Bachelor's Thesis

Influence of the epitaxial strain at the lateral thin film-stripe interface on the Ferromagnetic-Antiferromagnetic phase coexistence in FeRh

Final Thesis 12.09 MB

Author of thesis: Bc. Sára Hrdinová

Acad. year: 2023/2024

Supervisor: Ing. Oleksii Zadorozhnii

Reviewer: Ing. Mgr. Peter Kepič

Abstract:

An equiatomic alloy FeRh exhibits a phase transition from antiferromagnetic to ferromagnetic ordering at a temperature about 350 K. This makes it an ideal material for studying exchange interactions between ferromagnetic and antiferromagnetic ordering within a single material system. In this work, we investigate the relaxation of the compressive epitaxial strain caused by nanopatterning of the FeRh film. Relaxation of this strain leads to the stabilization of the ferromagnetic phase, which could lead to the formation of a suitable interface between the relaxed ferromagnetic nanostripe and the compressed antiferromagnetic film. In this bachelor thesis we first describe in detail the magnetic properties of the materials and properties of the FeRh alloy with the emphasis on the phase transformation. Our experiments supported by recent literature research include depositing FeRh layers with thicknesses of 36 nm and 180 nm. After patterning by electron beam lithography, the samples were characterised using Magnetic Force Microscopy. These measurements show that the strain relaxation is strongly dependent on the orientation of the nanostripe with respect to the crystal structure of the substrate. For an orientation of 0°, the strain relaxation is more pronounced than for 45°. The 36 nm thin FeRh layers do not form a continuous phase boundary between the nanostripe and the full film in 1000 nm wide nanostripes. Instead, they form small domains that are indicative of the strain distribution in the structure. Layers of 180 nm thickness show a phase coexistence between the nanopatterned stripe and the film for stripes longer than 25 µm. Well-defined interfaces have been achieved for stripe widths from 1250 nm to 300 nm. For structures with wider stripes, the ferromagnetic phase at the edges of the continuous layer merges with the ferromagnetic phase in the nanostripe, thereby curving the interface. This merging is gradually broken at the interface in the thinner stripe with dimensions 800 nm and 600 nm. The stripe of width 300 nm shows a clear interface between its sides.

Keywords:

Ferromagnetism, Antiferromagnetism, FeRh alloy, Phase transition, Epitaxial strain, Magnetic, Force Microscopy, MFM, Strain-induced phase transition

Date of defence

13.06.2024

Result of the defence

Defended (thesis was successfully defended)

znamkaAznamka

Grading

A

Process of defence

Po otázkách oponenta bylo dále diskutováno: Srovnání MFM a AFM z hlediska interakčního potenciálu vzorek-hrot. Studentka na otázky odpověděla.

Language of thesis

English

Faculty

Department

Study programme

Physical Engineering and Nanotechnology (B-FIN-P)

Composition of Committee

prof. RNDr. Tomáš Šikola, CSc. (předseda)
prof. RNDr. Jiří Spousta, Ph.D. (místopředseda)
prof. RNDr. Radim Chmelík, Ph.D. (člen)
prof. RNDr. Petr Dub, CSc. (člen)
prof. Ing. Jan Čechal, Ph.D. (člen)
prof. Ing. Miroslav Kolíbal, Ph.D. (člen)
doc. Mgr. Vlastimil Křápek, Ph.D. (člen)
doc. Ing. Stanislav Průša, Ph.D. (člen)
doc. Ing. Radek Kalousek, Ph.D. (člen)
doc. Ing. Miroslav Bartošík, Ph.D. (člen)
RNDr. Antonín Fejfar, CSc. (člen)

Supervisor’s report
Ing. Oleksii Zadorozhnii

During the course of this bachelor thesis the student has shown great interest in the topic, high degree of enthusiasm and was very adaptible and quick-learning. Unfortunately due to delays on our part with the lithographic procedure, student had very limited time to conduct the experiments on 180 nm thick films. However, for the short time spent on the Magnetic Force Microscopy we consider the experimental output to be of very high quality. 

Theoretical and state of the art parts of this thesis in my opinion cover all the necessary basics for understanding of metamagnetism in FeRh, phase transition phenomena as well as the influence of the strain on the phase transition.

Interpretation of the results was made difficult due to FeRh supercooling coming into play, which was discovered for 10 micrometre sized elements after the entire set of measurements was made. This made interpretation of the results more difficult than expected, however the conclusions of the thesis are still valid and applicable for our future measurements, and definitely provide useful information on strain influence on FeRh phase transition from antiferromagnetic to ferromagnetic state. 

With the above factors mentioned, we consider that within given parameters the student has performed very close to the best expectations and provided us with very valuable data as part of their bachelor's thesis.
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 B
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

Grade proposed by supervisor: A

Reviewer’s report
Ing. Mgr. Peter Kepič

The thesis starts very nicely. The author clearly describes a basic theory of magnetism and consistently shifts to classifying magnets with relevant graphs describing them. This graph is later well-used when introducing iron-rhodium alloy, an antiferromagnetic to ferromagnetic phase-change material. While phase transition is adequately described using a visually explicit phase diagram, a more detailed description of the hysteresis with a focus on heating and cooling temperatures is missing. Especially later in the state-of-the-art section, where the supercooling of nanostructures is revealed, and comparison to thin film/bulk temperatures is crucial. Moreover, switching between degrees of Celsius and Kelvins in the text makes understanding harder. The methods section is adequately brief. However, there is a slight imbalance between the first two methods (Magnetron and VSM) and AFM. While AFM is, in my opinion, overexplained, Magnetron and VSM especially miss a more detailed description of operation. In the last section, I admire the huge amount of samples the author measured and the work she did. Sadly, weaker interpretation of results and sentence arrangement make it hard to understand these results and their impact. Specifically, nanostructures are sometimes called nanowires, other times strips or stripes; results from images are followed by a discussion about fabrication/methods and then back to other results without explaining the meaning of previous results; FM and AFM contrasts are not adequately explained; therefore clusters and AFM domains discussed in the text are not visible (at least for me; should be highlighted). Despite the weaker interpretation on which the author should work in the future, I evaluate this thesis as very good, especially when considering the amount of time and work she had to dedicate during her bachelor year to study the problematics and measure the samples.
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 C
Využitelnost výsledků v praxi nebo teorii A
Logické uspořádání práce a formální náležitosti B
Grafická, stylistická úprava a pravopis B
Práce s literaturou včetně citací A
Topics for thesis defence:
  1. Epitaxial growth is highlighted as very important in the context of magnetic and electric properties during the phase transition of FeRh. Could the author explain why? What would happen when deposited on an amorphous substrate such as Fused Silica?
  2. Could the author specify contrast changes in MFM images during the FeRh phase transition? After that, could the author choose one of the measured areas of the 36 nm thick structures and properly describe and highlight the formation of the AFM cluster and transformation of the nanowire and patch?

Grade proposed by reviewer: B

Responsibility: Mgr. et Mgr. Hana Odstrčilová