| 1. |
- Turenne, Diego, et al.
(författare)
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Nonequilibrium sub–10 nm spin-wave soliton formation in FePt nanoparticles
- 2022
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 8:13
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic nanoparticles such as FePt in the L10 phase are the bedrock of our current data storage technology. As the grains become smaller to keep up with technological demands, the superparamagnetic limit calls for materials with higher magnetocrystalline anisotropy. This, in turn, reduces the magnetic exchange length to just a few nanometers, enabling magnetic structures to be induced within the nanoparticles. Here, we describe the existence of spin-wave solitons, dynamic localized bound states of spin-wave excitations, in FePt nanoparticles. We show with time-resolved x-ray diffraction and micromagnetic modeling that spin-wave solitons of sub–10 nm sizes form out of the demagnetized state following femtosecond laser excitation. The measured soliton spin precession frequency of 0.1 THz positions this system as a platform to develop novel miniature devices.
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| 2. |
- Jangid, Rahul, et al.
(författare)
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Extreme Domain Wall Speeds under Ultrafast Optical Excitation
- 2023
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 131:25
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved ultrafast EUV magnetic scattering was used to test a recent prediction of > 10 km/s domain wall speeds by optically exciting a magnetic sample with a nanoscale labyrinthine domain pattern. Ultrafast distortion of the diffraction pattern was observed at markedly different timescales compared to the magnetization quenching. The diffraction pattern distortion shows a threshold dependence with laser fluence, not seen for magnetization quenching, consistent with a picture of domain wall motion with pinning sites. Supported by simulations, we show that a speed of approximate to 66 km/s for highly curved domain walls can explain the experimental data. While our data agree with the prediction of extreme, nonequilibrium wall speeds locally, it differs from the details of the theory, suggesting that additional mechanisms are required to fully understand these effects.
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| 3. |
- Pancaldi, Matteo, 1989-, et al.
(författare)
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Spiral metamaterials for terahertz magnetic field enhancement
- 2019
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Ingår i: META 2019 Lisbon - Portugal. - : META Conference. ; , s. 1343-1344
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Konferensbidrag (refereegranskat)abstract
- We designed a class of spiral thin-film antennas for enhancing, in the near field, the incident terahertz (THz) magnetic field. Indeed, using existing laser-based THz sources, our metamaterial geometry allows generating magnetic fields of the order of 2 T over a time scale of few picoseconds, enabling the investigation of nonlinear ultrafast spin dynamics in table-top experiments.
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| 4. |
- Zhou Hagström, Nanna, 1993-, et al.
(författare)
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Megahertz-rate Ultrafast X-ray Scattering and Holographic Imaging at the European XFEL
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence, and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, we present the results from the first megahertz repetition rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL. We illustrate the experimental capabilities that the SCS instrument offers, resulting from the operation at MHz repetition rates and the availability of the novel DSSC 2D imaging detector. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative examples, providing an ideal test-bed for operation at megahertz rates. Nevertheless, our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range.
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| 5. |
- Zhou Hagström, Nanna, 1993-, et al.
(författare)
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Megahertz-rate ultrafast X-ray scattering and holographic imaging at the European XFEL
- 2022
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Ingår i: Journal of Synchrotron Radiation. - : International Union of Crystallography (IUCr). - 0909-0495 .- 1600-5775. ; 29, s. 1454-1464
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Tidskriftsartikel (refereegranskat)abstract
- The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, results from the first megahertz-repetition-rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL are presented. The experimental capabilities that the SCS instrument offers, resulting from the operation at megahertz repetition rates and the availability of the novel DSSC 2D imaging detector, are illustrated. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative, providing an ideal test-bed for operation at megahertz rates. Our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range.
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| 6. |
- Zhou Hagström, Nanna, 1993-, et al.
(författare)
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Symmetry-dependent ultrafast manipulation of nanoscale magnetic domains
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 106:22
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Tidskriftsartikel (refereegranskat)abstract
- Femtosecond optical pumping of magnetic materials has been used to achieve ultrafast switching and recently to nucleate symmetry-broken magnetic states. However, when the magnetic order parameter already presents a broken-symmetry state, such as a domain pattern, the dynamics are poorly understood and consensus remains elusive. Here, we resolve the controversies in the literature by studying the ultrafast response of magnetic domain patterns with varying degrees of translation symmetry with ultrafast x-ray resonant scattering. A data analysis technique is introduced to disentangle the isotropic and anisotropic components of the x-ray scattering. We find that the scattered intensity exhibits a radial shift restricted to the isotropic component, indicating that the far-from-equilibrium magnetization dynamics are intrinsically related to the spatial features of the domain pattern. Our results suggest alternative pathways for the spatiotemporal manipulation of magnetism via far-from-equilibrium dynamics and by carefully tuning the ground-state magnetic textures.
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| 7. |
- Zhou Hagström, Nanna, 1993-
(författare)
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Ultrafast spin dynamics at the nanoscale : using coherent X-ray and terahertz radiation
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The field of ultrafast magnetism is driven by the growing need for faster and more efficient magnetic data storage, which comprises the vast majority of the digital information worldwide. However, after more than two decades of intense research, the understanding of the fundamental physical processes governing the transfer of angular momentum necessary for magnetic switching, is still lacking, partially hampered by the appropriate experimental tools. This situation is rapidly changing with the advent of X-ray free electron lasers (XFEL), which combine high temporal and spatial resolutions, necessary for a complete view of the physics at play. In the first work presented in this thesis, we demonstrate the capabilities of the recently built Spectroscopy and Coherent Scattering (SCS) instrument at the European XFEL. We perform ultrafast time-resolved small angle X-ray scattering (SAXS) on nanometre magnetic domains, combining ultrafast temporal resolution with high spatial resolution. We also demonstrate X-ray holographic reconstruction of similar magnetic domains. Our results show that the efficient data acquisition for holographic imaging is possible thanks to the MHz-operation of the European XFEL, paving the way for new studies and ultimately to create femtosecond movies of magnetism at the nanoscale. In the second work of this thesis, we describe a subsequent experiment at the SCS instrument, where we focus on the impact of symmetry breaking on the ultrafast dynamics of magnetic domains by looking at the diffracted SAXS data. Surprisingly, we observe a different ultrafast response depending on the anisotropy of the domains. We observe a clear contraction of the isotropic scattering ring in the reciprocal wavevector space (characteristic of randomly oriented domains), while no such contraction is observed in the anisotropic scattering pattern (distinctive of stripe-ordered domains). While the fundamental physical reason for the occurrence of the shift in wavevector space remains unexplained, we find that they correlate well with the domain symmetry. Our observation underlines the importance of symmetry as a critical variable for far-from-equilibrium dynamics. Finally, in the last work of the thesis, we look at the possibility of triggering ultrafast spin dynamics using intense THz magnetic field pulses. Typically, ultrafast spin dynamics is triggered using femtosecond lasers in the visible range. While readily available, these pulses cause highly non-equilibrium processes to take place because of the excitation energies in the eV range, comparable to the width of a typical electronic band. The potential excitation of all possible states within a band makes it difficult to disentangle which are the fundamental physical processes responsible for ultrafast demagnetization. On the other hand, radiation in the THz frequency range (meV energy range) directly couples to the magnetization without the risk of masking key processes. However, intense THz radiation is not easily generated because the relatively long wavelengths hamper the focusing capabilities due to the diffraction limit. To address this issue, we propose a metamaterial structure that enhances the THz magnetic field component of a free-space coupled THz field by more than one order of magnitude and exceeding the 1 T value. A table-top ultrafast time-resolved Faraday microscope setup with sub-micrometer resolution was built in order to investigate this experimentally.
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