| 1. |
- George, Sebastian, 1990-
(författare)
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Amorphous Magnetic Materials : A Versatile Foundation for Tomorrow’s Applications
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Amorphous magnetic materials exhibit a number of key differentiating properties with respect to crystalline magnets. In some cases, the differences may simply be in the values of macroscopic properties such as saturation magnetization, coercivity, Curie temperature, and electrical conductivity. Other cases are more fundamental, such as the possibility for many amorphous alloys to be produced with nearly arbitrary composition, something that is not always possible in crystal structures that may only be stable for certain specific compositions.Fundamentally, these properties arise due to the disordered arrangement of atoms in amorphous materials. However, this structure is challenging to probe and characterize, either experimentally or theoretically. A significant contribution of this thesis is the development of a new approach for studying the local atomic structure of amorphous materials, specifically amorphous SmCo and FeZr alloys. The strategy combines extended x-ray absorption spectroscopy (EXAFS) measurements with stochastic quenching (SQ) simulations in a way that provides more information than either method can offer alone. Additionally, this approach offers the potential for identifying any shortcomings in the theoretical models obtained via SQ.Having an accurate model of the atomic arrangement is not, however, a prerequisite for developing technical applications of amorphous magnetic materials. For that, it is sufficient to quantify those macroscopic properties that are relevant for a given application. Such is the value of the magnetic characterization of amorphous TbCo and CoFeZr alloy thin films presented here. Both investigations used methods such as vibrating sample magnetometry (VSM) and magneto-optic Kerr effect (MOKE) measurements to highlight the high tunability of the magnetic properties in these materials, which can be achieved simply by changing the chemical composition.The final portion of this thesis examines what can be achieved by combining amorphous SmCo and TbCo alloys together in bilayer structures. This is a step away from the alloy characterization studies, as it focuses on how new properties can be realized when multiple materials are brought together. MOKE measurements were used to identify the conditions under which the bilayers spontaneously become magnetized parallel to the film plane versus when the TbCo magnetization begins to tilt out of the plane. Further investigation combining x-ray circular magnetic dichroism (XMCD) measurements and micromagnetic simulations provided a depth-resolved model of the magnetization throughout the bilayers in the presence of a broad range of external field strengths and directions. These models also showed that the local magnetization just above and just below the SmCo/TbCo interface can be aligned either parallel or antiparallel to one another simply by varying the TbCo composition. This discovery offers a novel method for controlling the magnetic behavior in these materials, and may well be useful for all-optical switching or spintronics applications where amorphous TbCo alloys have already drawn attention.
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| 2. |
- Marattukalam, Jithin James
(författare)
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Tailoring structure and morphology during additive manufacturing of metallic components
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work described in this thesis explores the use of laser process parameters to functionalize the material properties by the control of microstructure and optimization of morphology in components by selective laser melting. The microstructure in amorphous and crystalline metallic alloy systems is influenced by changing the laser power density and scanning strategies respectively. A combination of X-ray/neutron diffraction and optical/electron microscopy is used to evaluate the microstructure and phase formation in SLM components. The influence of the microstructure on the mechanical properties of as-printed samples was investigated using hardness and uniaxial tensile testing methods. To begin with, the process parameters for selective laser melting of a Zr-based bulk metallic glass Zr59.3Cu28.8Al10.4Nb1.5 (trade name AMLOY-ZR01) are developed to obtain high density and crack-free bulk components. The influence of oxygen on the thermal stability and crystallization pathway in AMLOY-ZR01 was found to be significant in determining the formation of metastable crystalline phases within the amorphous matrix. It was also shown that the mechanical properties in AMLOY-ZR01 can be influenced by changing the amount of crystalline phases formed within the amorphous matrix. This was achieved by changing the laser power density during the SLM process. The alloy composition was also investigated for its biocompatibility, and the cell-material interactions under in-vitro test conditions showed no cytotoxic effect. These findings demonstrate that AMLOY-ZR01 is a promising candidate for orthopedic bio-implant applications. The latter half of this work demonstrates the influence of microstructure and crystallographic texture on the mechanical properties of 316L SS. This was achieved by changing the "laser scanning methodology" during the SLM process and a correlation between the applied scanning methodology and structure-property relation was identified. A single crystalline-like texture can be obtained using a bi-directional scanning methodology, whereas a fiber texture is achieved when rotating the laser scan vectors by 67° to melt consecutive powder bed layers. The mechanical properties of 316L SS are influenced by the type of laser scan used to fabricate the components, as it dictates the final grain orientation within the SLM samples. It is also shown that the scanning patterns can be altered during the SLM process to create position-specific crystallographic grain orientation within the component. This opens up the possibility to fabricate functionally graded components which contain a spatial variation in composition and/or microstructure for the specific purpose of controlling material properties. Finally, the functionalization of material properties through design of components by additive manufacturing was demonstrated by fabricating waveguides with the specific geometries.
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| 3. |
- Ravensburg, Anna L.
(författare)
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Order and interfaces in epitaxial heterostructures : Structure and magnetism
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work in this dissertation is devoted to investigating order and interfaces in epitaxial heterostructures. To achieve that the software tool box GenL was developed for simulating and fitting x-ray diffraction patterns from epitaxial thin films, which is used to access structural information on the length scales of interfaces and atomic bonds. Employing GenL, it is shown that a small lattice mismatch between substrate and epitaxial layer is not the sole origin of high crystal quality, as demonstrated for nearly strain-free epitaxial growth of tungsten on sapphire with a lattice mismatch of up to 19.4 %. Furthermore, it is discussed that electronic states at the substrate/film interface can have substantial significance for the crystal structure of an epitaxial layer. For instance, despite a nearly mismatch-free interface of body-centered cubic iron on spinel, the presence of a boundary-induced interface layer with tetragonally distorted crystal structure is discovered, which has a profound impact on the magnetic properties. Finally, when creating multilayered structures, not only the interface states but the total structure is found to influence the physical properties, which is demonstrated for the interlayer exchange coupling in [Fe/MgO]Nsuperlattices.Note: This PhD thesis is partly based on the licentiate dissertation "Growth of high quality Fe thin films" by Anna L. Ravensburg, Uppsala University, 2022. Particularly parts of: Chapter 1, Sections 2.0, 2.1, 2.2, 3.0, 3.1, 3.2, 3.3, 5.1, and Fig. 2.6 are adapted from the licentiate thesis with minor edits and updates.
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| 4. |
- Stopfel, Henry, 1984-
(författare)
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Tailoring the magnetic order in mesoscopic spin systems
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mesoscopic spin systems can be designed and fabricated using modern nano-fabrication techniques. These systems can contain large numbers of patterned ferromagnetic elements, for which the shape will generally determine their effective mesospin dimensionality. The lateral arrangement of these mesospins can be further used to tune the interactions between them.With an appropriate choice of material, it is possible to define a temperature range where thermal fluctuations of these mesospins are experimentally accessible. To actively define this range, we use δ-doped Palladium, a three-layer system of Palladium—Iron—Palladium, for which the Curie-temperature scales with the Iron layer thickness. The patterned mesoscopic elements used in this work have a stadium-like shape that promotes a single magnetic domain state, thus making these islands behave as one-dimensional Ising-like mesospins that can be observed using magnetic imaging techniques.We investigate the impact on the magnetic order resulting from modifications of the square spin ice geometry. By adding, removing and merging elements in the square artificial spin ice architecture, energy-landscape variations can be realized. Firstly, an added interaction modifier is used to equilibrate the interactions between the mesospins at the vertex level, which can restore the degenerate ground state of the square spin ice model. Secondly, the removal of elements can lead to topologically frustrated spin systems, as not all building blocks can simultaneously be in their lowest energy state. Furthermore, the merging results in multiple element sizes in the mesospin system. As the magnetization reversal barrier is dependent on the element size, these mesospin systems have different energy barriers. The thermal ordering process in such a system differs from a single-size element system with its unique energy barrier. Using reciprocal space analysis tools like the magnetic spin structure factor we show that systems with multiple element sizes achieve a higher short-range order then their single-size element references. The magnetic order in mesoscopic spin systems could successfully be tailored by modifications of the lattice geometry.
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| 5. |
- Ravensburg, Anna Lena
(författare)
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Growth of high quality Fe thin films : A study of the effect of mismatch strain on the physical properties of Fe
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work in this licentiate is devoted to investigating the epitaxial growth of thin Fe layers on MgAl2O4 (001) and MgO (001) substrates using dc magnetron sputtering. The aim is to qualitatively and quantitatively determine the crystal quality of the grown Fe layers depending on their thickness, substrate material, and selected deposition parameters. The effect of the crystal quality on the magnetic and electronic transport properties is discussed. The structural characterization of the epitaxial Fe thin films is carried out by x-ray reflectometry and diffraction as well as transmission electron microscopy. X-ray scattering measurements and analysis with related models allow for a quantitative determination of layering, crystal quality, and strain profiles in the growing Fe. Magnetic properties are determined using a combination of longitudinal magneto-optical Kerr effect measurements, Kerr microscopy, and scanning electron microscopy with polarization analyser. Electronic transport properties are characterized by four-point probe measurements of the thin films. The epitaxial growth of Fe is found to be highly substrate dependent: Fe layers grown on MgAl2O4 have a significantly higher crystal quality, as compared to Fe grown on MgO. The difference in crystal quality is attributed to different strain states in Fe, which is supported by theoretical calculations of the critical thickness on both substrates. Moreover, an anomalous elastic response in Fe at the thin film limit is found. The magnetic properties of Fe are weakly reflecting the differences in crystal quality of the Fe layers. However, the difference in crystal quality affects the electronic transport properties. The results of this study on epitaxial Fe layers can provide insights into strain and defect engineering in Fe thin films, which can additionally serve as model systems for finite size effects.
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| 6. |
- Sahlberg, Martin, 1981-
(författare)
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Light-Metal Hydrides for Hydrogen Storage
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Demands for zero greenhouse-gas emission vehicles have sharpened with today’s increased focus on global warming. Hydrogen storage is a key technology for the implementation of hydrogen powered vehicles. Metal hydrides can claim higher energy densities than alternative hydrogen storage materials, but a remaining challenge is to find a metal hydride which satisfies all current demands on practical usability. Several metals store large amounts of hydrogen by forming a metal hydride, e.g., Mg, Ti and Al. The main problems are the weight of the material and the reaction energy between the metal and hydrogen. Magnesium has a high storage capacity (7.6 wt.% hydrogen) in forming MgH2; this is a slow reaction, but can be accelerated either by minimizing the diffusion length within the hydride or by changing the diffusion properties. Light-metal hydrides have been studied in this thesis with the goal of finding new hydrogen storage compounds and of gaining a better understanding of the parameters which determine their storage properties. Various magnesium-containing compounds have been investigated. These systems represent different ways to address the problems which arise in exploiting magnesium based materials. The compounds were synthesized in sealed tantalum tubes, and investigated by in situ synchrotron radiation X-ray powder diffraction, neutron powder diffraction, isothermal measurements, thermal desorption spectroscopy and electron microscopy. It is demonstrated that hydrogen storage properties can be improved by alloying magnesium with yttrium or scandium. Mg-Y-compounds decompose in hydrogen to form MgH2 nano-structures. Hydrogen desorption kinetics are improved compared to pure MgH2. The influence of adding a third element, gallium or zinc has also been studied; it is shown that gallium improves hydrogen desorption from YH2. ScAl1-xMgx is presented here for the first time as a hydrogen storage material. It absorbs hydrogen by forming ScH2 and Al(Mg) in a fully reversible reaction. It is shown that the hydrogen desorption temperature of ScH2 is reduced by more than 400 °C by alloying with aluminium and magnesium.
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| 7. |
- Skovdal, Björn Erik
(författare)
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Phase transitions in magnetic metamaterials
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Magnetic metamaterials consisting of arrays of densely packed, two-dimensional nanoscale magnetic islands have degrees of freedom on two separate length scales: inside the islands, and among them. These degrees of freedom can be tuned by e.g. size, shape, island separation and lattice geometry. The material can thereby be tailored to display behavior corresponding to conventional universality classes, wherein small elongated islands behave like Ising spins and circular ones behave like XY-spins. Making the islands larger promotes inner degrees of freedom in the form of inner magnetic textures. Some of these textures, such as magnetic vortices in circular islands, have a critical impact on the interaction between the islands and therefore also on the global order.In this thesis, the interplay between the inner textures and island-island interactions is explored, anticipating the emergence of behavior beyond that of conventional universality classes. A transition temperature between static and dynamic inner textureswas found in systems with elongated islands. In arrays of circular islands, a collapse from metastable collinear islands to vortex islands was observed, with a dependence on both island size and lattice orientation. Finally, a model was created based on key aspects of the circular islands, and using Monte Carlo calculations, an exotic phase diagram with a tricritical point and first order phase transitions was found. The transition is caused by a mutual dependence on the degrees of freedom inside, and among the elements. The experimental and numerical results presented in this thesis signify the existence of such phase transitions in the multiscale material.
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| 8. |
- Ahlberg, Martina, 1980-
(författare)
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Critical Phenomena and Exchange Coupling in Magnetic Heterostructures
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The continuous phase transition in thin magnetic films and superlattices has been studied using the magneto-optical Kerr effect (MOKE) and polarized neutron scattering (PNR). It has been shown that the critical behavior of amorphous thin films belonging to the 2D XY universality class can be described within the same theory as crystalline sample. This means that quenched disorder only serves as a marginal perturbation in systems with this symmetry. The connection between interlayer exchange coupling and the observed critical behavior in Fe/V superlattices was explored. The results prove that the origin of unusually high values of the exponent β can be traced to a position dependence of the magnetization at elevated temperatures. The magnetization of the outermost layers within the superlattice shows a more pronounced decrease at lower temperatures, compared to the inner layers, which in turn have a more abrupt decrease in the vicinity of the critical temperature. This translates to a high exponent, especially when the layers are probed by a technique where more weight is given to the layers close to the surface, e.g.MOKE. The interlayer exchange coupling as a function of spacer thickness and temperature was also studied in its own right. The data was compared to the literature, and a dependence on the thickness of the magnetic layers was concluded. The phase transition in amorphous FeZr/CoZr multilayers, where the magnetization emanates from ferromagnetic proximity effects, was investigated. Even though the determined exponents of the zero-field magnetization, the susceptibility and the critical isotherm did not correspond to any universality class, scaling plots displayed an excellent data collapse. Samples consisting of Fe δ-layers (0.3-1.4 monolayers) embedded in Pd were studied using element-specific resonant x-ray magnetic scattering. The magnetization of the two constituents showed distinctly different temperature dependences.
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| 9. |
- Carstensen, Hauke
(författare)
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Self-assembly of magnetic particles
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Self-assembly is the spontaneous formation of larger structures from small building blocks. This process is driven and determined by the interactions between the constituents. Examples of self assembly are found almost everywhere and, in particular, biological systems in general rely on a hierarchical formation of structures over a range of length scales. Technologically, self-assembly can be used to form mesoscopic structures and artificial crystals. In the case of particles with micrometer size suspended in a liquid phase, it is possible to use optical microscopy for the the investigation of self-assembly.In this thesis, the self-assembly of microbeads with tunable magnetic interactions is studied, based on the statistic analysis of microscope images and computer simulations. Magnetic and non-magnetic microbeads are suspended in a ferrofluid, which is a dispersion of magnetic nanoparticles in water. As a result, the magnetic properties of the microbeads in the ferrofluid are altered and can be described by effective magnetic susceptibilities and magnetic dipole moments, which can be tuned continuously. The liquid is confined between glass slides and effectively the microbeads are studied in a 2D geometry under a magnetic field, applied either in- or out-of-plane. The resulting structures are detected by image analysis algorithms, analyzed and correlated to the dipolar interaction between the beads, as well as to macroscopic quantities, like the particle density and ratio. For the in-plane field a phase transition from square to hexagonal lattice is observed. This phase transition is explained by the change in dipole interaction between the microbeads as the moments change from anti-parallel to parallel alignment. For the out-of-plane field the situation becomes diverse and more phases appear. It turns out that the phase formation in this case is strongly dependent on the bead ratio, density and interactions.We identify regions in the phase diagram, where isolated beads, percolated structures, and crystals dominate. To cover a wide parameter range the experiments are complemented by computer simulations. The tools developed in this thesis enable us to construct phase diagrams extracted from direct imaging and dependence on the extracted relevant parameters.
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| 10. |
- Goetz, Inga Katharina, 1992-
(författare)
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Local structure and composition : in additively manufactured bulk metallic glasses and composites
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Additive manufacturing enables the production of complex multi-material geometries and bulk metallic glass parts beyond their critical casting thickness. The local modification of structure, composition, and properties is explored in this thesis with the aim to design novel composite materials and functional gradients. The present work contributes to the process understanding required to produce bulk metallic glass composites in the laser powder bed fusion process. The investigated material systems include zirconium and iron based metallic glasses, metallic glass - nitride composites, as well as laser nitriding of titanium.The onset of devitrification of metallic glasses due to the processing or post-processing heat treatments induces the formation of nanoscale clusters. Features of > 1 nm can be detected and reliably distinguished from random fluctuations by atom probe tomography. The progression of crystallisation depends on the oxygen content of the samples and the applied heating or cooling rates. In-depth understanding of the crystallisation processes can be used to optimise both compositions and processing conditions. The process atmosphere contains reactive species such as oxygen or nitrogen, which can be incorporated during different stages of processing by surface oxidation of the powder or substrate pieces as well as by reactions with the gas during processing. While an inert gas atmosphere with a low residual oxygen content is sufficient to hinder reactions with residual oxygen, a nitrogen atmosphere can be used for local laser nitriding and, thus, the fabrication of metallic glass – ceramic composites. Due to the decompositions of nitrides formed in the preceding process steps and the tendency of a metallic glass matrix to crystallise, which limits the processing conditions, the incorporation of nitrogen is restricted to the first few hundred nm from the surface for a Zr-based amorphous alloy. In titanium, as a crystalline example, nitrogen is incorporated throughout the molten pool.The nitride composites exhibit increased hardness depending on the local nitride fraction, which can thus be used to fabricate specific property gradients within or on a printed piece. With optimised process parameters, the amorphous fraction of a printed Fe-based bulk metallic glass can be tailored for improved soft magnetic properties.
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