Opponent: Professor Florian Banhart, Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), Université de Strasbourg, Strasbourg, France
Supervisor: Professor Jonas Weissenrieder, Ljus och materiens fysik; Professor Oscar Tjernberg, Ljus och materiens fysik
The ability to manipulate magnetization at femtosecond and nanometer scales is crucial for future advancements in spintronics and high-speed magnetic data storage. Ultrafast magnetization dynamics, driven by femtosecond laser pulses, have garnered significant attention due to their potential applications in energy-efficient magnetic switching, ultrafast spintronic devices, and high-density data storage, particularly relevant for the development of next-generation memory and logic devices, where speed, miniaturization, and energy efficiency are critical. Recent studies have demonstrated that ultrafast laser pulses can induce phenomena such as femtosecond demagnetization, all-optical switching, and coherent spin wave excitation, providing new avenues for magnetic control at ultrafast timescales. However, despite these advances, key challenges remain in understanding the fundamental mechanisms governing ultrafast magnetization dynamics, particularly in complex magnetic systems and magnetization dynamics influenced by confined light, structured light, and geometric constraints, where spatially localized interactions play a crucial role. While existing optical techniques such as the magneto-optical Kerr effect (MOKE) and X-ray magnetic circular dichroism (XMCD) have been widely used to investigate ultrafast magnetization dynamics, their spatial resolution is fundamentally limited by the diffraction limit, preventing detailed real-space observation of localized ultrafast magnetic phenomena. On the other hand, Lorentz electron microscopy has proven to be a powerful technique for imaging static magnetic structures such as domain walls and skyrmions, but its application to ultrafast magnetization dynamics has been relatively underdeveloped. The emergence of ultrafast Lorentz electron microscopy (UEM), which combines transmission electron microscopy with femtosecond pump-probe techniques, overcomes these limitations by providing both high temporal and spatial resolution. This approach enables the direct visualization of real-space magnetization dynamics beyond the constraints of optical-probe techniques, offering a novel perspective on localized investigation on ultrafast magnetization phenomena.
Previous studies utilizing UEM have primarily focused on ultrafast dynamics of topological magnetic textures, leaving many aspects of ultrafast magnetization dynamics unexplored. To address this gap, this thesis expands the application of UEM to investigate laser-driven ultrafast magnetization dynamics beyond topological structures, including plasmon-mediated ultrafast demagnetization, all-optical domain wall writing, and optically engineered spin resonance modes. By utilizing structured light excitation and ultrafast Lorentz imaging, this work provides new insights into the fundamental mechanisms governing ultrafast magnetization dynamics, enabling precise control of magnetization at sub-picosecond timescales and sub-micrometer spatial resolution.
The experimental investigations in this thesis focus on three key areas. First, ultrafast demagnetization dynamics in permalloy (Ni₈₀Fe₂₀) thin films are studied in the presence of surface plasmon polaritons (SPPs), revealing a strong spatial modulation of demagnetization amplitude. By coupling SPPs to a ferromagnetic thin film Ni80Fe20, localized enhancements of demagnetization are observed near the sample edge, where plasmonic hotspots significantly influence the spatial profile of magnetization quenching. The results also demonstrate that SPP-induced demagnetization exhibits a characteristic beat pattern with a periodicity of approximately 17 μm, suggesting an interplay between propagating SPP waves and magnetization dynamics. These findings establish a direct link between plasmonic excitations and ultrafast magnetic responses, highlighting the potential for nanoscale magnetoplasmonic control.
Second, we present the first direct visualization in real space of ultrafast, all‑optical domain‑wall (DW) writing together with its nonequilibrium formation pathway. We tracked, both spatially and temporally, how a disordered magnetization configuration evolves into a well‑defined stripe‑like DW configuration within only a few picoseconds. Strikingly, during this transformation we observe an intermediate regime, manifested as a highly asymmetric and transient DW contrast. We also reveal a narrow fluence range where a short‑lived DW structure briefly forms and spontaneously collapses within roughly 10 ps, indicating the presence of a previously unidentified nonequilibrium in‑plane spin texture. We established a theoretical framework supported by multiscale micromagnetic simulations to interpret the observed DW nucleation dynamics. We found that DW nucleation is driven by both localized spin textures and the global drive generated by partial magnetization reversal through local all‑optical switching. The results of multiscale micromagnetic simulations are able to reproduce the Lorentz contrast of the intermediate state observed in the experiments. This DW formation pathway differs fundamentally from previously reported DW writing mechanisms and points to a promising strategy for achieving DW writing at speeds hundreds of times faster than approaches driven by magnetic fields, spin torques, or strain.
Finally, the thesis demonstrates the ultrafast creation of antiferromagnetic (AFM) coupling in the all-optical switching induced synthetic ferrimagnetic (SyF)-like configuration which can remain stable for at least nanoseconds, revealing the presence of coexisting spin resonance modes. Experimental results show the emergence of an acoustic resonance mode, whose frequency varies with applied magnetic field, alongside an optical resonance mode, which remains constant at approximately 4 GHz. These spin resonance modes are indicative of an engineered synthetic SyF-like state, wherein neighboring spin-up and spin-down domains are optically induced and AFM coupled. Furthermore, we can tune the coupling and decoupling of these two modes by adjusting the magnetic field and fluence, significantly influencing the amplitude and frequency of the two spin oscillation modes. The ability to manipulate this AFM coupled spin dynamics using ultrafast optical excitation opens new possibilities for engineered spintronic devices and magnonic applications.
In summary, this thesis advances ultrafast Lorentz electron microscopy for direct spatiotemporal visualization of ultrashort laser-driven magnetization dynamics, laying the groundwork for controlling localized ultrafast spin dynamics and potentially driving future innovations in spintronics and nanoscale magnetic manipulation.