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1. Magnetic Skyrmions with Unconventional Helicity Polarization in a Van Der Waals Ferromagnet

   https://doi.org/10.1002/adma.202204163  

   Zhang, Chenhui ; Liu, Chen ; Zhang, Senfu ; Zhou, Bojian ; Guan, Chaoshuai ; Ma, Yinchang ; Algaidi, Hanin ; Zheng, Dongxing ; Li, Yan ; He, Xin ; et al ( September 2022 , Advanced Materials)

   Skyrmion helicity, which defines the spin swirling direction, is a fundamental parameter that may be utilized to encode data bits in future memory devices. Generally, in centrosymmetric ferromagnets, dipole skyrmions with helicity of −π/2 and π/2 are degenerate in energy, leading to equal populations of both helicities. On the other hand, in chiral materials where the Dzyaloshinskii–Moriya interaction (DMI) prevails and the dipolar interaction is negligible, only a preferred helicity is selected by the type of DMI. However, whether there is a rigid boundary between these two regimes remains an open question. Herein, the observation of dipole skyrmions with unconventional helicity polarization in a van der Waals ferromagnet, Fe_5−δGeTe₂, is reported. Combining magnetometry, Lorentz transmission electron microscopy, electrical transport measurements, and micromagnetic simulations, the short‐range superstructures in Fe_5−δGeTe₂resulting in a localized DMI contribution, which breaks the degeneracy of the opposite helicities and leads to the helicity polarization, is demonstrated. Therefore, the helicity feature in Fe_5−δGeTe₂is controlled by both the dipolar interaction and DMI that the former leads to Bloch‐type skyrmions with helicity of ±π/2 whereas the latter breaks the helicity degeneracy. This work provides new insights into the skyrmion topology in van der Waals materials.

    

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2. Ferrimagnetic Skyrmions in Topological Insulator/Ferrimagnet Heterostructures

   https://doi.org/10.1002/adma.202003380  

   Wu, Hao ; Groß, Felix ; Dai, Bingqian ; Lujan, David ; Razavi, Seyed Armin ; Zhang, Peng ; Liu, Yuxiang ; Sobotkiewich, Kemal ; Förster, Johannes ; Weigand, Markus ; et al ( July 2020 , Advanced Materials)

   Magnetic skyrmions are topologically nontrivial chiral spin textures that have potential applications in next‐generation energy‐efficient and high‐density spintronic devices. In general, the chiral spins of skyrmions are stabilized by the noncollinear Dzyaloshinskii–Moriya interaction (DMI), originating from the inversion symmetry breaking combined with the strong spin–orbit coupling (SOC). Here, the strong SOC from topological insulators (TIs) is utilized to provide a large interfacial DMI in TI/ferrimagnet heterostructures at room temperature, resulting in small‐size (radius ≈ 100 nm) skyrmions in the adjacent ferrimagnet. Antiferromagnetically coupled skyrmion sublattices are observed in the ferrimagnet by element‐resolved scanning transmission X‐ray microscopy, showing the potential of a vanishing skyrmion Hall effect and ultrafast skyrmion dynamics. The line‐scan spin profile of the single skyrmion shows a Néel‐type domain wall structure and a 120 nm size of the 180° domain wall. This work demonstrates the sizable DMI and small skyrmions in TI‐based heterostructures with great promise for low‐energy spintronic devices.

    

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3. 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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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. Assembling Diverse Skyrmionic Phases in Fe 3 GeTe 2 Monolayers

   https://doi.org/10.1002/adma.202107779  

   Xu, Changsong ; Li, Xueyang ; Chen, Peng ; Zhang, Yun ; Xiang, Hongjun ; Bellaiche, Laurent ( February 2022 , Advanced Materials)

   Skyrmionic magnetic states are promising in advanced spintronics. This topic is experiencing recent progress in 2D magnets, with, for example, a near 300 K Curie temperature observed in Fe₃GeTe₂. However, despite previous studies reporting skyrmions in Fe₃GeTe₂, such a system remains elusive, since it has been reported to host either Néel‐type or Bloch‐type textures, while a net Dzyaloshinskii–Moriya interaction (DMI) cannot occur in this compound for symmetry reasons. It is thus desirable to develop an accurate model to deeply understand Fe₃GeTe₂. Here, a newly developed method adopting spin invariants is applied to build a first‐principle‐based Hamiltonian, which predicts colorful topological defects assembled from the unit of Bloch lines, and reveals the critical role of specific forms of fourth‐order interactions in Fe₃GeTe₂. Rather than the DMI, it is the multiple fourth‐order interactions, with symmetry and spin–orbit couplings considered, that stabilize both Néel‐type and Bloch‐type skyrmions, as well as antiskyrmions, without any preference for clockwise versus counterclockwise spin rotation. This study also demonstrates that spin invariants can be used as a general approach to study complex magnetic interactions.

    

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