Although ferromagnets are found in all kinds of technological applications,
their natural occurrence is rather unusual because only few substances are known
to be intrinsically ferromagnetic at room temperature. In the past twenty years,
a plethora of new artificial ferromagnetic materials has been found by
introducing defects into non-magnetic host materials. In contrast to the
intrinsic ferromagnetic materials, they offer an outstanding degree of material
engineering freedom, provided one finds a type of defect to functionalize every
possible host material to add magnetism to its intrinsic properties. Still, some
controversial questions remain: What are the mechanisms behind these
ferromagnetic materials? Why are their magnetization values reported in the
literature so low? Are these materials really technologically relevant
ferromagnets?
In this work, we aim to provide a systematic investigation of the phenomenon. We
propose a universal scheme for the computational discovery of new artificial
functional magnetic materials, which is guided by experimental constraints and
based on first principles. The obtained predictions explain very well the
experimental data found in the literature. The potential of the method is
further demonstrated by the experimental realization of a truly 2D ferromagnetic
phase at room temperature, created in nominally non-magnetic TiO$_2$ films by
ion irradiation, which follows a characteristic 2D magnetic percolation
transition and exhibits a tunable magnetic anisotropy.
Furthermore, the technological relevance of these artificial ferromagnetic
materials, which comes to shine when one combines the engineered magnetic with
some of the intrinsic properties of the host material, is demonstrated by
creating a spin filter device in a ZnO host that generates highly spin-polarized
currents even at room temperature.:1 Introduction
2 Computational discovery of artificial ferromagnets
2.1 Ferromagnetism in solids
2.1.1 Exchange interaction and magnetic order
2.1.2 Artificial magnetism due to defects
2.2 Predicting defect structures from collision cascades
2.3 Finding magnetic defect candidates
2.4 Magnetic percolation
2.5 Magnetic phase diagram of anatase TiO 2 artificial ferromagnet
2.5.1 Defect creation in anatase TiO 2
2.5.2 Magnetic properties of dFP defects in anatase TiO 2
2.5.3 Constructing a magnetic phase diagram
2.6 Revisiting prior experimental results
3 Artificial ferromagnetism in TiO 2 hosts
3.1 Low energy ion irradiation
3.2 SQUID magnetometry
3.3 Experimental realization of an artificial ferromagnet in TiO2
4 Artificial magnetic monolayers and surface effects
4.1 Critical behavior and 2D magnetism
4.2 Magnetic anisotropy
4.2.1 Demagnetizing field and magnetic shape anisotropy
4.2.2 Magnetocrystalline anisotropy
4.3 Artificial ferromagnetic monolayer at TiO 2 surface with perpendicular magnetic anisotropy
4.4 DFT calculations of the defective anatase TiO 2 [001] surface
5 Spin transport through artificial ferromagnet interfaces
5.1 Artificial ferromagnetism in ZnO hosts
5.2 Spin filter effect at magnetic/non-magnetic interfaces in ZnO
5.2.1 The spin filter effect
5.2.2 Lithium and hydrogen doping in ZnO
5.2.3 Magneto-transport in artificial ferromagnetic Li:ZnO microwires
5.2.4 Spin transport through magnetic/non-magnetic interfaces
5.2.5 Minority spin filter effect
6 Conclusions and Outlook
Bibliography
Appendix:
A List of publications
B Computation inputs and codes
B.1 DFT electronic structure calculations - Fleur input files
B.2 Magnetic Percolation simulations
B.3 SQUID raw data analysis code
B.4 SRIM Monte Carlo binary collision code automation / Obwohl Ferromagnete in allen möglichen technischen Anwendungen zu finden sind,
ist ihr natürliches Vorkommen eher ungewöhnlich, da nur wenige Stoffe bekannt
sind, die bei Raumtemperatur intrinsisch ferromagnetisch sind. In den letzten
zwanzig Jahren wurde eine Fülle neuer künstlicher ferromagnetischer Materialien
durch das Einbringen von Defekten in nichtmagnetische Wirtsmaterialien entdeckt.
Im Gegensatz zu den intrinsischen ferromagnetischen Materialien bieten sie einen
herausragenden Grad an materialtechnischer Freiheit, vorausgesetzt man findet zu
jedem möglichen Wirtsmaterial einen passenden Typus von Defekten, um dessen
intrinsische Eigenschaften um Magnetismus zu ergänzen. Dennoch bleiben einige
kontroverse Fragen bislang unbeantwortet: Welche Mechanismen stehen hinter
diesen ferromagnetischen Materialien? Warum werden ihre Magnetisierungswerte in
der Literatur meist so niedrig angegeben? Sind diese Materialien wirklich
technologisch relevante Ferromagneten?
In dieser Arbeit wollen wir eine systematische Untersuchung des Phänomens
durchführen. Wir schlagen ein universelles ab-initio Protokoll für die
computergestützte Entdeckung von neuen künstlichen funktionalen magnetischen
Materialien vor, das sich an experimentellen Bedingungen orientiert. Die
erhaltenen Vorhersagen erklären die in der Literatur gefundenen experimentellen
Daten sehr gut. Wir demonstrieren die Wirksamkeit der Methode durch die
experimentelle Realisierung einer echten 2D-ferromagnetischen Phase bei
Raumtemperatur, die in nominell nicht-ma'-gne'-tischen TiO$_2$-Filmen durch
Ionenbestrahlung erzeugt wird. Die so entstehende ferromagnetische Phase folgt
einem charakteristischen zweidimensionalen magnetischen Perkolationsprozess und
weist eine steuerbare magnetische Anisotropie auf.
Weiterhin wird die technologische Relevanz dieser künstlichen ferromagnetischen
Materialien gezeigt, welche besonders zum Vorschein kommt, wenn man die
künstlichen magnetischen mit einigen der intrinsischen Eigenschaften des
Wirtsmaterials kombiniert, und zwar indem ein Spin-Filter Element auf Basis
eines ZnO-Wirts gebaut wird, das selbst bei Raumtemperatur hoch
spin-polarisierte Ströme erzeugt.:1 Introduction
2 Computational discovery of artificial ferromagnets
2.1 Ferromagnetism in solids
2.1.1 Exchange interaction and magnetic order
2.1.2 Artificial magnetism due to defects
2.2 Predicting defect structures from collision cascades
2.3 Finding magnetic defect candidates
2.4 Magnetic percolation
2.5 Magnetic phase diagram of anatase TiO 2 artificial ferromagnet
2.5.1 Defect creation in anatase TiO 2
2.5.2 Magnetic properties of dFP defects in anatase TiO 2
2.5.3 Constructing a magnetic phase diagram
2.6 Revisiting prior experimental results
3 Artificial ferromagnetism in TiO 2 hosts
3.1 Low energy ion irradiation
3.2 SQUID magnetometry
3.3 Experimental realization of an artificial ferromagnet in TiO2
4 Artificial magnetic monolayers and surface effects
4.1 Critical behavior and 2D magnetism
4.2 Magnetic anisotropy
4.2.1 Demagnetizing field and magnetic shape anisotropy
4.2.2 Magnetocrystalline anisotropy
4.3 Artificial ferromagnetic monolayer at TiO 2 surface with perpendicular magnetic anisotropy
4.4 DFT calculations of the defective anatase TiO 2 [001] surface
5 Spin transport through artificial ferromagnet interfaces
5.1 Artificial ferromagnetism in ZnO hosts
5.2 Spin filter effect at magnetic/non-magnetic interfaces in ZnO
5.2.1 The spin filter effect
5.2.2 Lithium and hydrogen doping in ZnO
5.2.3 Magneto-transport in artificial ferromagnetic Li:ZnO microwires
5.2.4 Spin transport through magnetic/non-magnetic interfaces
5.2.5 Minority spin filter effect
6 Conclusions and Outlook
Bibliography
Appendix:
A List of publications
B Computation inputs and codes
B.1 DFT electronic structure calculations - Fleur input files
B.2 Magnetic Percolation simulations
B.3 SQUID raw data analysis code
B.4 SRIM Monte Carlo binary collision code automation
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:75672 |
| Date | 10 August 2021 |
| Creators | Botsch, Lukas |
| Contributors | Universität Leipzig |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/acceptedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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