Wakefields and the corresponding frequency-domain phenomenon beam coupling impedance have been well studied for a number of years as a source of beam instabilities within particle accelerators. With the development of the Large Hadron Collider (LHC) and the large beam currents stored in the LHC during fills for physics production, wakefield driven instabilities and strong beam induced heating have become a limiting factors in luminosity production due to both instantaneous luminousity and the available time for collisions.In this thesis is presented an in depth study of the beam coupling impedance of two important (from an impedance and operational point of view) devices in the LHC; the collimation system and the injection kicker magnets (MKIs). These systems have both been sources of concern for the beam impedance of the LHC, the collimators due to their large transverse impedance and the MKIs due to the strong heating observed during the increased of beam current during operation in 2011 and 2012. The source of the heating for the MKIs is studied in depth, found to be power lost by the beam to wakefields in the MKIs. Simulations and measurements are used to characterise the impedance and localise the areas responsible for the high impedance, here the beam screen and ferrite yoke of the magnet; improvements are proposed to better screen the ferrite yoke and verified. A new RF damping system using ferrite for the collimation system is studied and compared to the existing RF damping system, focusing on the heating of the damping system. Highlights include a new method for measuring the quadrupolar and constant transverse impedances of an asymmetric structure using a coaxial wire technique is proposed and verified using computational simulations, and a study of the heat loss in a ferrite damped cavity, focusing on the location of the power loss for cavities being damped to varying degrees.

New and more efficient magnetic materials for energy applications are a big necessity for sustainable future. Whether the application is energy conversion or refrigeration, materials based on sustainable elements should be used, which discards all rare earth elements. For energy conversion, permanent magnets with high magnetisation and working temperature are needed whereas for refrigeration, the entropy difference between the non-magnetised and magnetised states should be large. For this reason, magnetic materials have been synthesised with high temperature methods and structurally and magnetically characterised with the aim of making a material with potential for large scale applications. To really determine the cause of the physical properties the connections between structure (crystalline and magnetic) and, mainly, the magnetic properties have been studied thoroughly. The materials that have been studied have all been iron based and exhibit properties with potential for the applications in mind. The first system, for permanent magnet applications, was Fe5SiB2. It was found to be unsuitable for a permanent magnet, however, an interesting magnetic behaviour was studied at low temperatures. The magnetic behaviour arose from a change in the magnetic structure which was solved by using neutron diffraction. Substitutions with phosphorus (Fe5Si1-xPxB2) and cobalt (Fe1-xCox)5PB2 were then performed to improve the permanent magnet potential. While the permanent magnetic potential was not improved with cobalt substitutions the magnetic transition temperature could be greatly controlled, a real benefit for magnetic refrigeration. For this purpose AlFe2B2 was also studied, and there it was found, conclusively, that the material undergoes a second order transition, making it unsuitable for magnetic cooling. However, the magnetic structure was solved with two different methods and was found to be ferromagnetic with all magnetic moments aligned along the crystallographic a-direction. Lastly, the origin of magnetic cooling was studied in Fe2P, and can be linked to the interactions between the magnetic and atomic vibrations.

Permanent magnets are essential components for many devices such as motors, which currently account for 45 % of global electricity consumption, generators and also superconducting magnetic bearings used for applications such as flywheel energy storage. But even the most powerful rare-earth magnets are limited to a remanent field of 1.4 T, whereas superconducting materials such as YBCO in their bulk form have the extraordinary ability to trap magnetic fields an order of magnitude higher, whilst being very compact. This gives them the potential to increase efficiency and allow significant volume and weight reductions for rotating machines despite the need for cooling. A new design of superconducting magnetic bearing has been developed which uses magnetized bulks as the field source, eliminating permanent magnets. Finite element modelling shows that the bulk – bulk design can achieve much higher force densities than existing permanent magnet – bulk designs, giving it potential to be used as a compact magnetic bearing. A system was created to magnetize bulks using a pulsed magnetic field down to 10 K and then measure levitation force. In proving the concept of the proposed design, the highest levitation forces ever reported between two superconducting bulks were measured, including a levitation force of 500 N between a 1.7 T magnetized YBCO bulk and a coaxial $MgB_{2}$ bulk tube. The biggest factor limiting the use of magnetized bulks in applications is magnetizing them in the first place. Using a pulsed magnetic field is most practical but generates excessive heat dissipation leading to a loss of flux in conventional bulk superconductors, which are 100% superconductor. Although multi-pulse techniques help maximise the trapped field, the poor thermal properties of bulk (RE)BCO are a limiting factor. New composite superconducting structures are reported which can overcome these problems by using high thermal conductivity materials, the motivation for which came from finite element modelling of the critical state coupled with heat transfer. In particular, composite structures created by cutting and stacking 12 mm wide (RE)BCO superconducting tape are shown experimentally to have exceptional field trapping ability due to superior thermal and mechanical properties compared to existing bulks. Up to 2 T was trapped in a stack of commercially available tape produced by SuperPower Inc. in the first reported pulsed magnetization of such a stack. Over 7 T was trapped between two stacks using field cooling at 4.2 K, the highest field yet trapped in such a sample.

In order for molecular magnetic materials to become applicable, they must retain their magnetisation at reasonable temperatures, which can be achieved with high energy barriers for spin reversal and high blocking temperatures. In the field of Single-Molecule Magnets (SMMs), over the last decade, the main focus has shifted from large spin complexes to highly anisotropic systems which have displayed record energy barriers. There are two main methods of increasing magnetic anisotropy in a complex: i) Choosing a metal ion that boasts high magnetic anisotropy then coupling two such ions through magnetic interactions to induce large global anisotropy, and ii) maintain a low spin or use a mononuclear complex while minimising quantum tunnelling of the magnetisation by controlling the geometric features of the metal ion. Both strategies are equally valid and have been explored in this thesis using dinuclear lanthanide as well as mononuclear 3d complexes. In the pursuit of high-barrier SMMs via alignment of anisotropy axes, two dinuclear, quadruple-stranded helicates and one mesocate were isolated and are described in detail herein, both structurally and magnetically. Furthermore, theoretical calculations have been performed to determine the energies of Kramers doublets on each DyIII centre to derive magneto-structural correlations. To induce magnetic interactions between DyIII ions, a centrosymmetric dinuclear SMM was synthesised. Investigation of the crucial DyIII…DyIII interaction as well as its effect on the quantum tunnelling of the magnetisation has been carried out using ab initio calculations and magnetic dilution studies. Using the same system, a method of greatly enhancing the energy barriers in SMMs has been developed. It involves modifying the coordinating ligands to include electron withdrawing groups in order to yield more anisotropic metal ions. The energy barrier for spin reversal has been increased 7-fold in one case. While lanthanide chemistry has proven to be quite versatile and promising, a new branch of nanomagnets is currently being pursued: mononuclear 3d complexes as SMMs. The advantages of 3d metals include high anisotropy per ion, low spin (as anisotropy decreases with increasing spin), well-understood electronic structures and clear correlations between geometry and magnetic anisotropy. The structural and magnetic properties of three complexes based on CoII and terpyridine ligands as well as a seven-coordinate CoII complex with positive anisotropy are discussed at length. The unique slow relaxation dynamics and spin crossover behaviour has been followed using DFT and ab initio calculations, as well as EPR and magnetic dilution studies. Overall, this thesis describes the efforts taken to synthesise high-barrier nanomagnets through understanding the origins and mechanisms of slow magnetic relaxation in both lanthanide and 3d metal complexes.

Molecular Nanomagnets have been proposed as plausible candidates in a variety of futuristic applications. Thorough understanding of the magnetic properties of these systems is therefore necessary to develop devices that include such units. The aim of this thesis is to synthesise and structurally and magnetically characterise a range of systems that could be used as elementary units in three proposed applications such as: data storage devices, magnetic refrigerants and qubits for quantum computing. A series of mixed 3d/4f metal complexes were synthesised through solvothermal reactions and characterised by X-ray single crystal diffraction and SQUID magnetometry. Through indirect methods it was possible to obtain high magnetic entropy change for some systems. It was also possible to obtain some insight into the magnetic interactions within the systems through modelling the magnetic data. The role of the 4f-4f and 3d-4f interactions in two sets of molecules is also described. The first study is in an asymmetric dysprosium dimer, where through a range of experimental techniques and advanced theoretical methods, such ab-initio calculations we are able to explain the role of the intramolecular interactions and their effect on the SMM properties of this system. Similarly, insight into the role of the 3d-4f interactions is achieved through the observation of the magnetic behaviour of a family of 27 tetranuclear systems, though SQUID data and ab-initio calculations. Finally, chemical functionalization of a well-proposed qubits, namely {Cr7Ni} and subsequent reaction with a redox active metal ion, CoII/III, two {Cr7Ni} systems are linked. The magnitude of the exchange interaction between the {Cr7Ni}-CoII-{Cr7Ni} was determined through Electron Paramagnetic Resonance. Furthermore, by chemical oxidation/reduction of the cobalt between paramagnetic and diamagneticstates, i.e. CoII and CoIII respectively, we demonstrate that the interaction can be switched ON/OFF. This characteristic makes of these systems candidates to function as a SWAP gate.

The introductory Chapter to this thesis outlines fundamental aspects of 4f lanthanide(III) coordination chemistry, in particular compounds that possess the intriguing properties of slow relaxation of magnetisation, (or the ability to behave as single-molecule magnets, SMMs). The recent renaissance into the study of the magnetic behaviour of 4f-coordination complexes has led to the consideration of utilising organometallic precursors for the development of novel lanthanide containing compounds, which may possess interesting magnetic properties, subsequently forming the basis of Chapter Two. In Chapter Two, the syntheses and structures of the novel lithiated thiolate ligand, lithium triphenylsilylthiolate, (Ph3SiS-Li) (2.1), and the sulfur-bridged, dimetallic dysprosium(III) and gadolinium(III) complexes [(MeCp)2Dy(µ-SSiPh3)]2 (2.2) and [(MeCp)2Gd(µ-SSiPh3)]2 (2.3), are described in detail. The structural and physical properties of these compounds are analysed through NMR, elemental analysis and SQUID magnetometry, alongside supporting theoretical calculations to reveal that compound 2.2 is the first dimetallic, sulfur-bridged SMM reported, giving an energy barrier to the reversal of magnetisation of Ueff = 192 ± 5 K.56bChapter Three reports on the structural development of a series of lanthanide monomers, exhibiting the general motif [Ln(OSiPh3)3(THF)3] (where Ln = Dy(3.4), Er(3.5), Ho(3.6), Gd(3.7), Tb(3.8)), exploiting the siloxide ligand Ph3SiOH through two novel synthetic routes. This Chapter also provides new analytical insight to these complexes by exploring their magnetic properties through a series of SQUID measurements and through the analysis of their electronic properties using air sensitive, variable temperature optical absorption spectroscopy. Compounds 3.4 and 3.5 were revealed to be SMMs, with 3.5 having a much higher thermal barrier to the reversal of magnetisation, Ueff = ~ 28 K, than 3.4, which are supported by theoretical analysis. Chapter Four describes the utility of ligand 2.1 and Ph3SiOH in the context of 3d transition metal cyclopentadienyl chemistry, outlining the syntheses and structures of three distinct compounds; the trimetallic, [Cp2Mn3(µ-OSiPh3)4](4.7), the hetero-cubane tetramer [CpMn(µ-SSiPh3)]4 (4.8) and the dimetallic thiolate-bridged [CpCr(µ-SSiPh3)]2 (4.9) compound. These compounds were formed in reactions exploiting organometallic manganocene and chromocene precursors. Magnetic susceptibility measurements were conducted on these compounds to gain further insight into their structural properties. The magnetic exchange coupling constants for Mn(II) compounds 4.7 and 4.8 were J = - 4.4 cm-1 and J = - 3.0 cm-1 respectively. Furthermore, having demonstrated the use of metal-cyclopentadienyl building blocks in the synthesis of novel SMMs, Chapter Five discusses the possibility of further advancement on the development of this class of magnetic molecules.

Obtaining new insights into the behavior of free-living marine organisms is fundamental for conservation efforts and anticipating the impact of climate change on marine ecosystems. Despite the recent advances in biotelemetry, collecting physiological and behavioral parameters of underwater free-living animals remains technically challenging. In this thesis, we develop the first magnetic underwater animal monitoring system that utilizes Tunnel magnetoresistance (TMR) sensors, the most sensitive solid-state sensors today, coupled with flexible magnetic composites. The TMR sensors are composed of CoFeB free layers and MgO tunnel barriers, patterned using standard optical lithography and ion milling procedures. The short and long-term stability of the TMR sensors has been studied using statistical and Allan deviation analysis. Instrumentation noise has been reduced using optimized electrical interconnection schemes. We also develop flexible NdFeB-PDMS composite magnets optimized for applications in corrosive marine environments, and which can be attached to marine animals. The magnetic and mechanical properties are studied for different NdFeB powder concentrations and the performance of the magnetic composites for different exposure times to sea water is systematically investigated. Without protective layer, the composite magnets loose more than 50% of their magnetization after 51 days in seawater. The durability of the composite magnets can be considerably improved by using polymer coatings which are protecting the composite magnet, whereby Parylene C is found to be the most effective solution, providing simultaneously corrosion resistance, flexibility, and enhanced biocompatibility. A Parylene C film of 2μm thickness provides the sufficient protection of the magnetic composite in corrosive aqueous environments for more than 70 days. For the high level performance of the system, the theoretically optimal position of the composite magnets with respect to the sensing direction of the sensor has been estimated using finite element modeling software. The magnetic sensing system has been practically implemented for monitoring the belly size of a model fish and for monitoring the behavior of the largest living bivalve, giant clam (Tridacna maxima) in an aquarium. In both of these experiments, the sensing system showed a high performance, indicating its potential for novel marine monitoring applications.

abstract: Transition metal di- and tri-halides (TMH) have recently gathered research attention owing to their intrinsic magnetism all the way down to their two-dimensional limit. 2D magnets, despite being a crucial component for realizing van der Waals heterostructures and devices with various functionalities, were not experimentally proven until very recently in 2017. The findings opened up enormous possibilities for studying new quantum states of matter that can enable potential to design spintronic, magnetic memory, data storage, sensing, and topological devices. However, practical applications in modern technologies demand materials with various physical and chemical properties such as electronic, optical, structural, catalytic, magnetic etc., which cannot be found within single material systems. Considering that compositional modifications in 2D systems lead to significant changes in properties due to the high anisotropy inherent to their crystallographic structure, this work focuses on alloying of TMH compounds to explore the potentials for tuning their properties. In this thesis, the ternary cation alloys of Co(1-x)Ni(x)Cl(2) and Mo(1-x)Cr(x)Cl(3) were synthesized via chemical vapor transport at a various stoichiometry. Their compositional, structural, and magnetic properties were studied using Energy Dispersive Spectroscopy, Raman Spectroscopy, X-Ray Diffraction, and Vibrating Sample Magnetometry. It was found that completely miscible ternary alloys of Co(1-x)Ni(x)Cl(2) show an increasing Néel temperature with nickel concentration. The Mo(1-x)Cr(x)Cl(3) alloy shows potential magnetic phase changes induced by the incorporation of molybdenum species within the host CrCl3 lattice. Magnetic measurements give insight into potential antiferromagnetic to ferromagnetic transition with molybdenum incorporation, accompanied by a shift in the magnetic easy-axis from parallel to perpendicular. Phase separation was found in the Fe(1-x)Cr(x)Cl(3) ternary alloy indicating that crystallographic structure compatibility plays an essential role in determining the miscibility of two parent compounds. Alloying across two similar (TMH) compounds appears to yield predictable results in properties as in the case of Co(1-x)Ni(x)Cl(2), while more exotic transitions, as in the case of Mo(1-x)Cr(x)Cl(3), can emerge by alloying dissimilar compounds. When dissimilarity reaches a certain limit, as with Fe(1-x)Cr(x)Cl(3), phase separation becomes more favorable. Future studies focusing on magnetic and structural phase transitions will reveal more insight into the effect of alloying in these TMH systems. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2020

The master’s thesis deals with the design of a synchronous motor with embedded magnets. First of all, the general theory of synchronous motor, which is focused on field-weakening drive, is described. Then a 6-pole and a 8-pole version of the motor with embded magnet, which has 65 kW and 3000 rpm is complexly designed. Motors are compared with each other and with a motor with surface magnets, that was also designed in this work. Finally, all motors are compared with a mafufactured and measured motor.

In diesem Beitrag wird der Einfluss der Formfreiheit von gespritzten kunststoffgebundenen Permanentmagneten (PM) auf die Parameter eines Memory Motors, einer neuartigen Motortopologie, die es ermöglicht den PM-Fluss mit kurzen Stromimpulsen zu regeln. Mithilfe numerischer FEM-Simulationen einer in der Literatur vorgeschlagenen Memory Motor Topologie wurde demonstriert, dass der Einsatz der Magnete mit komplexer Form eine Verringerung der Drehmomentwelligkeit ermöglicht.