Journal articles: 'Van der Waals magnets'

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Xu, Hang, Shengjie Xu, Xun Xu, Jincheng Zhuang, Weichang Hao, and Yi Du. "Recent advances in two-dimensional van der Waals magnets." Microstructures 2, no. 2 (2022): 2022011. http://dx.doi.org/10.20517/microstructures.2022.02.

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Two-dimensional (2D) magnets have evoked tremendous interest within the research community due to their fascinating features and novel mechanisms, as well as their potential applications in magnetic nanodevices. In this review, state-of-the-art research into the exploration of 2D magnets from the perspective of their magnetic interaction and order mechanisms is discussed. The properties of these magnets can be effectively modulated by varying the external parameters, such as the charge carrier doping, thickness effect, pressure and strain. The potential applications of heterostructures of these 2D magnets in terms of the interlayer coupling strength are reviewed, and the challenges and outlook for this field are proposed.

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Verzhbitskiy, Ivan, and Goki Eda. "Electrostatic control of magnetism: Emergent opportunities with van der Waals materials." Applied Physics Letters 121, no. 6 (August 8, 2022): 060501. http://dx.doi.org/10.1063/5.0107329.

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Since the first reports on the observation of magnetic order in atomically thin crystals of FePS3, CrI3, and CrGeTe3 in 2016 and 2017, there has been a greatly renewed interest in the magnetism of van der Waals (vdW) layered magnets. Due to their dimensionality and structure, ultrathin vdW magnets offer tantalizing prospects for electrostatic control of magnetism for energy-efficient spintronic logic and memory devices. Recent demonstrations revealed unusually high susceptibility of some vdW magnets to electrostatic fields and shed light on a path to room temperature devices, a long-standing goal in spintronics research. In this Perspective, we discuss the potential of different classes of vdW magnets for electrostatic control of magnetism by comparing their properties with those of non-vdW magnets such as dilute magnetic III–V semiconductors and perovskite manganites that have been intensively studied in the past two decades.

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Bedoya-Pinto, Amilcar, Jing-Rong Ji, Avanindra K. Pandeya, Pierluigi Gargiani, Manuel Valvidares, Paolo Sessi, James M. Taylor, Florin Radu, Kai Chang, and Stuart S. P. Parkin. "Intrinsic 2D-XY ferromagnetism in a van der Waals monolayer." Science 374, no. 6567 (October 29, 2021): 616–20. http://dx.doi.org/10.1126/science.abd5146.

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Taking the measure of a magnet The recent discovery of magnetism in two-dimensional (2D) materials has inspired efforts to understand its nature. Whereas the magnetism of monolayers of chromium iodide (CrI 3 ) can be understood in terms of out-of-plane magnetic anisotropy, the related material chromium chloride (CrCl 3 ) has spins that lie in the plane. Bedoya-Pinto et al . used molecular beam epitaxy to grow monolayers of CrCl 3 on graphene and studied its magnetic properties. Using x-ray magnetic circular dichroism measurements, the authors found that monolayer CrCl 3 is a ferromagnet, unlike bulk CrCl 3 , which is antiferromagnetic. The scaling of the signal in the critical region indicated that the material belongs to the 2D-XY universality class, distinct from Ising magnetism, which some other 2D magnets exhibit. —JS

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Wang, Xiao, Jian Tang, Xiuxin Xia, Congli He, Junwei Zhang, Yizhou Liu, Caihua Wan, et al. "Current-driven magnetization switching in a van der Waals ferromagnet Fe3GeTe2." Science Advances 5, no. 8 (August 2019): eaaw8904. http://dx.doi.org/10.1126/sciadv.aaw8904.

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The recent discovery of ferromagnetism in two-dimensional (2D) van der Waals (vdW) materials holds promises for spintronic devices with exceptional properties. However, to use 2D vdW magnets for building spintronic nanodevices such as magnetic memories, key challenges remain in terms of effectively switching the magnetization from one state to the other electrically. Here, we devise a bilayer structure of Fe3GeTe2/Pt, in which the magnetization of few-layered Fe3GeTe2 can be effectively switched by the spin-orbit torques (SOTs) originated from the current flowing in the Pt layer. The effective magnetic fields corresponding to the SOTs are further quantitatively characterized using harmonic measurements. Our demonstration of the SOT-driven magnetization switching in a 2D vdW magnet could pave the way for implementing low-dimensional materials in the next-generation spintronic applications.

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Jin, Wencan, Zhipeng Ye, Xiangpeng Luo, Bowen Yang, Gaihua Ye, Fangzhou Yin, Hyun Ho Kim, et al. "Tunable layered-magnetism–assisted magneto-Raman effect in a two-dimensional magnet CrI3." Proceedings of the National Academy of Sciences 117, no. 40 (September 23, 2020): 24664–69. http://dx.doi.org/10.1073/pnas.2012980117.

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We used a combination of polarized Raman spectroscopy experiment and model magnetism–phonon coupling calculations to study the rich magneto-Raman effect in the two-dimensional (2D) magnet CrI3. We reveal a layered-magnetism–assisted phonon scattering mechanism below the magnetic onset temperature, whose Raman excitation breaks time-reversal symmetry, has an antisymmetric Raman tensor, and follows the magnetic phase transitions across critical magnetic fields, on top of the presence of the conventional phonon scattering with symmetric Raman tensors in N-layer CrI3. We resolve in data and by calculations that the first-order Ag phonon of the monolayer splits into an N-fold multiplet in N-layer CrI3 due to the interlayer coupling (N≥2) and that the phonons within the multiplet show distinct magnetic field dependence because of their different layered-magnetism–phonon coupling. We further find that such a layered-magnetism–phonon coupled Raman scattering mechanism extends beyond first-order to higher-order multiphonon scattering processes. Our results on the magneto-Raman effect of the first-order phonons in the multiplet and the higher-order multiphonons in N-layer CrI3 demonstrate the rich and strong behavior of emergent magneto-optical effects in 2D magnets and underline the unique opportunities of spin–phonon physics in van der Waals layered magnets.

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Blei, M., J. L. Lado, Q. Song, D. Dey, O. Erten, V. Pardo, R. Comin, S. Tongay, and A. S. Botana. "Synthesis, engineering, and theory of 2D van der Waals magnets." Applied Physics Reviews 8, no. 2 (June 2021): 021301. http://dx.doi.org/10.1063/5.0025658.

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Sun, Yu-Yun, Liang-Qing Zhu, Zhongyao Li, WeiWei Ju, Shi-Jing Gong, Ji-Qing Wang, and Jun-Hao Chu. "Electric manipulation of magnetism in bilayer van der Waals magnets." Journal of Physics: Condensed Matter 31, no. 20 (March 14, 2019): 205501. http://dx.doi.org/10.1088/1361-648x/ab03ec.

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Jiang, Shengwei, Jie Shan, and Kin Fai Mak. "Electric-field switching of two-dimensional van der Waals magnets." Nature Materials 17, no. 5 (March 12, 2018): 406–10. http://dx.doi.org/10.1038/s41563-018-0040-6.

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Tong, Qingjun, Fei Liu, Jiang Xiao, and Wang Yao. "Skyrmions in the Moiré of van der Waals 2D Magnets." Nano Letters 18, no. 11 (October 4, 2018): 7194–99. http://dx.doi.org/10.1021/acs.nanolett.8b03315.

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Hu, Liang, Jian Zhou, Zhipeng Hou, Weitao Su, Bingzhang Yang, Lingwei Li, and Mi Yan. "Polymer-buried van der Waals magnets for promising wearable room-temperature spintronics." Materials Horizons 8, no. 12 (2021): 3306–14. http://dx.doi.org/10.1039/d1mh01439k.

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Thiel, L., Z. Wang, M. A. Tschudin, D. Rohner, I. Gutiérrez-Lezama, N. Ubrig, M. Gibertini, E. Giannini, A. F. Morpurgo, and P. Maletinsky. "Probing magnetism in 2D materials at the nanoscale with single-spin microscopy." Science 364, no. 6444 (April 25, 2019): 973–76. http://dx.doi.org/10.1126/science.aav6926.

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The discovery of ferromagnetism in two-dimensional (2D) van der Waals (vdW) crystals has generated widespread interest. Making further progress in this area requires quantitative knowledge of the magnetic properties of vdW magnets at the nanoscale. We used scanning single-spin magnetometry based on diamond nitrogen-vacancy centers to image the magnetization, localized defects, and magnetic domains of atomically thin crystals of the vdW magnet chromium(III) iodide (CrI3). We determined the magnetization of CrI3monolayers to be ≈16 Bohr magnetons per square nanometer, with comparable values in samples with odd numbers of layers; however, the magnetization vanishes when the number of layers is even. We also found that structural modifications can induce switching between ferromagnetic and antiferromagnetic interlayer ordering. These results demonstrate the benefit of using single-spin scanning magnetometry to study the magnetism of 2D vdW magnets.

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Hejazi, Kasra, Zhu-Xi Luo, and Leon Balents. "Noncollinear phases in moiré magnets." Proceedings of the National Academy of Sciences 117, no. 20 (May 4, 2020): 10721–26. http://dx.doi.org/10.1073/pnas.2000347117.

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We introduce a general framework to study moiré structures of two-dimensional Van der Waals magnets using continuum field theory. The formalism eliminates quasiperiodicity and allows a full understanding of magnetic structures and their excitations. In particular, we analyze in detail twisted bilayers of Néel antiferromagnets on the honeycomb lattice. A rich phase diagram with noncollinear twisted phases is obtained, and spin waves are further calculated. Direct extensions to zigzag antiferromagnets and ferromagnets are also presented. We anticipate the results and formalism demonstrated to lead to a broad range of applications to both fundamental research and experiments.

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Zhang, Wen, Ping Kwan Johnny Wong, Rui Zhu, and Andrew T. S. Wee. "Van der Waals magnets: Wonder building blocks for two‐dimensional spintronics?" InfoMat 1, no. 4 (November 11, 2019): 479–95. http://dx.doi.org/10.1002/inf2.12048.

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Idzuchi, Hiroshi, Andres E. Llacsahuanga Allcca, Amanda Victo Haglund, Xing-Chen Pan, Takuya Matsuda, Katsumi Tanigaki, David Mandrus, and Yong P. Chen. "On the Optical Properties of Cr2Ge2Te6 and Its Heterostructure." Condensed Matter 8, no. 3 (July 14, 2023): 59. http://dx.doi.org/10.3390/condmat8030059.

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Recently, there has been a growing interest in two-dimensional van der Waals (vdW) magnets owing to their unique two-dimensional magnetic phenomena and potential applications. Most vdW ferromagnets have the Curie temperature below room temperature, highlighting the need to explore how to enhance their magnetism. In our previous report, we successfully increased the Curie temperature of the prototypical vdW magnet Cr2Ge2Te6 using a NiO overlayer. In layered materials, the presence of wrinkles is often observed and evaluating them using optical microscopy proves to be useful; however, there have been limited investigations into the optical constants of vdW magnets, hampering progress in understanding their optical properties. In this study, we present the optical constants of Cr2Ge2Te6 obtained through ellipsometry measurements. To account for the presence of wrinkles, we model a vacuum region between the substrate and the vdW magnet, and we calculate the reflectivity as a function of wavelength and vacuum thickness to visualize the optical image. Furthermore, we discuss the relationship between the optical constants and the electronic structure of the material.

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CHUNG, Jae-Ho. "Dirac Magnons and Topological Energy Gaps in Honeycomb Magnets." Physics and High Technology 29, no. 6 (June 30, 2020): 8–13. http://dx.doi.org/10.3938/phit.29.019.

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Fermionic Dirac particles have long been known to exist in electronic materials exhibiting linear dispersion relations within energy-momentum spectra, such as two-dimensional graphene. Recently, a bosonic version of Dirac particles was predicted to appear in the magnon excitation structures of honeycomb magnets. In this article, we review theoretical predictions of topological Dirac magnons in honeycomb ferromagnets, and their experimental observations using inelastic neutron scattering in Cr-based van der Waals materials.

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Han, Myung-Geun, Yimei Zhu, and Sang-Wook Cheong. "In situ cryo-electron microscopy of two-dimensional van der Waals magnets." Microscopy and Microanalysis 27, S1 (July 30, 2021): 326–28. http://dx.doi.org/10.1017/s1431927621001732.

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Gong, Cheng, and Xiang Zhang. "Two-dimensional magnetic crystals and emergent heterostructure devices." Science 363, no. 6428 (February 14, 2019): eaav4450. http://dx.doi.org/10.1126/science.aav4450.

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Magnetism, originating from the moving charges and spin of elementary particles, has revolutionized important technologies such as data storage and biomedical imaging, and continues to bring forth new phenomena in emergent materials and reduced dimensions. The recently discovered two-dimensional (2D) magnetic van der Waals crystals provide ideal platforms for understanding 2D magnetism, the control of which has been fueling opportunities for atomically thin, flexible magneto-optic and magnetoelectric devices (such as magnetoresistive memories and spin field-effect transistors). The seamless integration of 2D magnets with dissimilar electronic and photonic materials opens up exciting possibilities for unprecedented properties and functionalities. We review the progress in this area and identify the possible directions for device applications, which may lead to advances in spintronics, sensors, and computing.

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Afanasiev, Dmytro, Jorrit R. Hortensius, Mattias Matthiesen, Samuel Mañas-Valero, Makars Šiškins, Martin Lee, Edouard Lesne, et al. "Controlling the anisotropy of a van der Waals antiferromagnet with light." Science Advances 7, no. 23 (June 2021): eabf3096. http://dx.doi.org/10.1126/sciadv.abf3096.

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Van der Waals magnets provide an ideal playground to explore the fundamentals of low-dimensional magnetism and open opportunities for ultrathin spin-processing devices. The Mermin-Wagner theorem dictates that as in reduced dimensions isotropic spin interactions cannot retain long-range correlations, the long-range spin order is stabilized by magnetic anisotropy. Here, using ultrashort pulses of light, we control magnetic anisotropy in the two-dimensional van der Waals antiferromagnet NiPS3. Tuning the photon energy in resonance with an orbital transition between crystal field split levels of the nickel ions, we demonstrate the selective activation of a subterahertz magnon mode with markedly two-dimensional behavior. The pump polarization control of the magnon amplitude confirms that the activation is governed by the photoinduced magnetic anisotropy axis emerging in response to photoexcitation of ground state electrons to states with a lower orbital symmetry. Our results establish pumping of orbital resonances as a promising route for manipulating magnetic order in low-dimensional (anti)ferromagnets.

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CHOE, Sug-Bong, Jun Woo CHOI, and Seo Hyoung CHANG. "Skyrmions in Heterostructures." Physics and High Technology 29, no. 6 (June 30, 2020): 21–27. http://dx.doi.org/10.3938/phit.29.021.

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Skrymions are topological spin textures in chiral magnetic materials and can provide new possibilities for next-generation devices. Real-space Berry curvature can play an important role in controlling the motion of skrymions. Here, we introduce ways to measure Dzyaloshinskii–Moriya interaction, which is closely related to the formation of skrymions, and to determine atomic positions by using X-ray scattering. We also review brief studies on skyrmions in two-dimensional (2D) magnetic heterostructures and the Moiré of van der Waals 2D magnets.

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Song, Dongsheng, Fengshan Zheng, and Rafal E. Dunin-Borkowski. "Prospect for measuring two-dimensional van der Waals magnets by electron magnetic chiral dichroism." Ultramicroscopy 234 (April 2022): 113476. http://dx.doi.org/10.1016/j.ultramic.2022.113476.

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Laraoui, Abdelghani, and Kapildeb Ambal. "Opportunities for nitrogen-vacancy-assisted magnetometry to study magnetism in 2D van der Waals magnets." Applied Physics Letters 121, no. 6 (August 8, 2022): 060502. http://dx.doi.org/10.1063/5.0091931.

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Exploring and understanding magnetism in two-dimensional (2D) van der Waals (vdW) magnetic materials present a promising route for developing high-speed and low-power spintronics devices. Studying their magnetic properties at the nanoscale is challenging due to their low magnetic moment compared to bulk materials and the requirements of highly sensitive magnetic microscopy tools that work over a wide range of experimental conditions (e.g., temperature, magnetic field, and sample geometry). This Perspective reviews the applications of nitrogen-vacancy center (NV) based magnetometry to study magnetism in 2D vdW magnets. The topics discussed include the basics, advantages, challenges, and the usage of NV magnetometry.

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Sheckelton, John P., Kemp W. Plumb, Benjamin A. Trump, Collin L. Broholm, and Tyrel M. McQueen. "Rearrangement of van der Waals stacking and formation of a singlet state at T = 90 K in a cluster magnet." Inorganic Chemistry Frontiers 4, no. 3 (2017): 481–90. http://dx.doi.org/10.1039/c6qi00470a.

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Cantos-Prieto, Fernando, Alexey Falin, Martin Alliati, Dong Qian, Rui Zhang, Tao Tao, Matthew R. Barnett, Elton J. G. Santos, Lu Hua Li, and Efrén Navarro-Moratalla. "Layer-Dependent Mechanical Properties and Enhanced Plasticity in the Van der Waals Chromium Trihalide Magnets." Nano Letters 21, no. 8 (April 9, 2021): 3379–85. http://dx.doi.org/10.1021/acs.nanolett.0c04794.

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Dirnberger, Florian, Jiamin Quan, Rezlind Bushati, Geoffrey M. Diederich, Matthias Florian, Julian Klein, Kseniia Mosina, et al. "Magneto-optics in a van der Waals magnet tuned by self-hybridized polaritons." Nature 620, no. 7974 (August 16, 2023): 533–37. http://dx.doi.org/10.1038/s41586-023-06275-2.

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Yang, M., Q. Li, R. V. Chopdekar, R. Dhall, J. Turner, J. D. Carlström, C. Ophus, et al. "Creation of skyrmions in van der Waals ferromagnet Fe3GeTe2 on (Co/Pd)n superlattice." Science Advances 6, no. 36 (September 2020): eabb5157. http://dx.doi.org/10.1126/sciadv.abb5157.

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Magnetic skyrmions are topological spin textures, which usually exist in noncentrosymmetric materials where the crystal inversion symmetry breaking generates the so-called Dzyaloshinskii-Moriya interaction. This requirement unfortunately excludes many important magnetic material classes, including the recently found two-dimensional van der Waals (vdW) magnetic materials, which offer unprecedented opportunities for spintronic technology. Using photoemission electron microscopy and Lorentz transmission electron microscopy, we investigated and stabilized Néel-type magnetic skyrmion in vdW ferromagnetic Fe3GeTe2 on top of (Co/Pd)n in which the Fe3GeTe2 has a centrosymmetric crystal structure. We demonstrate that the magnetic coupling between the Fe3GeTe2 and the (Co/Pd)n could create skyrmions in Fe3GeTe2 without the need of an external magnetic field. Our results open exciting opportunities in spintronic research and the engineering of topologically protected nanoscale features by expanding the group of skyrmion host materials to include these previously unknown vdW magnets.

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Zheng, Huan, Can Huang, Fanrong Lin, Jiyu Fan, Hao Liu, Lei Zhang, Chunlan Ma, Caixia Wang, Yan Zhu, and Hao Yang. "Two-dimensional van der Waals ferromagnetic thin film CrTe₂ with high Curie temperature and metallic conductivity." Applied Physics Letters 122, no. 2 (January 9, 2023): 023103. http://dx.doi.org/10.1063/5.0130479.

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Two-dimensional van der Waals (2D vdW) materials have opened up an opportunity to explore an innovative spin-based magnetic nanodevice. However, controllable fabrication of 2D vdW ferromagnets with high Curie temperature remains challenging. In this paper, we reported the growth of 2D CrTe2 single-crystal films epitaxially on Al2O3 substrates using pulsed laser deposition. We find that it shows a typical paramagnetic–ferromagnetic (PM–FM) phase transition around 200 K. The precise Curie temperature and Weiss temperature are 189 and 206.7 K, respectively. The saturation magnetization reaches 73.64 emu/g for the film thickness of 30 nm. The critical exponent β = 0.329 indicates that the magnetic interactions obey the 3D-Ising model. Electronic transport measurement confirms that a CrTe2 film always remains a metallic behavior at 5 K [Formula: see text] T [Formula: see text] 320 K and the resistivity of room temperature is 1.5 mΩ/cm. The first-principles calculation uncovers that the FM ordering state mainly stems from an exchange coupling of the adjacent Cr-spin t2 g polarized electrons and the metallic conductivity is due to p–d orbital hybridization between Cr and Te atoms. This work would shed new light on studying large-scale growth of 2D magnets and developing 2D magnet-based nanodevices of room temperature.

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Huang, Xiaokun, Yunying Mo, Jinlin Xu, Jiangnan Hu, Xin Nie, Chao Chen, Jiaqian Liu, Xiangping Jiang, and Jun-Ming Liu. "Fe-intercalated Fe3GeTe2: Potential quasi-van der Waals magnets." Applied Physics Letters 123, no. 1 (July 3, 2023). http://dx.doi.org/10.1063/5.0152869.

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In recent years, with the fast development of magnetic devices for information technology, the demands of magnetic thin films with both high functional stability and integration flexibility rapidly increase. It is believed that building a magnet with complementary advantages of van der Waals (vdW) and non-vdW magnets, which can be described as a “quasi-vdW magnet,” will be highly appreciated. One may expect a quasi-vdW magnet to have chemical bonding between the neighboring sublayers for strong magnetic coupling, but to preserve clean surfaces with vdW feature for flexible interface engineering. For this purpose, an intercalation of magnetic atoms into the interlayer gaps of vdW magnets, as a powerful method for tuning the interlayer coupling, can be a practical approach. In this work, using the first-principles calculations, we study the potential to utilize the Fe-intercalation to transform the vdW magnets Fe3GeTe2 (FGT) into quasi-vdW magnets. As two extreme cases, it is revealed that: (i) the Fe-intercalated FGT bilayer Fe-[Fe3GeTe2]2 (Fe-[FGT]2) does have remarkable interlayer ferromagnetic coupling based on covalent bonding between the intercalated Fe atom and FGT monolayers and retains low exfoliation energy with vdW feature, suggesting that the Fe-[FGT]2 bilayer can be regarded as a quasi-vdW magnet; and (ii) the Fe-intercalation can transform the vdW FGT bulk into a non-vdW Fe-Fe3GeTe2 (Fe-FGT) bulk magnet. Accordingly, as for the intermediate cases, it is suggested that Fe-intercalated FGT multilayers (Fen−1-[FGT]n, n > 2) can also be potential quasi-vdW magnets, forming a family of magnetic thin films that provide alternative building blocks for microminiaturized magnetic devices.

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Klein, J., T. Pham, J. D. Thomsen, J. B. Curtis, T. Denneulin, M. Lorke, M. Florian, et al. "Control of structure and spin texture in the van der Waals layered magnet CrSBr." Nature Communications 13, no. 1 (September 15, 2022). http://dx.doi.org/10.1038/s41467-022-32737-8.

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AbstractControlling magnetism at nanometer length scales is essential for realizing high-performance spintronic, magneto-electric and topological devices and creating on-demand spin Hamiltonians probing fundamental concepts in physics. Van der Waals (vdW)-bonded layered magnets offer exceptional opportunities for such spin texture engineering. Here, we demonstrate nanoscale structural control in the layered magnet CrSBr with the potential to create spin patterns without the environmental sensitivity that has hindered such manipulations in other vdW magnets. We drive a local phase transformation using an electron beam that moves atoms and exchanges bond directions, effectively creating regions that have vertical vdW layers embedded within the initial horizontally vdW bonded exfoliated flakes. We calculate that the newly formed two-dimensional structure is ferromagnetically ordered in-plane with an energy gap in the visible spectrum, and weak antiferromagnetism between the planes, suggesting possibilities for creating spin textures and quantum magnetic phases.

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Akram, Muhammad, and Onur Erten. "Skyrmions in twisted van der Waals magnets." Physical Review B 103, no. 14 (April 13, 2021). http://dx.doi.org/10.1103/physrevb.103.l140406.

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Zhu, Wenxuan, Cheng Song, Lei Han, Tingwen Guo, Hua Bai, and Feng Pan. "Van der Waals lattice-induced colossal magnetoresistance in Cr2Ge2Te6 thin flakes." Nature Communications 13, no. 1 (October 28, 2022). http://dx.doi.org/10.1038/s41467-022-34193-w.

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AbstractRecent discovery of two-dimensional (2D) magnets with van der Waals (vdW) gapped layered structure prospers the fundamental research of magnetism and advances the miniaturization of spintronics. Due to their unique lattice anisotropy, their band structure has the potential to be dramatically modulated by the spin configuration even in thin flakes, which is still unexplored. Here, we demonstrate the vdW lattice-induced spin modulation of band structure in thin flakes of vdW semiconductor Cr2Ge2Te6 (CGT) through the measurement of magnetoresistance (MR). The significant anisotropic lattice constructed by the interlayer vdW force and intralayer covalent bond induces anisotropic spin-orbit field, resulting in the spin orientation-dependent band splitting. Consequently, giant variation of resistance is induced between the magnetization aligned along in-plane and out-of-plane directions. Based on this, a colossal MR beyond 1000% was realized in lateral nonlocal devices with CGT acting as a magneto switch. Our finding provides a unique feature for the vdW magnets and would advance its applications in spintronics.

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Houmes, Maurits J. A., Samuel Mañas‐Valero, Alvaro Bermejillo‐Seco, Eugenio Coronado, Peter G. Steeneken, and Herre S. J. van der Zant. "Highly Anisotropic Mechanical Response of the Van der Waals Magnet CrPS₄." Advanced Functional Materials, October 12, 2023. http://dx.doi.org/10.1002/adfm.202310206.

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AbstractSemiconducting van der Waals magnets exhibit a rich physical phenomenology with different collective excitations, as magnons or excitons, that can be coupled, thereby offering new opportunities for optoelectronic, spintronic, and magnonic devices. In contrast with the well‐studied van der Waals magnets CrI3 or Fe3GeTe2, CrPS4 is a layered metamagnet with a high optical and magnon transport anisotropy. Here, the structural anisotropy of CrPS4 above and below the magnetic phase transition is investigated by fabricating nanomechanical resonators. A large anisotropy is observed in the resonance frequency of resonators oriented along the crystalline a‐ and b‐axis, indicative of a lattice expansion along the b‐axis, boosted at the magnetic phase transition, and a rather small continuous contraction along the a‐axis. This behavior in the mechanical response differs from that previously reported in van der Waals magnets, as FePS3 or CoPS3, and can be understood from the quasi‐1D nature of CrPS4. The results pinpoint CrPS4 as a promising material in the field of low‐dimensional magnetism and show the potential of mechanical resonators for unraveling the in‐plane structural anisotropy coupled to the magnetic ordering that, in a broader context, can be extended to studying structural modifications in other 2D materials and van der Waals heterostructures.

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Chen, Guangyi, Shaomian Qi, Jianqiao Liu, Di Chen, Jiongjie Wang, Shili Yan, Yu Zhang, et al. "Electrically switchable van der Waals magnon valves." Nature Communications 12, no. 1 (November 1, 2021). http://dx.doi.org/10.1038/s41467-021-26523-1.

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AbstractVan der Waals magnets have emerged as a fertile ground for the exploration of highly tunable spin physics and spin-related technology. Two-dimensional (2D) magnons in van der Waals magnets are collective excitation of spins under strong confinement. Although considerable progress has been made in understanding 2D magnons, a crucial magnon device called the van der Waals magnon valve, in which the magnon signal can be completely and repeatedly turned on and off electrically, has yet to be realized. Here we demonstrate such magnon valves based on van der Waals antiferromagnetic insulator MnPS3. By applying DC electric current through the gate electrode, we show that the second harmonic thermal magnon (SHM) signal can be tuned from positive to negative. The guaranteed zero crossing during this tuning demonstrates a complete blocking of SHM transmission, arising from the nonlinear gate dependence of the non-equilibrium magnon density in the 2D spin channel. Using the switchable magnon valves we demonstrate a magnon-based inverter. These results illustrate the potential of van der Waals anti-ferromagnets for studying highly tunable spin-wave physics and for application in magnon-base circuitry in future information technology.

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Da̧browski, Maciej, Shi Guo, Mara Strungaru, Paul S. Keatley, Freddie Withers, Elton J. G. Santos, and Robert J. Hicken. "All-optical control of spin in a 2D van der Waals magnet." Nature Communications 13, no. 1 (October 10, 2022). http://dx.doi.org/10.1038/s41467-022-33343-4.

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AbstractTwo-dimensional (2D) van der Waals magnets provide new opportunities for control of magnetism at the nanometre scale via mechanisms such as strain, voltage and the photovoltaic effect. Ultrafast laser pulses promise the fastest and most energy efficient means of manipulating electron spin and can be utilized for information storage. However, little is known about how laser pulses influence the spins in 2D magnets. Here we demonstrate laser-induced magnetic domain formation and all-optical switching in the recently discovered 2D van der Waals ferromagnet CrI3. While the magnetism of bare CrI3 layers can be manipulated with single laser pulses through thermal demagnetization processes, all-optical switching is achieved in nanostructures that combine ultrathin CrI3 with a monolayer of WSe2. The out-of-plane magnetization is switched with multiple femtosecond pulses of either circular or linear polarization, while single pulses result in less reproducible and partial switching. Our results imply that spin-dependent interfacial charge transfer between the WSe2 and CrI3 is the underpinning mechanism for the switching, paving the way towards ultrafast optical control of 2D van der Waals magnets for future photomagnetic recording and device technology.

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Iturriaga, Hector, Luis M. Martinez, Thuc T. Mai, Adam J. Biacchi, Mathias Augustin, Angela R. Hight Walker, Mohamed Fathi Sanad, et al. "Magnetic properties of intercalated quasi-2D Fe3-xGeTe2 van der Waals magnet." npj 2D Materials and Applications 7, no. 1 (August 21, 2023). http://dx.doi.org/10.1038/s41699-023-00417-w.

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AbstractAmong several well-known transition metal-based compounds, cleavable van der Waals (vdW) Fe3-xGeTe2 (FGT) magnet is a strong candidate for use in two-dimensional (2D) magnetic devices due to its strong perpendicular magnetic anisotropy, sizeable Curie temperature (TC ~154 K), and versatile magnetic character that is retained in the low-dimensional limit. While the TC remains far too low for practical applications, there has been a successful push toward improving it via external driving forces such as pressure, irradiation, and doping. Here we present experimental evidence of a room temperature (RT) ferromagnetic phase induced by the electrochemical intercalation of common tetrabutylammonium cations (TBA+) into quasi-2D FGT. We obtained Curie temperatures as high as 350 K with chemical and physical stability of the intercalated compound. The temperature-dependent Raman measurements, in combination with vdW-corrected ab initio calculations, suggest that charge transfer (electron doping) upon intercalation could lead to the observation of RT ferromagnetism. This work demonstrates that molecular intercalation is a viable route in realizing high-temperature vdW magnets in an inexpensive and reliable manner, and has the potential to be extended to bilayer and few-layer vdW magnets.

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Schmitt, Maurice, Thibaud Denneulin, András Kovács, Tom G. Saunderson, Philipp Rüßmann, Aga Shahee, Tanja Scholz, et al. "Skyrmionic spin structures in layered Fe5GeTe2 up to room temperature." Communications Physics 5, no. 1 (October 18, 2022). http://dx.doi.org/10.1038/s42005-022-01031-w.

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AbstractThe role of the crystal lattice, temperature and magnetic field for the spin structure formation in the 2D van der Waals magnet Fe5GeTe2 with magnetic ordering up to room temperature is a key open question. Using Lorentz transmission electron microscopy, we experimentally observe topological spin structures up to room temperature in the metastable pre-cooling and stable post-cooling phase of Fe5GeTe2. Over wide temperature and field ranges, skyrmionic magnetic bubbles form without preferred chirality, which is indicative of centrosymmetry. These skyrmions can be observed even in the absence of external fields. To understand the complex magnetic order in Fe5GeTe2, we compare macroscopic magnetometry characterization results with microscopic density functional theory and spin-model calculations. Our results show that even up to room temperature, topological spin structures can be stabilized in centrosymmetric van der Waals magnets.

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Li, Xin, Kuangyin Deng, and Benedetta Flebus. "Multitude of exceptional points in van der Waals magnets." Physical Review B 106, no. 21 (December 26, 2022). http://dx.doi.org/10.1103/physrevb.106.214432.

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Rhone, Trevor David, Romakanta Bhattarai, Haralambos Gavras, Bethany Lusch, Misha Salim, Marios Mattheakis, Daniel T. Larson, Yoshiharu Krockenberger, and Efthimios Kaxiras. "Artificial Intelligence Guided Studies of van der Waals Magnets." Advanced Theory and Simulations, April 16, 2023. http://dx.doi.org/10.1002/adts.202300019.

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Chyczewski, Stasiu Thomas, Ji Shi, Hanwool Lee, Paolo Ferrari Furlanetto, Kai Xu, Arend van der Zande, and Wenjuan Zhu. "Probing Antiferromagnetism in Exfoliated Fe₃GeTe₂ using Magneto-transport Measurements." Nanoscale, 2023. http://dx.doi.org/10.1039/d3nr01022h.

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Hu, Lei, Xuming Wu, Yulin Feng, Yuqi Liu, Zhiyuan Xu, and Guoying Gao. "Spin filtering effect, thermal spin diode effect and high tunneling magnetoresistance in the Au/GdI2/Au van der Waals junction." Nanoscale, 2022. http://dx.doi.org/10.1039/d2nr01757a.

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2D van der Waals magnets have been widely concerned in spintronics because of their unique electronic properties, no dangling bonds, and ultra-clean interfaces. However, most of them possess lower Curie...

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Seo, Junho, Eun Su An, Taesu Park, Soo-Yoon Hwang, Gi-Yeop Kim, Kyung Song, Woo-suk Noh, et al. "Tunable high-temperature itinerant antiferromagnetism in a van der Waals magnet." Nature Communications 12, no. 1 (May 14, 2021). http://dx.doi.org/10.1038/s41467-021-23122-y.

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AbstractDiscovery of two dimensional (2D) magnets, showing intrinsic ferromagnetic (FM) or antiferromagnetic (AFM) orders, has accelerated development of novel 2D spintronics, in which all the key components are made of van der Waals (vdW) materials and their heterostructures. High-performing and energy-efficient spin functionalities have been proposed, often relying on current-driven manipulation and detection of the spin states. In this regard, metallic vdW magnets are expected to have several advantages over the widely-studied insulating counterparts, but have not been much explored due to the lack of suitable materials. Here, we report tunable itinerant ferro- and antiferromagnetism in Co-doped Fe4GeTe2 utilizing the vdW interlayer coupling, extremely sensitive to the material composition. This leads to high TN antiferromagnetism of TN ~ 226 K in a bulk and ~210 K in 8 nm-thick nanoflakes, together with tunable magnetic anisotropy. The resulting spin configurations and orientations are sensitively controlled by doping, magnetic field, and thickness, which are effectively read out by electrical conduction. These findings manifest strong merits of metallic vdW magnets as an active component of vdW spintronic applications.

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Wang, Zi-Ao, Xiaomin Zhang, Wenkai Zhu, Faguang Yan, Pengfei Liu, Zhe Yuan, and Kaiyou Wang. "From Negative to Positive Tunnel Magnetoresistance in van der Waals Fe₃GeTe₂/Cr₂Ge₂Te₆/Fe₃GeTe₂ Junctions." Chinese Physics Letters, June 2, 2023. http://dx.doi.org/10.1088/0256-307x/40/7/077201.

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Abstract The emergent van der Waals magnetic material is a promising component for spintronic devices with novel functionalities. Here, we report a transition from negative to positive magnetoresistance in Fe3GeTe2/Cr2Ge2Te6/Fe3GeTe2 van der Waals allmagnetic tunnel junctions with increasing the applied bias voltage. A negative magnetoresistance is observed first in Fe3GeTe2/Cr2Ge2Te6/Fe3GeTe2 tunnel junctions, where the resistance with antiparallel aligned magnetization of two Fe3GeTe2 electrodes is lower than that with parallel alignment, which is due to the opposite spin polarizations of two Fe3GeTe2 electrodes. With the bias voltage increasing, the spin polarization of the biased Fe3GeTe2 electrode is changed so that the spin orientations of two Fe3GeTe2 electrodes are the same. Our experimental observations are supported by the calculated spin-dependent density of states for Fe3GeTe2 electrodes under a finite bias. The significantly bias voltage-dependent spin transport properties in van der Waals magnetic tunnel junctions open a promising route for designing electrical controllable spintronic devices based on van der Waals magnets.

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Song, Tiancheng, Eric Anderson, Matisse Wei-Yuan Tu, Kyle Seyler, Takashi Taniguchi, Kenji Watanabe, Michael A. McGuire, et al. "Spin photovoltaic effect in magnetic van der Waals heterostructures." Science Advances 7, no. 36 (September 3, 2021). http://dx.doi.org/10.1126/sciadv.abg8094.

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Khela, Maya, Maciej Da̧browski, Safe Khan, Paul S. Keatley, Ivan Verzhbitskiy, Goki Eda, Robert J. Hicken, Hidekazu Kurebayashi, and Elton J. G. Santos. "Laser-induced topological spin switching in a 2D van der Waals magnet." Nature Communications 14, no. 1 (March 13, 2023). http://dx.doi.org/10.1038/s41467-023-37082-y.

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AbstractTwo-dimensional (2D) van der Waals (vdW) magnets represent one of the most promising horizons for energy-efficient spintronic applications because their broad range of electronic, magnetic and topological properties. However, little is known about the interplay between light and spin properties in vdW layers. Here we show that ultrafast laser excitation can not only generate different type of spin textures in CrGeTe3 vdW magnets but also induce a reversible transformation between them in a topological toggle switch mechanism. Our atomistic spin dynamics simulations and wide-field Kerr microscopy measurements show that different textures can be generated via high-intense laser pulses within the picosecond regime. The phase transformation between the different topological spin textures is obtained as additional laser pulses are applied to the system where the polarisation and final state of the spins can be controlled by external magnetic fields. Our results indicate laser-driven spin textures on 2D magnets as a pathway towards reconfigurable topological architectures at the atomistic level.

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Fu, ZhuangEn, Piumi I. Samarawickrama, John Ackerman, Yanglin Zhu, Zhiqiang Mao, Kenji Watanabe, Takashi Taniguchi, et al. "Tunneling current-controlled spin states in few-layer van der Waals magnets." Nature Communications 15, no. 1 (May 1, 2024). http://dx.doi.org/10.1038/s41467-024-47820-5.

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AbstractEffective control of magnetic phases in two-dimensional magnets would constitute crucial progress in spintronics, holding great potential for future computing technologies. Here, we report a new approach of leveraging tunneling current as a tool for controlling spin states in CrI3. We reveal that a tunneling current can deterministically switch between spin-parallel and spin-antiparallel states in few-layer CrI3, depending on the polarity and amplitude of the current. We propose a mechanism involving nonequilibrium spin accumulation in the graphene electrodes in contact with the CrI3 layers. We further demonstrate tunneling current-tunable stochastic switching between multiple spin states of the CrI3 tunnel devices, which goes beyond conventional bi-stable stochastic magnetic tunnel junctions and has not been documented in two-dimensional magnets. Our findings not only address the existing knowledge gap concerning the influence of tunneling currents in controlling the magnetism in two-dimensional magnets, but also unlock possibilities for energy-efficient probabilistic and neuromorphic computing.

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Wang, Chong, Yuan Gao, Hongyan Lv, Xiaodong Xu, and Di Xiao. "Stacking Domain Wall Magnons in Twisted van der Waals Magnets." Physical Review Letters 125, no. 24 (December 8, 2020). http://dx.doi.org/10.1103/physrevlett.125.247201.

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Park, Eugene, John P. Philbin, Hang Ch, Joshua J. Sanchez, Connor Occhialini, Georgios Varnavides, Jonathan B. Curtis, et al. "Anisotropic 2D van der Waals Magnets Hosting 1D Spin Chains." Advanced Materials, May 25, 2024. http://dx.doi.org/10.1002/adma.202401534.

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AbstractThe exploration of one‐dimensional (1D) magnetism, frequently portrayed as spin chains, constitutes an actively pursued research field that illuminates fundamental principles in many‐body problems and applications in magnonics and spintronics. The inherent reduction in dimensionality often leads to robust spin fluctuations, impacting magnetic ordering and resulting in novel magnetic phenomena. Here, we explore structural, magnetic, and optical properties of highly anisotropic two‐dimensional (2D) van der Waals antiferromagnets that uniquely host spin chains. First‐principles calculations reveal that the weakest interaction is interchain, essentially leading to 1D magnetic behavior in each layer. With the additional degree of freedom arising from its anisotropic structure, we engineer the structure by alloying, varying the 1D spin chain length using electron beam irradiation, or twisting for localized patterning, and calculate spin textures, predicting robust stability of the antiferromagnetic ordering. Comparing with other spin chain magnets, we anticipate these materials to bring fresh perspectives on harvesting low‐dimensional magnetism.This article is protected by copyright. All rights reserved

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Zollitsch, Christoph W., Safe Khan, Vu Thanh Trung Nam, Ivan A. Verzhbitskiy, Dimitrios Sagkovits, James O’Sullivan, Oscar W. Kennedy, et al. "Probing spin dynamics of ultra-thin van der Waals magnets via photon-magnon coupling." Nature Communications 14, no. 1 (May 5, 2023). http://dx.doi.org/10.1038/s41467-023-38322-x.

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AbstractLayered van der Waals (vdW) magnets can maintain a magnetic order even down to the single-layer regime and hold promise for integrated spintronic devices. While the magnetic ground state of vdW magnets was extensively studied, key parameters of spin dynamics, like the Gilbert damping, crucial for designing ultra-fast spintronic devices, remains largely unexplored. Despite recent studies by optical excitation and detection, achieving spin wave control with microwaves is highly desirable, as modern integrated information technologies predominantly are operated with these. The intrinsically small numbers of spins, however, poses a major challenge to this. Here, we present a hybrid approach to detect spin dynamics mediated by photon-magnon coupling between high-Q superconducting resonators and ultra-thin flakes of Cr2Ge2Te6 (CGT) as thin as 11 nm. We test and benchmark our technique with 23 individual CGT flakes and extract an upper limit for the Gilbert damping parameter. These results are crucial in designing on-chip integrated circuits using vdW magnets and offer prospects for probing spin dynamics of monolayer vdW magnets.

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Wu, Dezhao, Meng Ye, Haowei Chen, Yong Xu, and Wenhui Duan. "Giant and controllable nonlinear magneto-optical effects in two-dimensional magnets." npj Computational Materials 10, no. 1 (April 22, 2024). http://dx.doi.org/10.1038/s41524-024-01266-x.

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AbstractThe interplay of polarization and magnetism in materials with light can create rich nonlinear magneto-optical (NLMO) effects, and the recent discovery of two-dimensional (2D) van der Waals magnets provides remarkable control over NLMO effects due to their superb tunability. Here, based on first-principles calculations, we reported giant NLMO effects in CrI3-based 2D magnets, including a dramatic change of second-harmonics generation (SHG) polarization direction (90°) and intensity (on/off switch) under magnetization reversal and a 100% SHG circular dichroism effect. We further revealed that these effects could not only be used to design ultra-thin multifunctional optical devices but also to detect subtle magnetic orderings. Remarkably, we analytically derived conditions to achieve giant NLMO effects and proposed general strategies to realize them in 2D magnets. Our work not only uncovers a series of intriguing NLMO phenomena but also paves the way for both fundamental research and device applications of ultra-thin NLMO materials.

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Suzuki, Ryuki, Tenghua Gao, Hiroki Nakayama, and Kazuya Ando. "Extrinsic anomalous Hall effect in van der Waals ferromagnet Fe5GeTe2." AIP Advances 13, no. 5 (May 1, 2023). http://dx.doi.org/10.1063/5.0112456.

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We investigate the anomalous Hall effect in a van der Waals material Fe5GeTe2. We find a distinct difference in the temperature dependence of the anomalous Hall effect associated with the evolution of magnetic states in Fe5GeTe2 films. In the low-temperature region, the anomalous Hall conductivity changes with the longitudinal conductivity, which highlights the substantial contribution from the extrinsic mechanism. The extracted skew scattering coefficient in the Fe5GeTe2 films is an order of magnitude larger than that in transition metal ferromagnets. This result sheds light on the role of the extrinsic mechanism in the anomalous Hall effect in van der Waals magnets.

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Tabataba-Vakili, Farsane, Huy P. G. Nguyen, Anna Rupp, Kseniia Mosina, Anastasios Papavasileiou, Kenji Watanabe, Takashi Taniguchi, et al. "Doping-control of excitons and magnetism in few-layer CrSBr." Nature Communications 15, no. 1 (June 3, 2024). http://dx.doi.org/10.1038/s41467-024-49048-9.

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AbstractMagnetism in two-dimensional materials reveals phenomena distinct from bulk magnetic crystals, with sensitivity to charge doping and electric fields in monolayer and bilayer van der Waals magnet CrI3. Within the class of layered magnets, semiconducting CrSBr stands out by featuring stability under ambient conditions, correlating excitons with magnetic order and thus providing strong magnon-exciton coupling, and exhibiting peculiar magneto-optics of exciton-polaritons. Here, we demonstrate that both exciton and magnetic transitions in bilayer and trilayer CrSBr are sensitive to voltage-controlled field-effect charging, exhibiting bound exciton-charge complexes and doping-induced metamagnetic transitions. Moreover, we demonstrate how these unique properties enable optical probes of local magnetic order, visualizing magnetic domains of competing phases across metamagnetic transitions induced by magnetic field or electrostatic doping. Our work identifies few-layer CrSBr as a rich platform for exploring collaborative effects of charge, optical excitations, and magnetism.

Journal articles on the topic 'Van der Waals magnets'

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