Posudky závěrečné kvalifikační práce

Posudek vedoucího

Uhlíř, Vojtěch

The master thesis of Ján Hnilica focuses on developing and testing an alternative material system for the conceptual race-track memory. The major advantage of the studied synthetic ferrimagnet based on multilayer Gd/Co configurations lies in its high tunability enabling adjustment of the key magnetic characteristics on demand. The effort is aimed at increasing the stability of the domain wall upon electric current stimuli and consequently, the efficiency of domain wall motion. In order to test these hypotheses, the first goal was to carefully optimize the configuration of the multilayer system and subsequently fabricate microstripes using lithography. Second, by constructing and exploiting a dedicated platform for simultaneous electric transport and magnetic imaging experiments to investigate the domain wall mobility close to the angular momentum compensation. Ján has demonstrated a diligent and persistent approach when executing the project, which allowed achieving valuable results matching the theoretical predictions. The detailed content of the manuscript with a logical and fluent structure shows that the student acquired a deep insight into the subject. Overall, the results are above expectations with respect to the outlined goals, and therefore without hesitation, I recommend the thesis be defended.

Dílčí hodnocení
Kritérium	Známka	Body	Slovní hodnocení
Splnění požadavků a cílů zadání	A
Postup a rozsah řešení, adekvátnost použitých metod	A
Vlastní přínos a originalita	A
Schopnost interpretovat dosažené výsledky a vyvozovat z nich závěry	A
Využitelnost výsledků v praxi nebo teorii	A
Logické uspořádání práce a formální náležitosti	A
Grafická, stylistická úprava a pravopis	A
Práce s literaturou včetně citací	A
Samostatnost studenta při zpracování tématu	A

Navrhovaná známka: A

Posudek oponenta

Hrabec, Aleš

The master thesis by Ing. Jan Hnilica deals with the current-induced magnetization dynamics in ferrimagnets based on tailored Gd/Co multilayers. While most of the studies performed on ferrimagnets are performed on alloys, here the work focuses on multilayered artificial systems. An extensive and systematic work is dedicated to the development of multilayered films with desired magnetic and electric properties. This allowed building a system suitable to the available experimental environment to achieve the most efficient domain wall motion in the vicinity of the angular momentum compensation. The domain wall dynamics is then systematically studied at various temperatures. The manuscript itself is divided into a logic sequence of chapters guiding the reader to the final experimental chapter. The phenomena and experimental results are nicely illustrated by own sketches and drawings, which I appreciate. From experimental point of view, I really like the results presented in Fig. 5.5, nicely showing that the whole idea of the domain wall dynamics enhancement around angular momentum compensation clearly works. While I think that this work is a good start of a larger study, it presents a nice set of experimental results with certain degree of understanding and theoretical description. Given the amount of work to acquire all the experimental data and the analysis, I am more than happy to recommend this work to be defended at an examination committee. I have also a few questions to be answered:

Dílčí hodnocení
Kritérium	Známka	Body	Slovní hodnocení
Splnění požadavků a cílů zadání	A
Postup a rozsah řešení, adekvátnost použitých metod	A
Vlastní přínos a originalita	A
Schopnost interpretovat dosaž. výsledky a vyvozovat z nich závěry	B
Využitelnost výsledků v praxi nebo teorii	A
Logické uspořádání práce a formální náležitosti	A
Grafická, stylistická úprava a pravopis	A
Práce s literaturou včetně citací	B

Navrhovaná známka: A

Otázky

• In equation (3.15), you describe the domain wall velocity dependence on the electric current and temperature in the so-called creep regime. This includes parameters such as depinning current density. This parameter can be extracted from the velocity curves if other parameters are known: see for example Díaz Pardo, Phys. Rev. B. 100 (2019). Therefore, in this sense, I wonder how the depinning current densities are extracted, for example, in Fig. 5.2 or 5.3. It seems that the definition of depinning current has changed throughout the thesis. Nevertheless, there should still be a scientific ground to extract the ‘depinning currents’.
• The flow regime is fitted with a linear dependence, sometimes under some very challenging conditions (e.g. Fig. 5.2(a)). How is the fitting range chosen? It seems a bit deliberate to me. Did you make sure that the log(v)=J^-1/4 slope deviates from the creep law - in Fig 5.2(b) only part of the data is shown.
• From theoretical point of view, the relation for the domain wall mobility is cited on page 31 without any bibliographic reference. However, Thiaville, EPL (2012) derived a different relation where the domain wall mobility depends also on DMI. Is there some estimation of the DMI constant in the prepared films?
• I am confused about the definition of the Walker breakdown field in the case of a domain wall driven by the spin-orbit torques. Here normally no breakdown is observed, only a saturation in terms of linear increase of the velocity (as also correctly noted on page 31). I thus wonder where this term in this thesis comes from.
• A reasonable effort was dedicated to the impedance matching of the samples. However, no experimental data of the pulse shapes are shown in the thesis, only sketches, which I find a pity. Can you show a few pulse shapes depending on the current density and length of the pulse?
• The manuscript lacks an outlook chapter, which brings me to my next question. What would be the next steps you would try if you had another year to work on this project? I will also attach the manuscript with a few remarks.