| 1. |
- Čiučiulkaitė, Agnė, MSc, 1991-
(författare)
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Dynamics in magnetic metamaterials
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Metamaterials are artificially created structures with properties that are not found in nature. They can be tailored to achieve desired response to external excitations such as external electric and magnetic fields, as well as to enhance materials’ optical or magnetic activity. Magnetic metamaterials comprised of arrays of sub-micrometer sized magnetic elements, can be used for a range of different applications, such as magnonic crystals and building blocks for magnetic memory elements.In this work, pathways for tuning magnetization dynamics are explored. Different magnetic metamaterials containing arrays of sub-micrometer sized elements, refered to as nanomagnets, were used as model systems for these explorations. The nano-magnets comrising these arrays are of two kinds: XY-rotors, with a magnetization direction rotating freely in-plane of a disk, and Ising-like spins, pointing along either of the two allowed magnetization directions. The Ising-like spins can be realized in either in-plane or out-of-plane magnetized materials.Collective magnetization dynamics were investigated in square arrays of coupled nanomagnets. Studies revealed that nanomagnet’s magnetization state cannot always be approximated by a ridig mesospin approximation. Instead, it was demonstrated that upon an external perturbation, such as an external magnetic or thermal field, internal magnetization experience texture excitations. The observed texture excitations have implications on the nanomagnet coupling in an array.Arrays, where collective phenomena emerge via excited plasmon resonances, were used for investigations of light-induced dynamics processes. Studies revealed importance of an array design for the observation of magneto-optical activity enhancement and more eÿcient ultrafast magnetization dynamics. It was reported that in arrays containing truncated Au nanocones with TbCo tip, enhancement of TbCo demagnetization can be achieved via resonant light illumination.Utilizing the array concept and magnetic anisotropy of a material, it is possible to create metamaterials, where a range of magnetization dynamics regimes can be investigated. Inter-element spacing defines on which lengthscale the nanomagnets will be coupled and which effects can be utilized for tuning system’s magneto-optical response and excited magnetization dynamics.
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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- Strandqvist, Nanny
(författare)
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Modulating magnetic interactions : in metamaterials and amorphous alloys
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is focused on exploring and modulating magnetic interactions in metamaterials and amorphous alloys along one-, two-, and three-dimensions. First, thin films of alternating Fe and MgO are adapted to modulate magnetic interactions along one dimension. At the remanent state, the Fe layers exist in an antiferromagnetic order, achieved by interlayer exchange coupling originating from spin-polarized tunneling through the MgO layers. Altering the number of repeats can tune the strength of the coupling. This is attributed to the total extension of the samples and beyond-nearest-neighbor interactions. Similarly, decreasing the temperature results in an exponential increase of the coupling strength, accompanied by changes in the reversal character of the Fe layers and magnetic ground state.Next, magnetic modulations along two dimensions are investigated using lithographically patterned metamaterial consisting of arrays with mesospins - i.e., circular islands. Mesospins have degrees of freedom on two separate length scales, within and between the islands. Changing their size and lateral arrangement alters their behavior. The magnetic texture in small elements can be described as collinear with XY-like behavior, while larger islands result in magnetic vortices. Allowing the islands to interact by densely packing them in a square lattice alters the energy landscape. This is manifested by the interplay of intra- and inter-island interactions and leads to temperature-dependent transitions from a static to a dynamic state. The temperature dependence can be further altered by both element size and lattice orientation, leading to emergent behavior.The final part of this thesis explores the modulations of interactions in three dimensions through inherent disorder in magnetic amorphous alloys. The atomic distribution in amorphous alloys can be viewed as random. However, local composition at the nanometer scale is, in fact, homogeneous. Variations in the composition of amorphous CoAlZr alloys lead to changes in the local distribution of magnetic amorphous CoAlZr manifested by competing anisotropies. Finally, off-specular scattering performed on a magnetic amorphous FeZr alloy is used to investigate the compositional variations at the nanometer scale. Indeed, correlations are observed at low temperatures due to the sample relaxation.
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| 7. |
- Warnatz, Tobias
(författare)
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Magnetic Properties of Epitaxial Metal/Oxide Heterostructures
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work in this dissertation is devoted to tailoring and studying magnetic properties of epitaxial metal/oxide heterostructures. The aim is to understand the fundamental principles governing these properties and how they affect each other. The acquired knowledge can prove useful for the development of future spintronic devices. A variety of experimental techniques is used to fabricate and characterize the epitaxial structures. For fabrication, a combination of direct-current and radio-frequency sputtering is used, whereas x-ray reflectivity and diffraction measurements are the main tools for the structural characterization of the heterostructures. The magnetic characterization of these structures is done by a combination of longitudinal magneto-optical Kerr-effect measurements, Kerr-microscopy and polarized neutron reflectometry. First, it is shown how strain affects the magnetic properties of metal/oxide heterostructures by comparing Fe/MgO and Fe/MgAl2O4 superlattices. Subsequently, an antiferromagnetic interlayer exchange coupling in Fe/MgO superlattices is revealed and attributed to a spin-polarized-tunneling mechanism. The coupling strength can be tuned by changing the MgO thickness leading to the stabilization of different remanent states as well as to different reversal mechanisms. It is shown that the interlayer exchange coupling in Fe/MgO superlattices is a consequence of two distinct components. These components can be interpreted as beyond-nearest-neighbor interactions and a contribution arising from the total thickness of the heterostructures.The interlayer exchange coupling is further investigated via temperature dependent magnetization measurements. It is shown that different remanent states and reversal mechanisms occur at different temperatures. Furthermore, a large increase in interlayer exchange coupling strength with reduced temperature is revealed. Finally, it is shown that Fe84Cu16/MgO superlattices exhibit a reduced magnetocrystalline anisotropy and interlayer exchange coupling strength, as compared to pure Fe/MgO superlattices. Patterning such Fe84Cu16/MgO superlattices in circular islands leads to an increased saturation field with decreasing island diameter.
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