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      Magnetostatic twists in room-temperature skyrmions explored by nitrogen-vacancy center spin texture reconstruction

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          Abstract

          Magnetic skyrmions are two-dimensional non-collinear spin textures characterized by an integer topological number. Room-temperature skyrmions were recently found in magnetic multilayer stacks, where their stability was largely attributed to the interfacial Dzyaloshinskii–Moriya interaction. The strength of this interaction and its role in stabilizing the skyrmions is not yet well understood, and imaging of the full spin structure is needed to address this question. Here, we use a nitrogen-vacancy centre in diamond to measure a map of magnetic fields produced by a skyrmion in a magnetic multilayer under ambient conditions. We compute the manifold of candidate spin structures and select the physically meaningful solution. We find a Néel-type skyrmion whose chirality is not left-handed, contrary to preceding reports. We propose skyrmion tube-like structures whose chirality rotates through the film thickness. We show that NV magnetometry, combined with our analysis method, provides a unique tool to investigate this previously inaccessible phenomenon.

          Abstract

          The Dzyaloshinskii-Moriya interaction (DMI) is crucial to the stabilization of skyrmions but the contribution is not well understood. Here, the authors provide a methodology using the single electron spin of a nitrogen-vacancy center to image the fine structure of skyrmions which is attributed to the competition between the DMI and stray fields.

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          Real-space observation of a two-dimensional skyrmion crystal.

          Crystal order is not restricted to the periodic atomic array, but can also be found in electronic systems such as the Wigner crystal or in the form of orbital order, stripe order and magnetic order. In the case of magnetic order, spins align parallel to each other in ferromagnets and antiparallel in antiferromagnets. In other, less conventional, cases, spins can sometimes form highly nontrivial structures called spin textures. Among them is the unusual, topologically stable skyrmion spin texture, in which the spins point in all the directions wrapping a sphere. The skyrmion configuration in a magnetic solid is anticipated to produce unconventional spin-electronic phenomena such as the topological Hall effect. The crystallization of skyrmions as driven by thermal fluctuations has recently been confirmed in a narrow region of the temperature/magnetic field (T-B) phase diagram in neutron scattering studies of the three-dimensional helical magnets MnSi (ref. 17) and Fe(1-x)Co(x)Si (ref. 22). Here we report real-space imaging of a two-dimensional skyrmion lattice in a thin film of Fe(0.5)Co(0.5)Si using Lorentz transmission electron microscopy. With a magnetic field of 50-70 mT applied normal to the film, we observe skyrmions in the form of a hexagonal arrangement of swirling spin textures, with a lattice spacing of 90 nm. The related T-B phase diagram is found to be in good agreement with Monte Carlo simulations. In this two-dimensional case, the skyrmion crystal seems very stable and appears over a wide range of the phase diagram, including near zero temperature. Such a controlled nanometre-scale spin topology in a thin film may be useful in observing unconventional magneto-transport effects.
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            Skyrmions on the track.

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              Skyrmion Lattice in a Chiral Magnet

              Skyrmions represent topologically stable field configurations with particle-like properties. We used neutron scattering to observe the spontaneous formation of a two-dimensional lattice of skyrmion lines, a type of magnetic vortices, in the chiral itinerant-electron magnet MnSi. The skyrmion lattice stabilizes at the border between paramagnetism and long-range helimagnetic order perpendicular to a small applied magnetic field regardless of the direction of the magnetic field relative to the atomic lattice. Our study experimentally establishes magnetic materials lacking inversion symmetry as an arena for new forms of crystalline order composed of topologically stable spin states.
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                Author and article information

                Contributors
                yacoby@physics.harvard.edu
                Journal
                Nat Commun
                Nat Commun
                Nature Communications
                Nature Publishing Group UK (London )
                2041-1723
                13 July 2018
                13 July 2018
                2018
                : 9
                : 2712
                Affiliations
                [1 ]ISNI 000000041936754X, GRID grid.38142.3c, Department of Physics, , Harvard University, ; 17 Oxford Street, Cambridge, MA 02138 USA
                [2 ]GRID grid.455754.2, Harvard-Smithsonian Center for Astrophysics, ; 60 Garden Street, Cambridge, MA 02138 USA
                [3 ]ISNI 000000041936754X, GRID grid.38142.3c, John A. Paulson School of Engineering and Applied Sciences, , Harvard University, ; Cambridge, MA 02138 USA
                [4 ]ISNI 0000 0001 2341 2786, GRID grid.116068.8, Department of Materials Science and Engineering, , Massachusetts Institute of Technology, ; Cambridge, MA 02139 USA
                Author information
                http://orcid.org/0000-0002-3341-9946
                http://orcid.org/0000-0002-8451-3385
                http://orcid.org/0000-0002-6204-9948
                http://orcid.org/0000-0001-9158-7430
                Article
                5158
                10.1038/s41467-018-05158-9
                6045603
                30006532
                bf350ef7-b7a5-4c61-9972-b28976e09070
                © The Author(s) 2018

                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
                : 12 December 2017
                : 1 May 2018
                Funding
                Funded by: FundRef https://doi.org/10.13039/100000936, Gordon and Betty Moore Foundation;
                Award ID: GBMF4531
                Award Recipient :
                Funded by: FundRef https://doi.org/10.13039/100000015, U.S. Department of Energy (DOE);
                Award ID: DE-SC0012371
                Award Recipient :
                Funded by: FundRef https://doi.org/10.13039/501100001711, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (Swiss National Science Foundation);
                Award ID: P300P2-158417
                Award Recipient :
                Funded by: FundRef https://doi.org/10.13039/100000001, National Science Foundation (NSF);
                Award ID: DGE1144152
                Award Recipient :
                Funded by: FundRef https://doi.org/10.13039/501100001659, Deutsche Forschungsgemeinschaft (German Research Foundation);
                Award ID: BU 3297/1-1
                Award Recipient :
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