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- Neeraj, Kumar, 1991-
(författare)
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Terahertz spin dynamics in metallic thin film ferromagnets
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The past two decades have witnessed an increasing interest in understanding and controlling materials at the pico- and femtosecond time scales, the so-called ultrafast regime. Among the broad field of condensed matter physics, magnetism and magnetic materials have attracted much interest both from a fundamental and an applied perspective. The field of ultrafast magnetism is at the frontier of current physics research, with fundamental questions that are still unanswered but which have the potential of impacting the data storage technology upon which our digitalized world relies on. Ultrafast lasers in the visible range (i.e., with energies in the eV range) have been widely used to study ultrafast magnetization dynamics, but due to the relatively large photon energy, they create highly non equilibrium states which tend to mask the fundamental coupling processes leading to ultrafast demagnetization. However, the relatively recent appearance of intense coherent terahertz (THz) radiation (with photon energies in the meV range) offers a new way to understand and manipulate the magnetic order, and is receiving much attention in the research community. As a major part of this thesis, a table-top experimental setup for generating intense THz radiation has been developed for the purpose of carrying out pump-probe studies of thin ferromagnetic metallic films. The setup is capable of delivering state-of-the-art THz electric fields as large as 1 MV/cm, corresponding to 0.3 T magnetic fields which can directly couple to the magnetization to trigger ultrafast dynamics. The ultrafast magnetization dynamics is probed with the time resolved magneto-optical Kerr effect with a resolution of approximately 40 fs. Three main scientific results have been obtained with this thesis work. First, the experimental evidence, in the form of a spin nutation in the THz range, of inertial magnetization dynamics in thin film ferromagnets, which we could describe with a modified version of the textbook Landau Lifshitz-Gilbert (LLG) equation to include a realistic inertial tensor. Second, with this modified LLG equation, we performed simulations to study the role of inertia in the switching of the magnetization with picosecond magnetic field pulses. We found that inertia leads to a so-called ballistic switching which is more robust to the details of the magnetic field pulse. Third, we studied the influence of crystalline order on the charge and spin transport at terahertz rates. We found that while the charge scattering follows the degree of crystalline order in the film, the spin scattering is enhanced at intermediate crystalline phases which have not fully ordered, but where the magnetic anisotropy is largest.
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| 2. |
- 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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