Bachelor's Thesis

Magneto-optical imaging and analysis of magnetic domain microstructures

Final Thesis 10.7 MB

Author of thesis: Ing. Tomáš Molnár

Acad. year: 2020/2021

Supervisor: M.Sc. Jon Ander Arregi Uribeetxebarria, Ph.D.

Reviewer: Ing. Ondřej Wojewoda, Ph.D.

Abstract:

Magneto-optical effects are associated with the interaction of polarized light with magnetized media. Although discovered nearly two centuries ago, magneto-optics is a widely utilized tool for magnetic characterization due to its high sensitivity and versatility. For example, it enables visualization of magnetic domain structures in microscale magnetic systems via wide-field magneto-optical microscopy. The majority of research on this topic has employed linear effects in magnetization to image domains in ferromagnetic materials, where one measures an optical contrast for regions with opposite magnetization orientations. It has been recently shown that even antiferromagnetic materials can be studied using quadratic magneto-optical effects (Voigt effect), making it possible to visualize regions with different spin axis orientations. In this work, we perform a fundamental magneto-optical microscopy study of magnetic thin film and microstructure systems using linear and quadratic effects. Consequently, the microscale magnetization configuration of continuous and patterned magnetic thin films is interpreted via the image processing strategy developed in this thesis, leading to quantitative vector magnetometry. Moreover, the coexistence of the ferro- and antiferromagnetic phase domains in FeRh films was investigated using optical and magneto-optical microscopy.

Keywords:

vectorial magnetometry, magnetic domains, magneto-optics, Kerr effect, Voigt effect, FeRh, permalloy, phase transition

Date of defence

17.06.2021

Result of the defence

Defended (thesis was successfully defended)

znamkaAznamka

Grading

A

Process of defence

Po otázkách oponenta bylo diskutováno: vliv historie externího pole na stav magnetizace Student na otázku odpověděl. Rozprava proběhla v angličtině.

Language of thesis

English

Faculty

Department

Study programme

Applied Sciences in Engineering (B3A-P)

Field of study

Physical Engineering and Nanotechnology (B-FIN)

Composition of Committee

prof. RNDr. Tomáš Šikola, CSc. (předseda)
prof. RNDr. Miroslav Liška, DrSc. (místopředseda)
prof. RNDr. Bohumila Lencová, CSc. (člen)
doc. Ing. Stanislav Průša, Ph.D. (člen)
prof. RNDr. Petr Dub, CSc. (člen)
prof. RNDr. Radim Chmelík, Ph.D. (člen)
prof. RNDr. Jiří Spousta, Ph.D. (člen)
doc. Ing. Radek Kalousek, Ph.D. (člen)
RNDr. Antonín Fejfar, CSc. (člen)

The bachelor thesis by Tomáš Molnár deals with microscopy of magnetic domains using linear and quadratic magneto-optical effects. The theory behind electromagnetic waves and magneto-optics is precisely described in the first two chapters. In the experimental part, Mr. Molnár firstly employs imaging via linear magneto-optics to study magnetic domains in thin films and microstructures. He also developed a convenient Matlab-based software that automates image processing and provides quantitative magnetometry in a reliable way, with a performance comparable to commercial software. The additional part of the work on domain imaging via higher order magneto-optics is very valuable. The reported Voigt and gradient effect microscopy images are of remarkably high quality. In addition, interesting preliminary insights are given on the problem of antiferromagnetic domain imaging in metallic systems.

The student was independent and persistent during the working process, despite having a severely limited access to laboratory time due to general sanitary restrictions. He showed a very good handling of the Kerr microscope and a natural ability for it after little initial guidance. While minor aspects of the manuscript and graphics could be improved, the text is clear and well written. All the thesis goals have been accomplished by far and I would like to give the highest mark “A”.
Evaluation criteria Grade
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 B
Práce s literaturou včetně citací A
Samostatnost studenta při zpracování tématu A
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Grade proposed by supervisor: A

Reviewer’s report
Ing. Ondřej Wojewoda, Ph.D.

The bachelor thesis of Tomáš Molnár is dealing with magneto-optical imaging and analysis of the obtained data. In the first chapter author performed comprehensive literature research on the micromagnetic energies and various spin textures. The second chapter is devoted to the magneto-optical effects. In the following chapter, the author presents experimental results evaluated with self-developed code for vector magnetometry. The given procedure and code can be further used in the studies focused on micromagnetism. The self-developed code provides advanced options of filtering and processing; furthermore, it can be operated via a GUI, which enhances its versatility and feasibility.  In the last chapter, the author investigates the second-order magneto-optical effects in ferromagnetic and antiferromagnetic materials. 
The presented thesis is thoroughly written, the achieved results are comparable to the commercial solution. The results achieved on the system of the FeRh are innovative and can be further used in the following research.
All objectives of the bachelor thesis were accomplished, the thesis has good quality. Thus I recommend the work for defense with an overall grade of A.
Evaluation criteria Grade
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 A
Topics for thesis defence:
  1. Could be the remanent-magnetic state dependent on the history of the external field? What was the procedure to achieve states presented in Figure 3.3c)?
  2. In Figure 4.10 the change in phase of FeRh in dependence on temperature is presented. Could the author elaborate more on the underlying mechanisms of the change of reflectivity upon this phase transition?
  3. In Figure 4.12a) it seems that the antiferromagnetic phase is inhomogeneous. Could this cause the resulting contrast in Figure 4.12b)?
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Grade proposed by reviewer: A