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1. Strain-mediated voltage-controlled switching of magnetic skyrmions in nanostructures

   https://doi.org/10.1038/s41524-018-0119-2  

   Hu, Jia-Mian ; Yang, Tiannan ; Chen, Long-Qing ( November 2018 , npj Computational Materials)

   Magnetic skyrmions are swirling spin structures stabilized typically by the Dyzaloshinskii-Moriya interaction. The existing control of magnetic skyrmions has often relied on the use of an electric current, which may cause overheating in densely packed devices. Here we demonstrate, using phase-field simulations, that an isolated Néel skyrmion in a magnetic nanodisk can be repeatedly created and deleted by voltage-induced strains from a juxtaposed piezoelectric. Such a skyrmion switching is non-volatile, and consumes only ~0.5 fJ per switching which is about five orders of magnitude smaller than that by current-induced spin-transfer-torques. It is found that the strain-mediated skyrmion creation occurs through an intermediate vortex-like spin structure, and that the skyrmion deletion occurs though a homogenous shrinkage during which the Néel wall is temporarily transformed to a vortex-wall. These findings are expected to stimulate experimental research into strain-mediated voltage control of skyrmions, as well as other chiral spin structures for low-power spintronics.

    

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2. Ultrafast microscopy of a twisted plasmonic spin skyrmion

   https://doi.org/10.1063/5.0084482  

   Dai, Yanan ; Zhou, Zhikang ; Ghosh, Atreyie ; Kapoor, Karan ; Dąbrowski, Maciej ; Kubo, Atsushi ; Huang, Chen-Bin ; Petek, Hrvoje ( March 2022 , Applied Physics Reviews)

   We report a transient plasmonic spin skyrmion topological quasiparticle within surface plasmon polariton vortices, which is described by analytical modeling and imaging of its formation by ultrafast interferometric time-resolved photoemission electron microscopy. Our model finds a twisted skyrmion spin texture on the vacuum side of a metal/vacuum interface and its integral opposite counterpart in the metal side. The skyrmion pair forming a hedgehog texture is associated with co-gyrating anti-parallel electric and magnetic fields, which form intense pseudoscalar E·B focus that breaks the local time-reversal symmetry and can drive magnetoelectric responses of interest to the axion physics. Through nonlinear two-photon photoemission, we record attosecond precision images of the plasmonic vectorial vortex field evolution with nanometer spatial and femtosecond temporal (nanofemto) resolution, from which we derive the twisted plasmonic spin skyrmion topological textures, their boundary, and topological charges; the modeling and experimental measurements establish a quantized integer photonic topological charge that is stable over the optical generation pulse envelope.

    

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3. Multi-criticality and field induced non-BEC transition in frustrated magnets

   Sur, Shouvik ; Xu, Yi ; Li, Shuyi ; Gong, Shou-Shu ; Nevidomskyy, Andriy H. ( November 2022 , arXivorg)

   Frustrated spin-systems have traditionally proven challenging to understand, owing to a scarcity of controlled methods for their analyses. By contrast, under strong magnetic fields, certain aspects of spin systems admit simpler and universal description in terms of hardcore bosons. The bosonic formalism is anchored by the phenomenon of Bose-Einstein condensation (BEC), which has helped explain the behaviors of a wide range of magnetic compounds under applied magnetic fields. Here, we focus on the interplay between frustration and externally applied magnetic field to identify instances where the BEC paradigm is no longer applicable. As a representative example, we consider the antiferromagnetic J1−J2−J3 model on the square lattice in the presence of a uniform external magnetic field, and demonstrate that the frustration-driven suppression of the Néel order leads to a Lifshitz transition for the hardcore bosons. In the vicinity of the Lifshitz point, the physics becomes unmoored from the BEC paradigm, and the behavior of the system, both at and below the saturation field, is controlled by a Lifshitz multicritical point. We obtain the resultant universal scaling behaviors, and provide strong evidence for the existence of a frustration and magnetic-field driven correlated bosonic liquid state along the entire phase boundary separating the Néel phase from other magnetically ordered states. 

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

   https://doi.org/10.1038/s41467-018-05158-9  

   Dovzhenko, Y. ; Casola, F. ; Schlotter, S. ; Zhou, T. X. ; Büttner, F. ; Walsworth, R. L. ; Beach, G. S. D. ; Yacoby, A. ( July 2018 , Nature Communications)

   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.

    

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5. Eigenmodes of 1-kink skyrmions in cylindrical magnetic discs

   https://doi.org/10.1016/j.jmmm.2024.171761  

   Staggers, Trae L. ; Pollard, Shawn D. ( February 2024 , Journal of Magnetism and Magnetic Materials)

   The stability and resonance spectra associated with a domain wall skyrmion embedded within a Néel skyrmion, forming a 1-kink skyrmion, has been studied using micromagnetic simulations. We show that the 1-kink skyrmion is stable over a wide range of fields at moderate strengths of the Dzyaloshinskii-Moriya interaction. By exciting these structures with a broadband magnetic field excitation, we find complex resonance behavior deviating from that of a pure Néel skyrmion. For out-of-plane excitations, the 1-kink skyrmion demonstrates an additional mode relative to that of the Néel skyrmion at reduced amplitude. These consist of low frequency and high frequency modes associated with both a breathing mode and an oscillation of the embedded domain wall skyrmion. Following an in-plane excitation, both Néel and 1-kink skyrmions exhibit a counterclockwise rotational mode with similar frequencies, as well as a higher frequency mode associated with the interaction of the structure with the ferromagnetic background state. These results may help pave the way for exploration of more complex spin structures for magnetic-based microwave devices. 

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