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      Room-temperature intrinsic ferromagnetism in epitaxial CrTe 2 ultrathin films

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          Abstract

          While the discovery of two-dimensional (2D) magnets opens the door for fundamental physics and next-generation spintronics, it is technically challenging to achieve the room-temperature ferromagnetic (FM) order in a way compatible with potential device applications. Here, we report the growth and properties of single- and few-layer CrTe 2, a van der Waals (vdW) material, on bilayer graphene by molecular beam epitaxy (MBE). Intrinsic ferromagnetism with a Curie temperature ( T C) up to 300 K, an atomic magnetic moment of ~0.21  \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mu }_{{\rm{B}}}$$\end{document} /Cr and perpendicular magnetic anisotropy (PMA) constant ( K u) of 4.89 × 10 5 erg/cm 3 at room temperature in these few-monolayer films have been unambiguously evidenced by superconducting quantum interference device and X-ray magnetic circular dichroism. This intrinsic ferromagnetism has also been identified by the splitting of majority and minority band dispersions with ~0.2 eV at Г point using angle-resolved photoemission spectroscopy. The FM order is preserved with the film thickness down to a monolayer ( T C ~ 200 K), benefiting from the strong PMA and weak interlayer coupling. The successful MBE growth of 2D FM CrTe 2 films with room-temperature ferromagnetism opens a new avenue for developing large-scale 2D magnet-based spintronics devices.

          Abstract

          The emergence of two dimensional ferromagnetism suffers from an inherent fragility to thermal fluctuations, which typically restricts the Curie temperature to below room temperature. Here, Zhang et al present CrTe 2 thin films grown via molecular beam epitaxy with a Curie temperature exceeding 300 K.

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          A perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction.

          Magnetic tunnel junctions (MTJs) with ferromagnetic electrodes possessing a perpendicular magnetic easy axis are of great interest as they have a potential for realizing next-generation high-density non-volatile memory and logic chips with high thermal stability and low critical current for current-induced magnetization switching. To attain perpendicular anisotropy, a number of material systems have been explored as electrodes, which include rare-earth/transition-metal alloys, L1(0)-ordered (Co, Fe)-Pt alloys and Co/(Pd, Pt) multilayers. However, none of them so far satisfy high thermal stability at reduced dimension, low-current current-induced magnetization switching and high tunnel magnetoresistance ratio all at the same time. Here, we use interfacial perpendicular anisotropy between the ferromagnetic electrodes and the tunnel barrier of the MTJ by employing the material combination of CoFeB-MgO, a system widely adopted to produce a giant tunnel magnetoresistance ratio in MTJs with in-plane anisotropy. This approach requires no material other than those used in conventional in-plane-anisotropy MTJs. The perpendicular MTJs consisting of Ta/CoFeB/MgO/CoFeB/Ta show a high tunnel magnetoresistance ratio, over 120%, high thermal stability at dimension as low as 40 nm diameter and a low switching current of 49 microA.
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            Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit

            Since the discovery of graphene, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin–valley coupling, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-density waves, and topological semimetals with edge transport. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered; such a crystal would be useful in many technologies from sensing to data storage. Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin–Wagner theorem. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 kelvin is only slightly lower than that of the bulk crystal, 61 kelvin, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent physical properties typical of van der Waals crystals. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect, whereas in trilayer CrI3 the interlayer ferromagnetism observed in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as electrical control for realizing magnetoelectronics, and van der Waals engineering to produce interface phenomena.
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              Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals

              The realization of long-range ferromagnetic order in two-dimensional van der Waals crystals, combined with their rich electronic and optical properties, could lead to new magnetic, magnetoelectric and magneto-optic applications. In two-dimensional systems, the long-range magnetic order is strongly suppressed by thermal fluctuations, according to the Mermin–Wagner theorem; however, these thermal fluctuations can be counteracted by magnetic anisotropy. Previous efforts, based on defect and composition engineering, or the proximity effect, introduced magnetic responses only locally or extrinsically. Here we report intrinsic long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers, as revealed by scanning magneto-optic Kerr microscopy. In this magnetically soft, two-dimensional van der Waals ferromagnet, we achieve unprecedented control of the transition temperature (between ferromagnetic and paramagnetic states) using very small fields (smaller than 0.3 tesla). This result is in contrast to the insensitivity of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field leads to an effective anisotropy that is much greater than the near-zero magnetocrystalline anisotropy, opening up a large spin-wave excitation gap. We explain the observed phenomenon using renormalized spin-wave theory and conclude that the unusual field dependence of the transition temperature is a hallmark of soft, two-dimensional ferromagnetic van der Waals crystals. Cr2Ge2Te6 is a nearly ideal two-dimensional Heisenberg ferromagnet and so will be useful for studying fundamental spin behaviours, opening the door to exploring new applications such as ultra-compact spintronics.
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                Author and article information

                Contributors
                heliang@nju.edu.cn
                rzhang@nju.edu.cn
                biang@missouri.edu
                ybxu@nju.edu.cn
                Journal
                Nat Commun
                Nat Commun
                Nature Communications
                Nature Publishing Group UK (London )
                2041-1723
                3 May 2021
                3 May 2021
                2021
                : 12
                : 2492
                Affiliations
                [1 ]GRID grid.41156.37, ISNI 0000 0001 2314 964X, Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, , Nanjing University, ; Nanjing, China
                [2 ]GRID grid.134936.a, ISNI 0000 0001 2162 3504, Department of Physics and Astronomy, , University of Missouri, ; Columbia, MO USA
                [3 ]GRID grid.4970.a, ISNI 0000 0001 2188 881X, Department of Electronic Engineering, , Royal Holloway University of London, ; Egham, Surrey UK
                [4 ]GRID grid.453246.2, ISNI 0000 0004 0369 3615, New Energy Technology Engineering Laboratory of Jiangsu Provence & School of Science, , Nanjing University of Posts and Telecommunications, ; Nanjing, China
                [5 ]GRID grid.134936.a, ISNI 0000 0001 2162 3504, Department of Chemistry, , University of Missouri, ; Columbia, MO USA
                [6 ]GRID grid.64523.36, ISNI 0000 0004 0532 3255, Department of Physics, , National Cheng Kung University, ; Tainan, Taiwan
                [7 ]Center for Quantum Frontiers of Research and Technology (QFort), Tainan, Taiwan
                [8 ]GRID grid.134936.a, ISNI 0000 0001 2162 3504, Electron Microscopy Core Facility, , University of Missouri, ; Columbia, MO USA
                [9 ]GRID grid.134936.a, ISNI 0000 0001 2162 3504, Department of Mechanical and Aerospace Engineering, , University of Missouri, ; Columbia, MO USA
                [10 ]GRID grid.41156.37, ISNI 0000 0001 2314 964X, National Laboratory of Solid State Microstructures and Department of Physics, , Nanjing University, ; Nanjing, China
                [11 ]GRID grid.5685.e, ISNI 0000 0004 1936 9668, York-Nanjing Joint Centre (YNJC) for Spintronics and Nano Engineering, Department of Electronic Engineering, , The University of York, ; York, UK
                Author information
                http://orcid.org/0000-0001-7379-9894
                http://orcid.org/0000-0001-7750-1485
                http://orcid.org/0000-0003-1222-2527
                http://orcid.org/0000-0002-5279-0097
                http://orcid.org/0000-0003-0015-6331
                http://orcid.org/0000-0002-8793-9023
                http://orcid.org/0000-0002-7823-0725
                Article
                22777
                10.1038/s41467-021-22777-x
                8093203
                33941773
                46d3b8f1-4f4d-499f-827b-7ccab987c138
                © The Author(s) 2021

                Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

                History
                : 16 August 2020
                : 22 March 2021
                Funding
                Funded by: FundRef https://doi.org/10.13039/501100001809, National Natural Science Foundation of China (National Science Foundation of China);
                Award ID: 61427812
                Award Recipient :
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                © The Author(s) 2021

                Uncategorized
                two-dimensional materials,magnetic properties and materials
                Uncategorized
                two-dimensional materials, magnetic properties and materials

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