation from antiferromagnetic bilayer CrI3

Cited by 372 publications

References 37 publications

Coexistence of Magnetic Orders in Two-Dimensional Magnet CrI3

Coexistence of Magnetic Orders in Two-Dimensional Magnet CrI3

Orbitally-resolved ferromagnetism of monolayer CrI3

Tuning inelastic light scattering via symmetry control in the two-dimensional magnet CrI3

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Product

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Coexistence of Magnetic Orders in Two-Dimensional Magnet CrI₃

Coexistence of Magnetic Orders in Two-Dimensional Magnet CrI₃

Orbitally-resolved ferromagnetism of monolayer CrI₃