The bachelor thesis by Jakub Opršal deals with the investigation of ultrafast laser-induced magnetization control in magnetic materials. The state of the art in ultrafast magnetism is correctly summarized, with its physical principles presented to an adequate level. This introduction is followed by a brief theoretical overview of magnetic interactions and magneto-optical effects.

The principal aim of the thesis was to develop an optical setup for ultrashort laser pulse illumination of magnetic specimens under different light polarization conditions. The setup building efforts were shared together with his colleague Alexander Velič, while Mr. Opršal had a leading role in the task. The setup had to be built twice due to the laser safety system installation occurring in between (the first time, with assistance from the supervisor; the second time, with little or practically no assistance). The student showed a satisfactory progress in handling different aspects of the setup optimization. Furthermore, Mr. Opršal characterized the magnetic properties of a series of multilayer samples, identifying those that could be suitable for the laser illumination experiments.

Finally, the all-optical magnetization switching phenomenon was demonstrated by sweeping the pulsed laser on both ferromagnetic Co/Pt and ferrimagnetic Co/Gd specimens, and subsequently observing the modified magnetic domain patterns under the magneto-optical microscope. I believe this constitutes a remarkable result for a bachelor thesis.

The experimental efforts are correctly summarized in the thesis. As a side comment, the description sometimes lacks a slightly more developed discussion in comparison to the literature. The student was able to reflect by himself on these comparisons and to assess detailed aspects (such as the cumulative role of the light pulses), posing new interesting questions. However, such interesting aspects were omitted in the thesis, partially due to lack of time, partially due to the complexity of the topic.

While there are some minor weaknesses in the manuscript and in the level of depth offered in the discussion of the results, I believe that all the thesis goals have been accomplished by far and I would like to give the highest mark “A”.

The bachelor thesis of Jakub Opršal deals with ultrafast control of magnetic states. In the first chapter, the author performed comprehensive literature research on magnetism, emphasizing micromagnetic energies. The second chapter is aimed at Jones's formalism and magneto-optics effects. The following chapter describes the own-built setup for controlling the state of magnetization. Even though the chapter is relatively brief, the work done to achieve a functional setup is vast. The author has to work with laser class 4 (the highest class), which is extremely hard alone. In the last two chapters, the author performed experiments that proved the built setup's feasibility.
However, there are some language errors in the thesis (missing verb in the sentence, missing articles, wrong words, …), which lower the overall quality of the work. Furthermore, the author does not reference the figures in the main text, making the text difficult to understand. Also, at some points, the author makes statements but does not provide relevant references or satisfying reasoning to support them.
Nevertheless, the thesis accomplished all stated goals and will significantly contribute to the magneto-optical research in the CEITEC laboratories. Thus, I recommend the work for defense with an overall grade of B. Otázky k obhajobě:

1. The name of the thesis suggests ultrafast switching of the magnetization, but from the work presented in the thesis, it is not clear if it was achieved. Can the author discuss how to prove that the switch is “ultrafast”? What changes would be necessary to make in the built setup?
2. The imaging of the induced change in magnetic state is done ex-situ in a MOKE microscope. Can the author discuss the built setup modification to make the in-situ measurement possible?
3. In Figure 4.4, there are three hysteresis loops with different shapes. The reason why they have different characteristics is briefly discussed in the text, but can the author introduce some model which can describe these behaviors?