Ultrafast Dynamics of Magnetic Multilayer Films: Magneto-Optical Spectroscopy and Resonant Scattering in the Extreme Ultraviolet and Soft X-ray Spectral Regions

<p>Magnetism has been known for millennia. Yet, even today, our understanding of the interplay of various interactions and processes in a material that give rise to its magnetic properties and, in particular, are responsible for their dynamics is quite limited. Such understanding, however, is critical not only from the standpoint of fundamental research. From an applied perspective, there is a strong motivation to explore the possibility of utilizing the spin degree of freedom in the quest of improving the eciency, increasing the speed, and reducing the size of logic devices, which requires a detailed understanding of magnetic interactions and their dynamics. Part of the reason for a lack of such understanding is related to experimental challenges in capturing materials\textquoteright magnetism at its natural time and length scales. Recent advances in ultrafast extreme ultraviolet (EUV) and soft X-ray sources have enabled element-speci c studies of magnetic materials with nanometer spatial and femtosecond temporal resolution by use of magneto-optics.</p> <p>In this thesis, I present temporally and spatially resolved studies of ferromagnetic thin lms. In Chapter 4, I develop a new magneto-optical technique that allows a direct measurement of the full resonant complex magneto-optical coecient of a material with tabletop high harmonic (HHG) sources of EUV light. I apply it, along with a conventional magneto-optical technique, to a thin Co lm in a pump-probe experiment to study the dynamic response of the lm to a femtosecond laser pulse. By comparing the experimental results with theoretical predictions, a connection to the microscopic mechanisms of ultrafast demagnetization is made, and it is found that the transient magnetization dynamics in Co are mostly dominated by magnon excitations with possible smaller contributions from other mechanisms. In Chapter 5, I develop an approach for resonant magnetic scattering (RMS) on a tabletop with an HHG source that does not require iv wavelength-selective optics and instead relies on the resonant nature of the scattering process. I use this approach to study magnetic textures with spatial resolution and, by applying a eld to a ferromagnetic multilayer lm, observe a transition from a disordered network of stripe domains to an ordered lattice of magnetic vortices. In Chapter 6, I present the results of a dynamic soft X-ray RMS experiment on a disordered domain network performed at the Linear Coherent Light Source (LCLS) at SLAC. By directly applying the experimental data to a carefully simulated domain pattern, I capture laser-induced transient changes in the domains in real space and observe strong non-uniformities in the demagnetization across the sample, which, with the help of simulations, are attributed to a combined e ect of ultrafast spin-polarized currents and a gradient in the pump absorption throughout the thickness of the sample. In Chapter 7, I show preliminary simulations and provide an outlook towards time-resolved lensless magnetic spectro-microscopy with HHG sources.</p>
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University of Colorado Boulder
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