Nanometer scale observation of magnetization textures induced by ultrashort laser pulses

The manipulation of magnetic order with ultrashort laser pulses has become a vivid field in modern magnetism research. Laser pulses have been found to not only demagnetize magnetic materials on a femtosecond timescale, but even reverse the magnetization in certain (anti-)ferromagnetically coupled alloys and multilayer systems. Very recently, it was demonstrated that laser pulses create topological magnetic solitons called magnetic skyrmions in ferromagnetic multilayer systems. Discovering the mechanisms that drive the laser-induced magnetization effects as well as their speed is not only of fundamental interest, but also relevant for potential spintronic applications. In this thesis, I present static and time-resolved soft x-ray experiments investigating laser-induced spin textures, in particular magnetic skyrmions. The creation process of magnetic skyrmions induced by excitation with ultrashort laser pulses is mediated by a high-temperature fluctuation state where topological charge appears and disappears. In static x-ray imaging experiments, I studied the influence of different sample-intrinsic and external parameters on the final skyrmion state after the laser irradiation. I found that the external magnetic field is the parameter that influences the skyrmion state the most. In subsequent time-resolved small-angle x-ray scattering (SAXS) experiments, I studied the field dependence of the fluctuation state. This non-equilibrium state always appears after the laser excitation even if the applied magnetic field prevents the existence of skyrmions in the final state. Atomistic spin dynamics simulations associate the fluctuating state with a competition of skyrmion nucleation and decay which can be explained with an Arrhenius-like activation over an energy barrier. The corresponding relaxation times are strongly field and temperature dependent resulting in a freezing in of magnetic skyrmions at low magnetic fields, where the relaxation times are longer than the timescale of the temperature change during the laser excitation. Time-resolved imaging experiments have the potential to measure the optical skyrmion creation process in a more direct way than scattering experiments. For the experimental realisation of optical pump－x-ray probe imaging experiments, the established imaging methods can be used if a laser system can be integrated into the existing instruments. I will show a proof-of-principle experiment of such time-resolved pump-probe measurements recording the ultrafast demagnetization of a ferrimagnetic alloy with picosecond temporal and nanometer spatial resolution, using a scanning x-ray transmission microscope that was recently equipped with an optical laser system. By the development of suitable measures in the sample design to mitigate the heat load on the magnetic film, pump-probe imaging at MHz repetition rates becomes possible. The instrumentation and methods developed will facilitate imaging of ultrafast magnetization dynamics with nanometer spatial resolution in the future.

Author dc:creator

• Gerlinger, Kathinka Sandrine

Advisors dc:contributor.advisor

• Eisebitt, Stefan
• Pfau, Bastian

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Identifier URI

https://doi.org/10.14279/depositonce-17088

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/18729