Bachelor's Thesis

Development of the scanning time-resolved Kerr microscope

Final Thesis 2.84 MB

Author of thesis: Ing. Zdeněk Nekula

Acad. year: 2020/2021

Supervisor: Ing. Ondřej Wojewoda, Ph.D.

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

Abstract:

In recent days, magnetic materials, structures, and devices are getting more popular,
especially those related to spintronics. Research and development of such magnetic
samples require a way to observe magnetization with good spatial and temporal
resolution. Most dynamic changes of magnetization are realized in nanoseconds or even
faster. If we can detect these dynamic processes, we can reveal many exciting
magnetization features and add the fourth dimension to our experiments. We introduce a
scanning Kerr microscope working in two modes: static and dynamic. In static mode, our
microscope detects a direction of magnetization in a variable magnetic field. In dynamic
mode, we use a pump-probe method to reach a temporal resolution and observe a fast
evolution of magnetization.

Keywords:

Kerr microscope, Kerr effect, time resolution, pump-probe, vortex

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: difrakční limit rovnovážná hodnota rotace polarizace světla Student na otázky odpověděl.

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)

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

The bachelor thesis of Zdenek Nekula is dealing with the construction of a scanning-time-resolved magneto-optical microscope. In the first part author performed comprehensive literature research on the electromagnetic waves and the Kerr effect. The third chapter is dealing with the used optics and signal processing. The following chapter discusses the mechanical design of the developed microscope. In the last two chapters, the obtained results are presented for both static and dynamic measurements. The student showed diligence and independence in the construction and aligning of the optical setup.  The achieved results are of outstanding quality and comparable with the state-of-the-art setups around the world. Unfortunately, the quality of the text and scientific accuracy does not perfectly match the experimental part's standard.
Nevertheless, the author constructed and aligned the scanning magneto-optical microscope capable of spatially- and time-resolved quantitative magnetometry. All of the objectives were accomplished, and therefore 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 dosažené 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 B
Práce s literaturou včetně citací C
Samostatnost studenta při zpracování tématu A
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Grade proposed by supervisor: A

The bachelor thesis of Zdeněk Nekula deals with the development of a magneto-optical Kerr effect microscope to characterize dynamic magnetization processes at the microscale. The thesis firstly describes the fundamentals of electromagnetic waves and magneto-optical Kerr effects in detail.

Concerning experimental work, Mr. Nekula built a more advanced version of the scanning Kerr effect microscope. A new signal acquisition scheme is based on a smart strategy consisting of four-quadrant based detection, and the possibility to rotate the sample in a magnetic field is added. A user-friendly control and acquisition software is also incorporated. It is satisfactorily shown that scanning Kerr magnetometry is achieved with 500 nm spatial resolution. In addition, the time-resolved capability using a picosecond laser and a pump-probe setting (microwave pump, optical probe) is successfully implemented, a work showing a high command of advanced electronics and data acquisition. The measurements of vortex gyration dynamics in permalloy microdiscs are impressive and of remarkable quality.

I would like to give a very positive overall evaluation of the thesis. A very large amount of work has been done and all thesis objectives are accomplished by far. The thesis has a nice graphical layout and the text is generally clear. As a suggestion for improvement, I would point to occasional imprecisions in the text that make appear some concepts confusing. The language style is at times casual, and the organization could be slightly improved (for example, section 2.3 before 2.2). I would also recommend improving the style of references in future work.

Despite these minor inaccuracies, I believe that the work is of exceptionally high quality and hence I would like to grant the highest overall 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 dosaž. 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 C
Topics for thesis defence:
  1. - The achieved spatial resolution is 500 nm, which is very close to the fundamental diffraction limit (Rayleigh Criterion). In principle, it is possible to improve the spatial resolution by using a laser with lower wavelength (e.g., 200 nm or below). However, this strategy is problematic or non-advantageous for laboratory-based magneto-optical setups. Could you mention two or three (instrumental or fundamental) problems? Hint: what is the Kerr amplitude of Co, Ni or Fe at UV wavelengths? [see, for example Figure 1 in Physical Review Letters 30, 1329 (1973)].
  2. - The performance of the setup (how long do measurements take) is only indirectly mentioned (hours). A future challenge will be performing simultaneous space- and time-resolved experiments. Could you estimate how long a time-resolved measurement like the one in Fig. 6.8a would take upon implementing space-resolution? How to improve in this aspect?
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Grade proposed by reviewer: A