| 1. |
- Kampfrath, T., et al.
(author)
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Terahertz spin current pulses controlled by magnetic heterostructures
- 2013
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In: Nature Nanotechnology. - 1748-3387 .- 1748-3395. ; 8:4, s. 256-260
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Journal article (peer-reviewed)abstract
- In spin-based electronics, information is encoded by the spin state of electron bunches(1-4). Processing this information requires the controlled transport of spin angular momentum through a solid(5,6), preferably at frequencies reaching the so far unexplored terahertz regime(7-9). Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is used to drive spins(10-12) from a ferromagnetic iron thin film into a non-magnetic cap layer that has either low (ruthenium) or high (gold) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter(13) based on the inverse spin Hall effect(14,15), which converts the spin flow into a terahertz electromagnetic pulse. We find that the ruthenium cap layer yields a considerably longer spin current pulse because electrons are injected into ruthenium d states, which have a much lower mobility than gold sp states(16). Thus, spin current pulses and the resulting terahertz transients can be shaped by tailoring magnetic heterostructures, which opens the door to engineering high-speed spintronic devices and, potentially, broadband terahertz emitters(7-9).
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| 2. |
- Kuchkin, Vladyslav M., et al.
(author)
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Heliknoton in a film of cubic chiral magnet
- 2023
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In: Frontiers in Physics. - : FRONTIERS MEDIA SA. - 2296-424X. ; 11
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Journal article (peer-reviewed)abstract
- Cubic chiral magnets exhibit a remarkable diversity of two-dimensional topological magnetic textures, including skyrmions. However, the experimental confirmation of topological states localized in all three spatial dimensions remains challenging. In this paper, we investigate a three-dimensional topological state called a heliknoton, which is a hopfion embedded into a helix or conic background. We explore the range of parameters at which the heliknoton can be stabilized under realistic conditions using micromagnetic modeling, harmonic transition state theory, and stochastic spin dynamics simulations. We present theoretical Lorentz TEM images of the heliknoton, which can be used for experimental comparison. Additionally, we discuss the stability of the heliknoton at finite temperatures and the mechanism of its collapse. Our study offers a pathway for future experimental investigations of three-dimensional topological solitons in magnetic crystals.
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| 3. |
- Peters, L., et al.
(author)
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Correlation effects and orbital magnetism of Co clusters
- 2016
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In: PHYSICAL REVIEW B. - 2469-9950. ; 93:22
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Journal article (peer-reviewed)abstract
- Recent experiments on isolated Co clusters have shown huge orbital magnetic moments in comparison with their bulk and surface counterparts. These clusters hence provide the unique possibility to study the evolution of the orbital magnetic moment with respect to the cluster size and how competing interactions contribute to the quenching of orbital magnetism. We investigate here different theoretical methods to calculate the spin and orbital moments of Co clusters, and assess the performances of the methods in comparison with experiments. It is shown that density-functional theory in conventional local density or generalized gradient approximations, or even with a hybrid functional, severely underestimates the orbital moment. As natural extensions/corrections, we considered the orbital polarization correction, the LDA+U approximation as well as the LDA+DMFT method. Our theory shows that of the considered methods, only the LDA+DMFT method provides orbital moments in agreement with experiment, thus emphasizing the importance of dynamic correlations effects for determining fundamental magnetic properties of magnets in the nanosize regime.
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| 4. |
- Savchenko, Andrii S., et al.
(author)
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Chiral standing spin waves in skyrmion lattice
- 2022
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In: APL Materials. - : AIP Publishing. - 2166-532X. ; 10:7
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Journal article (peer-reviewed)abstract
- This work studies the resonance excitations of the three-dimensional skyrmions lattice in the finite thickness plate of an isotropic chiral magnet using spin dynamics simulations. We found that the absorption spectra and resonance modes differ from those predicted by the two-dimensional model and the model of the unconfined bulk crystal. The features observed on the spectra can be explained by the formation of chiral standing spin waves, which, contrary to conventional standing spin waves, are characterized by the helical profile of dynamic magnetization of fixed chirality that is defined by the Dzyaloshinskii-Moriya interaction. In this case, the dynamic susceptibility becomes a function of the plate thickness, which gives rise to an interesting effect that manifests itself in periodical fading of the intensity of corresponding modes and makes excitation of these modes impossible at specific thicknesses.
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| 5. |
- Savchenko, Andrii S., et al.
(author)
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Diversity of states in a chiral magnet nanocylinder
- 2022
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In: APL Materials. - : American Institute of Physics (AIP). - 2166-532X. ; 10:6
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Journal article (peer-reviewed)abstract
- The diversity of three-dimensional magnetic states in a FeGe nanocylinder is studied using micromagnetic simulations and off-axis electron holography in the transmission electron microscope. In particular, we report the observation of a dipole string-a spin texture composed of two coupled Bloch points-which becomes stable under geometrical confinement. Quantitative agreement is obtained between experimental and theoretical phase shift images by taking into account the presence of a damaged layer on the surface of the nanocylinder. The theoretical model is based on the assumption that the damaged surface layer, which results from focused ion beam milling during sample preparation, has similar magnetic properties to those of an amorphous FeGe alloy. The results highlight the importance of considering the magnetic properties of the surface layers of such nanoscale samples, which influence their magnetic states.
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| 6. |
- Du, Haifeng, et al.
(author)
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Interaction of Individual Skyrmions in a Nanostructured Cubic Chiral Magnet
- 2018
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In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 120:19
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Journal article (peer-reviewed)abstract
- We report direct evidence of the field-dependent character of the interaction between individual magnetic skyrmions as well as between skyrmions and edges in B20-type FeGe nanostripes observed by means of high-resolution Lorentz transmission electron microscopy. It is shown that above certain critical values of an external magnetic field the character of such long-range skyrmion interactions changes from attraction to repulsion. Experimentally measured equilibrium inter-skyrmion and skyrmion-edge distances as a function of the applied magnetic field shows quantitative agreement with the results of micromagnetic simulations. The important role of demagnetizing fields and the internal symmetry of three-dimensional magnetic skyrmions are discussed in detail.
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| 7. |
- Jakobsson, Adam, et al.
(author)
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Tuning the Curie temperature of FeCo compounds by tetragonal distortion
- 2013
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 103:10, s. 102404-
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Journal article (peer-reviewed)abstract
- Combining density-functional theory calculations with a classical Monte Carlo method, we show that for B2-type FeCo compounds, tetragonal distortion gives rise to a strong reduction of the Curie temperature T-C. The T-C monotonically decreases from 1575K (for c/a = 1) to 940K (for c/a = root 2). We find that the nearest neighbor Fe-Co exchange interaction is sufficient to explain the c/a behavior of the T-C. Combination of high magnetocrystalline anisotropy energy with a moderate TC value suggests tetragonal FeCo grown on the Rh substrate with c/a = 1.24 to be a promising material for heat-assisted magnetic recording applications.
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| 8. |
- Jin, Chiming, et al.
(author)
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Control of morphology and formation of highly geometrically confined magnetic skyrmions
- 2017
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In: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 8
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Journal article (peer-reviewed)abstract
- The ability to controllably manipulate magnetic skyrmions, small magnetic whirls with particle-like properties, in nanostructured elements is a prerequisite for incorporating them into spintronic devices. Here, we use state-of-the-art electron holographic imaging to directly visualize the morphology and nucleation of magnetic skyrmions in a wedge-shaped FeGe nanostripe that has a width in the range of 45-150 nm. We find that geometrically-confined skyrmions are able to adopt a wide range of sizes and ellipticities in a nanostripe that are absent in both thin films and bulk materials and can be created from a helical magnetic state with a distorted edge twist in a simple and efficient manner. We perform a theoretical analysis based on a three-dimensional general model of isotropic chiral magnets to confirm our experimental results. The flexibility and ease of formation of geometrically confined magnetic skyrmions may help to optimize the design of skyrmion-based memory devices.
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| 9. |
- Kuchkin, Vladyslav M., et al.
(author)
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Geometry and symmetry in skyrmion dynamics
- 2021
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In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 104:16
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Journal article (peer-reviewed)abstract
- The uniform motion of chiral magnetic skyrmions induced by a spin-transfer torque displays an intricate dependence on the skyrmions' topological charge and shape. We reveal surprising patterns in this dependence through simulations of the Landau-Lifshitz-Gilbert equation with Zhang-Li torque and explain them through a geometric analysis of Thiele's equation. Our results provide a universal geometrical understanding of the dependence of the skyrmion's velocity and Hall angle on the skyrmion's topological charge, shape, and orientation. The generality of our approach suggests the validity of our results for exchange-frustrated magnets, bubble materials, and other materials.
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| 10. |
- Kuchkin, Vladyslav M., et al.
(author)
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Magnetic skyrmions, chiral kinks, and holomorphic functions
- 2020
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In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 102:14
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Journal article (peer-reviewed)abstract
- We present a novel approach to understanding the extraordinary diversity of magnetic skyrmion solutions. Our approach combines an original classification scheme with efficient analytical and numerical methods. We introduce the concept of chiral kinks to account for regions of disfavored chirality in spin textures, and classify two-dimensional magnetic skyrmions in terms of closed domain walls carrying such chiral kinks In particular, we show that the topological charge of magnetic skyrmions can be expressed in terms of the constituent closed domain walls and chiral kinks Guided by our classification scheme, we propose a method for creating hitherto unknown magnetic skyrmions which involves initial spin configurations formulated in terms of holomorphic functions and subsequent numerical energy minimization We numerically study the stability of the resulting magnetic skyrmions for a range of external fields and anisotropy parameters, and provide quantitative estimates of the stability range for the whole variety of skyrmions with kinks We show that the parameters limiting this range can be well described in terms of the relative energies of particular skyrmion solutions and isolated stripes with and without chiral kinks.
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