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Time-resolved magnetic small angle X-ray scattering using a laser-driven plasma source
Schneider, Niklas
Magnetic small-angle X-ray scattering (mSAXS) in the soft X-ray range is a powerful tool for studying magnetism on its intrinsic time and length scales of picoseconds and nanometers. However, until now this technique has exclusively been available at large-scale facilities. Availability has also been limited by the short pulse durations required for pump-probe experiments, making investigations challenging.
In this thesis, I present the investigations on time-resolved, temperature- and magnetic-field dependence of magnetic domain dynamics at the iron L3- and gadolinium M5-edge of a [Gd(0.5)/Fe(0.4)]116 multilayer system. Utilizing a laboratory-based laser-driven plasma source providing sub-10 ps soft X-ray pulses, this thesis achieves two notable advances. Firstly, it enables time-resolved mSAXS experiments
and investigations into ultrafast demagnetization dynamics within a laboratory setting, by exploiting specific absorption edges. Secondly, it offers a means of investigating magnetic nanostructures at low temperatures with picosecond time resolution in a laboratory environment.
The objective of this thesis is to examine the relationship between laser-driven dynamics and the temperature-dependent ground state of the material. At the same time, the static and time-resolved experiments performed serve to demonstrate the performance of the source, with a focus on the demagnetization dynamics. Static, field-dependent observations include changes in domain growth and periodicity when
transitioning from antiferromagnetic (AFM)-aligned domains to ferromagnetic (FM) saturation. Furthermore, a reduction in domain periodicity was observed as the temperature decreased. This phenomenon is attributed to the temperature-dependent ground-state magnetization of the sample. The time-resolved experiments reveal variations in demagnetization amplitudes and
remagnetization time constants between Gd and Fe. Furthermore, a temperature dependent deformation of the domain is identified upon photoexcitation. This is characterized by decreasing domain periodicities at low temperatures and small delays, contrasting with increasing domain periodicities at high temperatures and low delays. At high delays, the domains exhibit larger periodicities at all temperatures.
To my knowledge, this phenomenon has been observed for the first time, and a theoretical model describing the dynamics is beyond the scope of this thesis. These results demonstrate the dependence of the demagnetization dynamics of the material on the magnetic ground state. They also break ground for studying ultrafast and temperature-dependent material dynamics in a flexible laboratory environment.
