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Typical thermodynamic systems have a small number of minimum-energy ground states into which the system strives to arrange itself when the temperature is sufficiently low. In some cases, however, competition between interactions leads to the suppression of conventional long-ranged magnetic ordering, through a mechanism known as frustration. One of the typical features is the appearance of a large number of degenerate (or nearly degenerate) minimal energy states, that can exhibit topological order. The dynamical properties of topological systems, in particular those that host fractionalised excitations, is currently a topic of intense research activity. This thesis focuses on one specific instance of a topological frustrated magnetic system: spin ice. Spin ice is one of the most thoroughly researched frustrated models, exhibiting exciting phenomena typical of topological matter - including an emergent gauge field description with fractionalised excitations that take the form of emergent magnetic monopoles. Highly accurate numerical methods and analytical techniques have been developed to model spin ice materials, and have facilitated a detailed understanding of their static properties. The primary aim of this thesis is to address some of the outstanding questions about the dynamics of classical spin ice materials. The focus will be on two dynamical properties: the magnetic relaxation time and the magnetic noise spectrum. Several independent experiments, some dating back over two decades, have found that the magnetic relaxation time in these materials diverges rapidly upon cooling - more rapidly than what any previous theory can explain without invoking extrinsic contributions such as surface effects, disorder, and temperature dependent microscopic time scales. Previous theories also predict that the magnetic noise spectrum of spin ice should be Lorentzian, with a power spectral density that decays as ν^(-2) with the frequency, ν. Here, noise spectra measured on a single crystal of Dy₂Ti₂O₇ are presented and shown to instead decay as ν^(−α), with an anomalous exponent α ≈ 1.5. Through a combination of extensive numerical modelling and fundamental arguments about the motion of the magnetic monopoles, I establish that the conventional model of spin ice dynamics cannot explain the anomalous magnetic noise. I resolve these issues by introducing a new model for the dynamics, which includes the effects of local symmetry on the flip rate of spins. The main insight the new model leads to is the emergence of dynamical fractal structures on which the magnetic monopoles are constrained to move. It is by hosting the monopole motion that the fractals bequeath the magnetic noise with an anomalous exponent and cause a slow-down of the relaxation. The spatial properties of the fractals thus manifest themselves in the time dependence of the magnetisation - and the anomalous dynamics observed in Dy₂Ti₂O₇ are evidence of fractal objects in a disorder-free, bulk crystal. Indeed, the new model dynamics quantitatively captures the experimentally observed noise spectrum and relaxation times, without the introduction of any additional fitting parameters. The dynamical fractals cannot be imaged directly, but do influence other dynamical properties, besides the already discussed magnetic noise and relaxation time. Using simulations and simplified models of analogous systems, I investigate how the fractals affect both equilibrium and out-of- equilibrium transport properties. The response of spin ice to an oscillating magnetic field is identified as one particularly promising area to explore in future experiments, with suggestions made for the field and temperature regimes in which fractals are likely to manifest themselves. An alternative spin ice model capable of reproducing the experimentally observed relaxation time and anomalous magnetic noise, but incompatible with the thermodynamics of spin ice materials, is also introduced. The model includes a specific choice of third-nearest-neighbour exchange interactions, which are frustrated independently of the usual spin ice interactions. These additional interactions are found to induce a phase transition between the normal spin ice phase and a spin nematic phase - a phase displaying rotational symmetry breaking without long-ranged order. In the "nematic spin ice" phase, low energy paths are formed along which magnetic monopoles move freely. These paths form anisotropic fractal clusters with properties akin to the dynamical fractals mentioned previously. Thus, the anomalous noise spectrum is again found to originate from the motion of magnetic monopoles on fractals.

The coexistence of different electronic orders in condensed matter systems is an actively explored area of experimental and theoretical research. This is both to advance understanding and, since different phases, like superconductivity or magnetism, can be utilised on the microscopic and macroscopic scale, for device applications. Exploration of the complex phase diagrams which emerge in many actinide materials requires careful synthesis of samples, as the microstructure and lattice defects of samples can affect thermodynamic and transport properties. Characterisation with microstructure probes and crystallographic diffraction is important for an informed interpretation of the resulting material properties. This thesis investigates the synthesis and characterisation of two uranium binary compounds, U2Ti and UTe2, with coexisting electronic correlations, paying particular attention to the effect of synthesis conditions on the microstructure, inclusion of other phases, and lattice disorder. A computational study of U2Ti predicted it to undergo a transition to charge density wave (CDW) order on the basis of density functional theory calculations of the phonon spectrum. A previously reported experimental investigation of the material identified it as a bulk superconductor (SC) at 0.38 K, but did not identify a CDW transition. Coexistence of SC and CDW order is especially interesting as both compete for states at the Fermi level and gap the Fermi surface, with the former leading to a zero resistance state and the latter giving rise to an insulating state (or decrease in conductivity) in many systems. The motivation for studying U2Ti was thus to investigate the predicted CDW order and the possible coexistence with superconductivity. Synthesis of U2Ti single crystals is challenging due to a disordered to ordered solid transition. Therefore, we explored the synthesis conditions and their impact on the properties of samples produced. Through a combination of thermodynamic, electrical transport and xray diffraction (XRD) measurements (conducted at the ESRF, Grenoble, France) we confirm the emergence of incommensurate CDW order at 71K followed by a commensuratation transition at 46 K. The measurements also suggest that at ambient pressure, U2Ti is not a bulk SC and previous identification of SC order in the system was due to inclusions. Energy dispersive x-ray (EDX) analysis is used to identify the inclusion of an α-U phase and, in combination with XRD measurements, we are able to identify the alignment of this inclusion with the bulk U2Ti lattice. UTe2 is an unconventional SC actively being studied to determine its order parameter and pairing mechanism. Due to the persistence of SC order in magnetic fields exceeding 40 T, the material is considered a candidate for spin triplet pairing, mediated by ferromagnetic fluctuations evidenced by the divergence of the dc magnetic susceptibility at low temperature. However, under pressures exceeding 1.7GPa antiferromagnetic order emerges upon the destruction of SC order. In order to make conclusive statements about the SC order pure samples of UTe2 are required, and the variation of thermodynamic and transport properties reported in the literature needed to be related to the growth conditions of samples. Therefore, the investigation presented in this thesis focused on departing from the standard growth conditions used widely in other studies to investigate the impact on sample properties. It is shown that varying the stoichiometry of the reactants in the chemical vapour transport growth of the compound can produce samples of higher superconducting transition temperature and lower residual heat capacity. This also impacts the magnetic susceptibility below 15K suggesting some impact on magnetic fluctuations. From magnetic susceptibility measurements on samples from different syntheses we are able to identify ferromagnetic impurities while, EDX measurements suggest how these impurities are distributed. We discuss the difficulty in identifying the differences in microstructural and crystallographic properties of samples with different SC transition temperatures, which remains an outstanding issue in the literature. The investigation of U2Ti has identified a new uranium based CDW compound and revealed that SC order previously identified is likely associated with inclusions and/or filamentary SC. The results motivate the further synthesis of bulk single crystal samples to explore the phase diagram of this material. The work on UTe2 has moved forward the understanding of the synthesis conditions on the superconducting and magnetic properties, as well as identifying inclusions. The results suggested that an early explanation of the superconducting order of UTe2 which involved pairing of only one spin channel of electrons, with a remaining unpaired electron liquid contributing to a residual Sommerfeld coefficient of heat capacity, is unlikely. The precise form of disorder in UTe2 that drives variations in the superconducting and magnetic properties remains an outstanding issue. Though it is determined, by XRD, that non-SC samples are compositionally distinct from SC samples due to the presence of uranium vacancies in the crystal structure. More broadly, the combination of these investigations contributes to the exploration of the coexistence of superconductivity and other electronic correlations (and order) in uranium containing compounds and motivates new areas of exploration.

Magnetic materials can host a rich variety of nanoscale magnetic structures with non-collinear spin orientations, known as topological spin textures. A plethora of intriguing phenomena entwines magnetism and topology, especially where a Dzyaloshinskii-Moriya interaction is present. Exploitations of the robust topological stability inherent in these textures promise applications in magnetic data storage and nanocomputing devices. To fully realise the potential of these spin structures, fundamental studies exploring the underlying mechanisms in real systems are essential with specific foci, including their formation, stability, transition, detection, and annihilation. The work presented in this thesis explores topological spin textures in the context of three-dimensional ferromagnets and addresses the ramifications of field-induced transitions, thermal stabilities, and associated emergent electromagnetic fields. In particular, from the three-dimensional perspective, fruitful insights are spotted by considering the depth-resolved complexity of magnetisation profiles. The first aspect of this study lies in the complex dynamics of extremely small topological defects - Bloch points, during the temporal evolutions of single topological spin textures, such as skyrmion tubes in bulk magnets and skyrmion chains in multilayers. Under the application of an external magnetic field, the transition mechanism is dominated by the combined effects of the Dzyaloshinskii-Moriya interaction and magnetostatic interaction. During the process, the Bloch-point propagation speeds are km/s. Meanwhile, the speed is periodically modulated by regular ''washboard'' potential wells that originate from the atomic arrangement, and oscillates with a THz frequency. Moreover, we report the thermal stabilities of such systems via exploration of minimum energy paths to other possible equilibrium states. It seeks to build upon the knowledge that topological spin textures can be stabilised as metastable states in a wide parameter space. The results show that the transitions are mediated by Bloch-point dynamics, where the presence of saddle points in the energy landscape is associated with the creation/destruction of Bloch points. It is notable that due to the discrete nature of magnetic systems, the topological protection property partially breaks, which leads to finite magnitudes of energy barriers. The resulting spatial and temporal non-uniformities of the magnetisation within the topological structure create emergent electromagnetic fields that act on conduction electrons. We reveal a phenomenological connection between the emergent magnetic field and three-dimensional topology, allowing unambiguous recognition of different textures based on their topological Hall signals. The transitions also bring drastic modifications to local emergent magnetic fields. It is shown that the ultra-fast propagation of a Bloch point radiates solenoidal emergent electric fields with MV/m magnitude and THz oscillation frequency. Using the understandings of the studies in this thesis, we propose several designs that seek to enable tuneabilities of thermal stabilities and ultra-fast transition phenomena. Relevant techniques are compatible with existing nanofabrication technologies. Thus, validation by future experiments should be within reach.

Magnetic materials play a key role in the advancement of modern technology, from data storage mediums and giant magnetoresistance read heads for information processing, to medical applications like drug delivery and magnetic resonance imaging. Their research and development have progressed towards ever-reducing spatial and magnetic footprints, largely motivated by promising novel properties emerging at reduced dimensions and the miniaturization of technology. Sustained research progress will therefore require the timely development of scalable probing techniques with enhanced sensitivities. State-of-the-art material research requirements of non-perturbative measurements, with resolutions of at least a hundred nanometers, and sensitivities down to single magnetic atomic layers, are becoming increasingly inaccessible by conventional spatial imaging techniques. In this respect, quantum sensing via nitrogen-vacancy centres in diamond promises to address contemporary and prospective imaging needs. Employed on a scanning probe platform, the scanning diamond quantum microscope is capable of nanoscale imaging with an unprecedented magnetic sensitivity down to the nano-Tesla regime and minimal magnetic back-action on the target. Scanning diamond quantum microscopy has been recently used to image a number of novel magnetic systems, however a large subset of materials, including various antiferromagnets and two-dimensional magnets, remains unexplored. Moreover, the technique is capable of secondary sensing modalities that are relatively underdeveloped and require systematic studies to understand their potentials and limitations. This thesis describes the work I have undertaken towards implementing a scanning diamond quantum microscope operational in cryogenic and ambient environments. I utilise this setup to develop imaging capabilities to study nanoscale non-trivial spin textures in topological (anti)ferromagnetic materials. The capabilities developed and physical insights revealed in this work establish the diamond quantum microscope as a powerful investigative tool for accelerating functional materials research towards future technologies.

Sodium-ion batteries (NIBs) are increasingly of interest in modern society as more affordable energy storage alternatives to lithium-ion batteries. At present, the electrochemical performance-the charge/discharge rate, lifetime and capacity-of NIBs is not optimised for practical applications and generally limited by the cathode material. To address these performance problems, a deeper understanding of the evolution of the chemical and electronic structure of NIB cathodes is required. In this thesis, Na₀.₆₇[Mg₀.₂₈Mn₀.₇₂]O₂, a layered cathode material exhibiting fast charge/discharge rates, a large voltage hysteresis and a high reversible capacity, is studied. Experimental techniques which probe both the local and bulk structure are employed, in conjunction with first-principles calculations. The superstructure of Na₀.₆₇[Mg₀.₂₈Mn₀.₇₂]O₂ is elucidated for the first time using synchrotron X-ray diffraction (XRD), total neutron scattering and high-frequency electron paramagnetic resonance spectroscopy (EPR). The effect of this superstructure on Na⁺-ion mobility (charge/discharge rates) is explored using variable-temperature solid-state ²³Na nuclear magnetic resonance (NMR) spectroscopy. Simulation of these spectra enables rationalisation of the relative mobilities of Na⁺ in different local environments. Using operando and ex situ XRD, as well as ex situ ²³Na NMR spectroscopy and first principles transition-state searching calculations, electrochemically-induced phase transformations are then identified, revealing that Mg²⁺ migration takes place during charge and contributes significantly to the observed hysteresis. Finally, the charge compensation mechanism is presented. Previous reports have attributed the large reversible capacity of Na₀.₆₇[Mg₀.₂₈Mn₀.₇₂]O₂ to redox reactions involving oxide anions, O²⁻, but the mechanism remains unclear. Through a combination of spectroscopic techniques (¹⁷O and ²⁵Mg NMR, EPR and X-ray absorption spectroscopy), bulk magnetic susceptibility measurements and first-principles calculations, the origin of the high capacity is identified with the formation of delocalised electronic states between Mn and O. These states are generated by Mg²⁺ migration and stabilised by strong antiferromagnetic interactions. The properties of Na₀.₆₇[Mg₀.₂₈Mn₀.₇₂]O₂-the superstructure, phase transformations, Na⁺ ion mobility and charge compensation mechanism-are summarised and a set of design rules for long lifetime, fast charging and high capacity NIB cathode materials is presented.

Neutron scattering is an important research technique that has furnished a wealth of information about condensed matter systems ever since Ernest O. Wollan and Clifford G. Shull first demonstrated its ability to probe the structure of polycrystalline samples, such as NaCl, on the Clinton Pile-a graphite moderated nuclear reactor built for research purposes at Oak Ridge, Tennessee. Since those early successful demonstrations, the experimental technique has been continuously improved such that it is now regularly used as a research tool by scientists in a plethora of different fields to glean information about the structure or dynamics of their samples. An example of these advances in the use of neutron scattering came when Bertram N. Brockhouse invented the triple-axis spectrometer, which was an instrument that facilitated the study of lattice dynamical behaviour within condensed matter systems. Without this innovation it would be much more challenging to study phonons-the quasiparticles representing the collective vibrations of atoms-within the perovskites and iron chalcogenides that were researched for this thesis. This thesis details how neutron scattering has been used as a tool to study the structure and lattice dynamical behaviour of the relaxor ferroelectric, PbMg1/3Nb2/3O3, and the iron-based chalcogenide, Fe1+yTe. The study of PbMg1/3Nb2/3O3 was to investigate the structure of the higher energy phonon modes, complementing previous research that successfully characteristed the lower energy acoustic and optical phonons of this crystal, and to use information obtained from neutron spectroscopy data to compare the lattice dynamical behaviour of this crystal with ordered perovskites, such as SrTiO3. An interest in studying the higher energy phonons has been aroused because the disorder within PbMg1/3Nb2/3O3, such as the presence of short-range polar nano-regions between the ferroelectric Curie temperature and the so-called Burns temperature (the temperature at which the short-range polar order emerges), leads to the energy broadening and dampening of the transverse optical phonons and triggers their precipitous collapse into the acoustic phonon branch. Previous neutron scattering studies suggested that the band of phonon scattering located at energy scales where the higher energy longitudinal and transverse optical phonons were predicted to be present were due to the occupancy disorder on the B-site of the perovskite between the Mg and Nb atoms. However, in this thesis investigations of the longitudinal and transverse scattering channels revealed lattice dynamical behaviour similar to those expected in SrTiO3, which has no occupancy disorder on the B-site. The hypothesis of disorder within the higher energy modes is further negated by studying the structure factors at various reciprocal lattice vectors that reveals a close agreement with a model for the lattice dynamics that involves the motion of the oxygen and B-site ions. The second study within this thesis investigates the softening of the transverse acoustic phonons within the Fe1+yTe system using time-of-flight neutron spectroscopy. This iron-based system has a complex structural, magnetic and electronic order that depends on the concentration of interstitial Fe atoms within the crystal structure and is often compared with other iron-based systems that share similar physical properties, such as FeSe, underdoped Ba(Fe0.94Co0.03)2As2 and optimally doped Ba(Fe0.94Co0.06)2As2. Studies of these systems revealed softening of the acoustic phonons close to the Brillouin zone centre (q∼ 0) that is indicative of the role that nematic order of the electronic charge has on the lattice dynamics within their crystal structures. However, investigations into the softening of the transverse acoustic phonon within low-y Fe1+yTe reveal that the softening within this system is mostly contained to the Brillouin zone edge and that the entire phonon branch is being driven to lower energies by scattering channels that open up as the tetragonal-monoclinic phase transition temperature is approached. The origin of the scattering channels is unknown, but the electronic nematic order could still be playing a role, albeit one that differs in effect to that observed in the other iron-based systems. The final study is a theoretical investigation into the origin of the bicollinear double stripe antiferromagnetic order that is observed in Fe1+yTe with a low interstitial iron concentration. The hypothesis is that this complicated magnetic ground state comes about due to the interplay between the spin and orbital degrees of freedom within the crystal due to the Jahn-Teller effect and the small energy scale associated with the splitting of the dXz and dY z orbitals in the magnetic (S=1) Fe atoms. This builds on previous theoretical studies which proposed models of the magnetic interactions within the FeTe single crystals that negated the possibility of an interplay between the spin and orbital degrees of freedom and hoped to explain the double stripe magnetic order by a model that included either the spin or orbital physics but not both. A Rayleigh-Schrödinger perturbative model has been developed that includes the second and fourth order virtual hopping of the electrons on neighbouring magnetic Fe atoms surrounded by tetrahedra of non-magnetic Te atoms. A domain space for each of the Fe atoms in the S=1 spin state has 3 degrees of freedom for the spin projection and 2 degrees of freedom for the orbital projection meaning that the resultant Hamiltonian that couples two neighbouring magnetic Fe atoms has a 36×36 matrix structure. This effective Hamiltonian leads to interactions that include spin-only terms, orbital pseudo-spin only terms and terms that describe the interaction between both the spin and orbital physics. The spin operators include single-ion anisotropy, an Ising term that correlates spin projections, an Ising-like term that correlates "mid-planeness" and a spin exchange term. The orbital pseudo-spin operators include a single-site "orbital transverse field" term and a two-site "orbital transverse field" term. The interplay between the spin and orbital degrees of freedom is produced by the multiplication of the spin-only and orbital pseudo-spin only terms in the effective Hamiltonian.

Molecular magnets have been demonstrated to be promising candidates for quantum-coherent nanodevices, due to their long relaxation times and design flexibility. Building and scaling such devices presents several challenges. In this thesis, two problems related to scalability of molecular magnet architectures are investigated using electron spin resonance. Using electric fields, rather than magnetic fields, to operate quantum devices, would radically improve addressability of individual spins. Electric field sensitivities of several molecular magnets, including frustrated Cu₃ triangles, Cr₇Ni and Cr₇Mn rings and crystals of HoW₁₀ complexes, are investigated through pulse sequences incorporating static electric fields. We find spin-electric couplings of various magnitudes, significant enough to manipulate the parameters of respective Hamiltonians to enable selective excitation and we demonstrate such excitation in the HoW₁₀ molecule. We identify a path towards improving electric field sensitivities, through coupling structural distortions exhibiting high electric polarizability with the molecular spin Hamiltonian. To operate assemblies of multiple molecular magnets as quantum devices, robust multiqubit gates, able to modify the amount of entanglement in the system, need to be implemented. Quantum state tomography may be employed to investigate the practical fidelity of such a gate and real entanglement it generates. Focusing on a weakly coupled dimer of two spin-½ molecules, we propose an implementation of a two-qubit entangling gate followed by quantum state tomography using ensemble electron spin resonance. We investigate the limitations and requirements of the gate and the tomography procedure through theoretical arguments, simulations and experiments. We conclude that while challenging, reaching and demonstrating actual entanglement is within our capabilities using ensembles of molecular magnet dimers.

Permanent magnetic materials are of fundamental importance to the modern world, utilised in fields as broad as computers, cars, and MRI machines. Their importance is set to increase as the world move towards sustainable energy and away from fossil fuels. A seamless switch requires an increase in magnet production, and an improvement in performance. Rare-earth reduced permanent magnets are considered a solution to these two problems. This thesis investigates the impact of chemical and morphological changes on the phase stability of rare-earth reduced hard permanent magnets. New methodologies for investigating the position preference of atomic substitutions and dopants have been applied to the RT12 (R = Rare-Earth, T = Transition metal) phase group. This work demonstrates that substitution of the transition metal for titanium in NdFe12, SmFe12, and SmCo12, decreases the cohesive energy, and therefore increases the stability of the structure up to 8Ti at.%. Through analysis of substitution positions it is demonstrated this is tied to a structural effect, derived from a switch in the symmetry of preferential substitution positions. To gauge the manufacturing feasibility of one of these phases, computational investigations of the melting temperature of NdFe12 at various pressures were performed using a Solid Liquid coexistence methodology applied in Molecular Dynamics. Pair potentials used for this work were generated by a genetic algorithm potential fitting methodology, which has application beyond the RT12 phase group. Finally, a new methodology for understanding grain morphology is presented, which takes into consideration the shape, surfaces, and interfaces of cyrstalline grain structures. This methodology is tested on the FePt L10 structure, which is able to produce stable magnetic grains at nanometer sizes, due to it's magnetic anisotropy of Ha = 6-10 MJ/m . This work shows that at grain sizes between 3-9nm, the morphology of the grains is dominated by surface energy, and will result in structures with {111} planes as their primary faces. This result has implications for the design of next generation hard drives.

An essential part of Hard Disk Drive (HDD) Areal Density Capability (ADC) growth is improving the linear resolution of the reader. Increasing reader shield saturation magnetisation is an attractive means to enable this as dynamic permeability would be expected to improve, thereby providing greater shielding from down-track bits. However, candidate materials tend to have poorly controlled magnetic properties, leading to the risk of reader instability. The use of thick seedlayers to control the magnetic properties of high moment shields is impractical due to reader shield-to-shield spacing constraints. In this work, ultra-thin (≤1nm) underlayers of Ni79Fe21 and Ru are shown to provide a pronounced reduction in the coercivity of both Ni45Fe55 and Co70Fe30 and establish a uniaxial anisotropy. Film stress is also significantly reduced. TEM and Fresnel-mode Lorentz TEM imaging of Ni79Fe21 seeded Ni45Fe55 reveal significant reductions in grain size and magnetic ripple dispersion, no significant texture is measured. It is proposed that initial underlayer island growth provides nucleation sites for the high moment material, resulting in a refinement of the grainsize distribution and an averaging of the local anisotropies. Ni45Fe55 layers seeded with thin Ni79Fe21 incorporated into synthetically antiferromagnetically (SAF) coupled shield structures produce reversal behaviour that matches that of SAF shields comprising Ni79Fe21. Read heads with high moment shields were fabricated and exhibit a 2.7% gain in linear resolution, in agreement with micromagnetic simulations. It is proposed that the reduced magnetisation deflection required by higher moment shields results in an improved dynamic response to media field. It has been demonstrated that use of ultra-thin Ni79Fe21 underlayers provides a practical path to realising highly permeable thin films of Ni45Fe55 with a well-defined uniaxial anisotropy, that when used in SAF shield structures provides improved HDD reader linear resolution.

This PhD focuses on the basic physics of epitaxial transition metal oxides for the development of new materials for spintronics and superconducting spintronics. The starting point of the project was to investigate the possibility of using the magnetic non-collinearity hosted in SrRuO3 to enhance conversion between spin-0 singlet and spin-1 triplet Cooper pairs in an epitaxial oxide ferromagnetic Josephson junction. This starting point has led to four main results: 1. Experiment: anti-ferromagnetic coupling in SrRuO3/La0.7Ca0.3MnO3. These heterostructures are grown on different substrates with different epitaxial strains. By measuring magnetic minor loops as a function of field-cool strength, I see an effective field acting on the softer La0.7Ca0.3MnO3 layer consistent with an anti-ferromagnetic exchange coupling from the SrRuO3. The strength of this coupling varies with the substrate used and essentially disappears when a non-magnetic spacer layer is added. The coupling combined with differences in magnetic anisotropy between layers is thought to create a non-collinear spin texture in the SrRuO3. 2. Review: the ambiguity between topological and inhomogeneous Hall effects. Topological Hall effect measurements are commonly used in the spintronics community as a signature of chiral spin textures. In this comprehensive literature review I argue this is not a reliable method since similar Hall signals can be created by experimental problems or an inhomogeneous anomalous Hall effect. 3. Experiment: two-channel anomalous Hall effect in SrRuO3. Using transport measurements, volume magnetometry, and magnetic force microscopy, I show that the Hall effect anomaly often attributed to skyrmions in ultra-thin SrRuO3 is instead likely caused by single unit cell thickness variations in films resulting from substrate miscut creating an inhomogeneous anomalous Hall effect. 4. Theory: numerical modelling of non-collinear ferromagnetic Josephson junctions. I develop a numerical model of magnetic Josephson junctions using Matsubara Green functions in the (clean) tunnelling limit. In agreement with prior literature, I find magnetic non-collinearity in junctions gives finite triplet currents which can show 0-π transitions versus ferromagnet strength or angle, and chiral junctions give a Josephson current with zero phase difference between superconducting layers. This model allows the exploration the junction properties as a function of temperature, ferromagnet strength, angle, and Rashba spin-orbit coupling strength, using more realistic electronic dispersion relations and using superconductors with s- or d-wave symmetry.

This thesis reports the structural and magnetic behaviour of complex lanthanide oxides exhibiting low-dimensional and/or geometrically frustrated magnetism. Materials displaying these phenomena are well studied as hosts for novel magnetic states such as spin liquids, and for applications such as magnetic refrigeration. In this thesis, three families of materials were studied in order to probe relationships between their structural and magnetic properties. First, the lanthanide calcium oxyborates Ca4LnO(BO3)3, Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, were synthesised using solid-state ceramic methods. In Ca4LnO(BO3)3 the magnetic Ln3+ ions form quasi-one-dimensional chains. The crystal structure was investigated using powder X-ray and neutron diffraction and the bulk magnetic properties were determined by magnetic susceptibility and heat capacity. There is evidence for short-range ordering in Ca4TbO(BO3)3 at 3.6 K but no long- or short-range order above 1.8 K in the other compounds. The magnetocaloric performance of Ca4GdO(BO3)3 and Ca4HoO(BO3)3 was found to be comparable with the well-known magnetic refrigerants Gd3Ga5O12 and Dy3Ga5O12 respectively. Secondly, the structural behaviour of the barium borates Ba3Ln(BO3)3 was probed as a function of temperature and lanthanide ion. A new low-temperature phase of Ba3Tb(BO3)3 was observed with the hexagonal crystal structure previously known only for Ln = Dy-Lu. In this phase the magnetic Tb3+ ions form quasi-two-dimensional triangular layers. The new phase was examined using powder diffraction and magnetometry and found to be strongly geometrically frustrated: low-temperature neutron diffraction indicates the absence of long-range magnetic order above 75 mK. Muon spectroscopy above 1.5 K also shows an absence of long-range order. Additional experiments are required for further investigation into the possible quantum spin liquid ground state. Finally, bulk magnetic properties are reported for the lanthanide orthotantalates LnTaO4, Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er. The lanthanide ions in LnTaO4 form an elongated diamond-like lattice which can theoretically host unusual magnetic states in the presence of competing magnetic interactions. Neutron diffraction revealed long-range antiferromagnetism in TbTaO4 while the remaining compounds do not order above 1.8 K. This thesis reports previously unexplored fundamental magnetic properties of several different lanthanide materials, demonstrating the rich variety of magnetic behaviour accessible via chemical substitution of the lanthanide ions in solid-state compounds.

An emerging field of research in recent years has been magnonics, the manipulation of coherent spin excitations, spin-waves, in magnetically ordered materials. Recent advances in experimental techniques for high-frequency magnetisation dynamics and the advent of micromagnetic simulations has led to the propositions of functional magnetic devices based upon the control of spin-waves. This thesis presents work for characterisation and future development of high-speed magnonic devices derived from micromagnetic simulations, and numerical techniques for the solution of the Landau-Lifshitz equation for micromagnetic simulations in the finite-difference time-domain approach. In chapter 3, spin-waves were controlled in the propagation along a thin film magnonic waveguide via resonant scattering from a mesoscale chiral magnetic resonator, in the backwards volume, forwards volume and Damon-Eshbach geometries. The scattering interaction demonstrated non-reciprocity associated with devices acting as spin-wave diodes. Additionally, such devices demonstrated the possibility of phase-shifting. The results obtained were numerically fit and interpreted in terms of a phenomenological model of resonant chiral scattering. The origin of the chiral coupling was discussed in terms of the stray field. In chapter 4, the phenomenon of spin-wave confinement, wavelength conversion and Möbius mode formation was demonstrated in the backwards volume configuration of thin-film magnetic waveguides. The presence of magnetic field gradients or thickness gradients modified the position of the Γ-point of the dispersion relation for Backwards Volume Dipolar-Exchange Spin-Waves (BVDESW), such that back-scattering and wavelength conversion occurred from the field/thickness gradients due to the "valleys" of the spin-wave dispersion. This work highlights a basis for not only experimental observation of such phenomena, but the potential for devices based upon valleytronics, an exploitation of the valley degree-of-freedom due to the spin-wave dispersion. In chapter 5, motivated by numerical error encountered in previous work in the thesis, the validity of implicit methods formulated for the numerical solution of the Landau-Lifshitz equation for finite-difference time-domain micromagnetic simulations were demonstrated. The implicit methods were tested for single spin precession in an external field, the μMAG standard problems and additional test cases. A source of numerical instability in explicit integration methods, numerical stiffness in systems of differential equations, was demonstrated to occur in existing explicit numerical methods, applied to the Landau-Lifshitz equation, common to popular micromagnetic software. The stability of implicit methods was demonstrated to be advantageous over explicit methods in micromagnetic scenarios where numerical stiffness could occur. Additionally, it was demonstrated that the quality of the numerical results was improved compared to explicit methods when the implicit method possessed L-stability, a damping of stiff, high wave number spin waves in the simulation.

Single-molecule magnets (SMMs) are a classification of molecular nanomagnets which can display slow relaxation of magnetisation. This is manifested in magnetic properties that are not reliant upon the cooperative interactions across domains of spins. This thesis aims to synthesise and characterise lanthanide organometallic complexes and to investigate the resultant magnetic properties. 4f/3d isocarbonyl complexes have previously been shown to display single-molecule magnet behaviour with large energy barriers to reversal of magnetisation, as a result of cooperative effects between the two individual metal centres facilitated by the isocarbonyl link. Isocarbonyl bridging ligands present an opportunity to explore the structural and electronic effects of bridging systems in relation to SMMs, as polymetallic complexes may provide higher blocking temperatures and energy barriers. Here, research is focused on utilising the heavier d-block metal tungsten, in combination with 4f elements to investigate the effect on both the structural and magnetic properties in order to expand on the understanding of isocarbonyl SMMs. The development of easily handled lanthanide organometallic building blocks, based around the 10π-aromatic-8-membered cyclooctatetraene dianion ([COT]²⁻), for future SMM design are also synthesised and their solid-state properties characterised. Though K₂COT and some COT ligated species can be extremely air sensitive and pyrophoric, the materials reported in this thesis showed no pyrophoricity and a reduced, but not dispelled, air sensitivity relative to previously reported work. The separated-ion pairs are subjected to investigations of their magnetic properties, which is informative for future efforts to synthesise [COT]²⁻-based SMMs. The second example of lanthanide inverse-sandwich pentalene complexes are synthesised and their solid-state structures are investigated, alongside computational analysis that demonstrates the change in direction of the principle magnetic axis. The precursor complexes are also characterised structurally and magnetically, to afford further insight into the effects of the ligands upon magnetic properties.

Cyclic π-conjugated organic ligands are widely used in rare-earth organometallic chemistry, and our group in particular has shown that cyclopentadienyl ligands are extremely effective in the synthesis of high-performance single-molecule magnets (SMMs). These remarkable magnetic properties are due to the strong axial ligand field that the monoanionic cyclopentadienyl ligands provide, with recent results including the record-breaking SMM [Dy(η⁵-C₅^iPr₅)(η⁵-C₅Me₅)]⁺. This thesis focuses on the coordination chemistry of a dianionic cyclobutadienyl ligand in rare-earth chemistry, i.e. [C₄(SiMe₃)₄]²⁻. My work on the fundamental coordination chemistry of the cyclobutadienyl ligand towards rare-earth elements and the reactivity of the new complexes is presented herein, in addition to the SMM properties of exemplar systems. The work presented in this thesis shows that cyclobutadienyl ligands can effectively replace cyclopentadienyl ligands in structurally similar rare-earth complexes, resulting in improved SMM properties. This thesis extends our understanding of magneto-structural correlations currently used in the design of high-performance organometallic rare-earth SMMs, and demonstrates that cyclobutadienyl-ligated SMMs should have the potential to surpass the current records set by bis-cyclopentadienyl dysprosium SMMs.

Magnetic systems are the cornerstone of contemporary life, with their unavoidable use in everyday routines. This fundamental dependency drives the exploration for materials that exhibit both advantageous and distinctive magnetic properties. At the same time, familiar materials are subjected to ever more intelligent experimentation: further understanding the more complex magnetic phenomena present. Additionally, computational advancements have led to an increase in the complexity of theoretical examinations. Thus, first-principles calculations stand uniquely poised, ready with ever-increasing improvements, to understand the characteristics of these magnetic materials. Double-perovskites exhibit complex and beneficial magnetic relationships, for instance, colossal magnetoresistance. The synthesised Tl2NiMnO6 experimentally broke the trend by manifesting ferromagnetic insulation. Firstprinciple calculations using electronic band structures, density of states and the effective masses of holes and electrons have predicted the material to be a ferromagnetic semiconductor. Effective masses of holes and electrons revealed insight into the transport properties, unveiling anisotropy potentially linked to the colossal magnetoresistance present in the material. Molecular magnets are versatile in their application of spintronics, making them a popular choice for investigating. We complete first-principles calculations on the Cr10(OMe)20(O2CCMe3)10 (Cr10), and Cr8F8(CH3)3CCO2H (Cr8) molecular rings to investigate the different magnetic states, using Cr8 as a comparison for the frustrated next-nearest neighbour interaction in Cr10. We use ionic constraints to enforce D5 symmetry in the Cr10 molecule, allowing for reliable extraction of the exchange interaction parameter and comparable spin-densities. The Cr10 molecule possesses a ferromagnetic ground state with the potential for non-co-linear spin-wave excitations whose wavelength is commensurate with the ring. The iron pnictides superconductors, FeAs and FeSe, simultaneously exhibit complex magnetic interactions, forming a spin-wave. We perform electronic band structure calculations on three competing magnetic arrangements with the meta-GGA, rSCAN, utilising a cold-smearing scheme, revealing, for FeAs, a band gap with Fermi level located inside: completely different from previous first principle band structures. Further reported is the dependency of the strength of magnetism exhibited to the number of magnetic sites present and the direction of the magnetic arrangement.

A model complex of dysprosium, Dy(N2O2C7H11)3 (Dy2) was built by successive capping of the peripheral aromatic ring of a butterfly-shaped dysprosium complex of a schiff base ligand, Dy(N4O5C14H11)3 (Dy1). The structural modifications were carried out in order to investigate the effect of the chemical surroundings like aromaticity on the observation of single-molecule magnet (SMM) behaviour in a lanthanide-based complex (Dy1). Experimental techniques were combined with theoretical tools to investigate the dynamics of magnetic properties of the Dy1 and compared with the theoretical results for Dy2 to gain insight on the contribution of covalency, crystal field effect and the role of aromaticity in stabilizing the excited magnetic levels of a single-molecule magnet. Dy1 showed frequency-dependent slow magnetic relaxation characteristics of a single-molecule magnet with and without applied dc field and a blocking temperature of about 8 K. The obtained results showed that despite f−electrons having weak interactions with ligand field, it is possible to tune the magnetic properties of lanthanide-based complexes using ring currents, control of covalency and peripheral ligand substitution. Furthermore, di-nuclear acetate bridged lanthanide complexes with two different structural motifs, Er1 = [Er(CH3COO)(CH3COO)2(H2O)2]2 · 4H2O and [Ln(CH3COO)(CH3COO)(CH3COO)(H2O)CH3COOH]2 · 2CH3COOH (Ln = Er (Er2), Y0.8Er0.2 (Er3)) were characterised. The solvent effects on the structure, electronic and magnetic properties were studied by experiments and theoretical methods. The tetraacetate-bridged erbium (Er2) showed a shorter intra-molecular Er - Er distance of 3.878 Å compared to the doubly-bridged counterpart (Er1) with Er - Er intra-molecular separation of 4.152 Å. Er2 exhibited weak ferromagnetic ordering at very low temperature in the dc magnetic measurement due to the short proximity of the Er centres. A field-induced slow magnetic relaxation for spin reversal characteristics of single-molecule magnet behaviour with relaxation dynamics dominated by Orbach process was observed for Er2. Er1 showed very fast slow magnetic relaxation dominated by quantum tunnelling of magnetization as evidenced in the Cole-Cole plot and the observed plateau in the susceptibility curve. The role of inter-molecular interactions between Er centres was investigated using yttrium diluted sample (Er3). The enhanced magnetic property of Er2 and Er3 over Er1 is attributed to the structural changes accompanying the choice of synthetic solvents used as well as the stronger interactions between Er and oxygen donor atoms of the acetate/acetic acid over that of water molecule as ligands. The present study presents an interesting result on solvent effects in the design of single-molecule magnets. In addition to the peripheral, aromatic, solvent effect and accompanying lanthanide-lanthanide coupling investigated, the role of hetero-metallic Pd-Ln bonding interaction was investigated using a set of tetranuclear acetate-bridged palladium-lanthanide complexes of the formula [Pd2Ln2(H2O)2(AcO)10] · 2AcOH (AcO = CH3COO- , Ln = Ce, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm and Yb) by theoretical means and compared with experimental results for the lanthanide series to establish an interesting alternating trend in the observation of slow magnetic relaxation between Kramers and non-Kramers ions along the series. The role of axial perturbation, electron-cloud distortion and utility of metal as a ligand to upset the electronic properties of lanthanide complexes and their implication to the observation of slow magnetic relaxation were investigated. Results showed that transition metal-lanthanide bonding interaction presents a unique way to control the ligand field of lanthanide at varying degrees while confirming the role of rigidity in SMM design.

The focus of this thesis is to understand a variety of emergent materials properties that are driven by quantum phenomena. A range of materials are investigated where the crystal, electronic and magnetic structures are delicately coupled to give rise to intriguing macroscopic properties. Computational methods are the primary tool for tackling these problems. This is largely centred around quantum mechanical, ab initio calculations using density functional theory (DFT), but also includes mean-field analyses and stochastic simulations of a model Hamiltonian. Tin telluride, SnTe, is a crystalline topological insulator with potential applications in a new generation of spintronics devices. In practice, SnTe shows a low temperature ferroelectric distortion and contains a large number of bulk carriers from Sn vacancies, so stands as a rare example of coexistence between metallicity and ferroelectricity. The implications of these effects for topological and transport properties require accurate modelling of the electronic structure. Here, in close collaboration with experiment, the evolution of the Fermi surface across this structural transition is probed by calculating quantum oscillation frequencies. The image analysis tools of Mathematica were exploited to develop a code for computing these frequencies from DFT electronic structure calculations. Agreement between experiment and theory is crucially dependent on the crystal structure. Calcium ruthenate, Ca2RuO4, is a layered perovskite compound with a rich phase diagram, which can be traced back to its strongly correlated electronic structure. Hydrostatic pressure can be used as a means to manipulate its crystal structure, which encourages several interesting effects. In particular, Ca2RuO4 undergoes anomalous expansion of the c axis, and a first-order structural transition coupled with a Mott insulator-metal transition. Here, this pressure response is investigated with DFT+U calculations, which account for the importance of electron correlation by adding an on-site Hubbard-like repulsion term. This work presents the first fully self-consistent electronic structure for Ca2RuO4, obtained from optimised crystal structures along a sequence of pressures. This appreciation of the coupling between lattice and electronic degrees of freedom sheds some light on its unusual phase diagram. The insulator-metal transition is reproduced and naturally coincides with a structural transition and associated orbital order. For the metallic phase, a complex energetic landscape with several competing phases emerges. Uranium gold, UAu2, is a heavy fermion metal with a complex spin-density-wave (SDW) phase at low temperature. This phase consists of frustrated, incommensurate ordering that is very robust in external fields, but suppressed by pressure to reveal unconventional superconductivity. The origin of this exotic magnetic ordering is of interest here, which is explored by several different approaches with DFT+U. Both itinerant and local-moment pictures of the magnetism are entertained. Fermi surface nesting is shown to be ineffective, but mapping the system to a Heisenberg model and computing effective exchange interactions identifies an instability towards modulated order. In addition to these material-specific investigations, the treatment of longitudinal magnetic fluctuations in computational methods is studied. Magnetic fluctuations are important for understanding materials on the border between itinerant and local-moment magnetism. The continuous-spin Ising (CSI) model is investigated here as a phenomenological model of these fluctuations. Using a bespoke simulation technique this model is extensively explored, firstly on a cubic ferromagnet and secondly on a highly frustrated, stacked triangular lattice with a variety of interaction topologies. The introduction of fluctuations is shown to alter the ground state of the prototypical frustrated triangular lattice, and to enhance the transition temperature of more severely frustrated systems. In all of these cases, different degrees of freedom in the system couple to one another, either to make an accurate theoretical description difficult, or to give rise to unexpected emergent properties. Both SnTe and Ca2RuO4 represent examples of delicately coupled crystal and electronic structures. In SnTe, the tuning of the crystal structure is vital to correctly describe the Fermi surface, while in Ca2RuO4 the Ru orbital structure very directly determines the structural properties, and drives the unusual pressure response. UAu2 introduces a non-trivial magnetic structure, which requires careful treatment of electron-electron interactions to locate. In the study of the CSI model, altering the interaction topology, which acts as a proxy for manipulating the underlying electronic structure, has drastic implications for the role of magnetic fluctuations in the system.

The use of magnetic fields has the potential to tailor the orientation and distribution of dispersed particles within polymer composites to create materials with anisotropic properties. However, limited literature exists on anisotropic nanocomposite hydrogels prepared under low magnetic fields, which are of interest for various soft and wet applications. This research reports polyacrylamide/montmorillonite-iron oxide (PAAm/MMT-IO), and poly(N-isopropylacrylamide)/MMT-IO (PNIPAM/MMT-IO) nanocomposite hydrogels prepared via low magnetic fields showing anisotropic mechanical properties. The magnetic field was studied as the parameter that produces anisotropy by controlling the orientation and distribution of the dispersed phase. MMT-IO nanohybrids at a 2:1 clay to magnetite weight ratio showed suitable magnetic properties and surface characteristics to be used in the preparation of PAAm/MMT-IO and PNIPAM/MMT-IO nanocomposite hydrogels. These materials showed swelling ratios that depended on the nanohybrid content, porous morphology with nanohybrid particles distributed along the applied magnetic field and enhanced Young's modulus, ultimate tensile strength, strain at break and toughness compared to pristine polymer hydrogel. The magnetic field was found to produce anisotropy, with superior mechanical properties along the direction of a 20 mT magnetic field produced by a couple of N35 Neodymium magnets and reduced properties on the transverse direction, compared to samples prepared in absence of a magnetic field. The Young's modulus was estimated through the Halpin-Tsai and Mori-Tanaka micromechanical models, which had not been applied for this kind of materials. The potential of PNIPAM/MMT-IO nanocomposite hydrogels was demonstrated through the design and fabrication of self-shaping multilayer structures that displayed complex deformations when subjected to a change in temperature due to their anisotropy and smart behaviour. The development of these anisotropic nanocomposite hydrogels could be useful in fields such as soft robotics and mechanobiology, where controlled deformations are desirable within a wet medium.

Systems that undergo metamagnetic phase transitions are of great academic interest, both from a fundamental science perspective and for their potential to be implemented in devices across a broad spectrum of technologies, such as in spintronics, data storage, logic, and magnetocalorics. One such system is equiatomic FeRh, which features an antiferromagnetic to ferromagnetic phase transition at ~100 degrees Celcius. The characteristics of this phase transition are extremely sensitive to a range of conditions such as strain state, composition, and applied magnetic field. Thus, there are multiple avenues through which the spatial distribution of magnetic phases can be tailored to suit a particular function. This thesis describes work to fabricate and characterise thin film structures incorporating FeRh in which the magnetic behaviour is controllably modulated, either through atomic disordering, exchange coupling, or magnetoelectric coupling. For each system studied, polarised neutron reflectometry (PNR) was used to determine the evolution of the magnetisation as a function of depth through the thin films. This work demonstrates that by irradiating FeRh thin films with Ne+ ions of varying fluence, it is possible to generate interfaces between regions of different magnetic phase distributions, allowing simple magnetic bilayers or a uniform magnetisation profile to be engineered. Here, PNR data are combined with positron annihilation lifetime spectroscopy and conversion electron Mossbauer spectroscopy to elucidate a correlation between the magnetic modifications and the induced disorder. The interlayer exchange coupling in an FePt/FeRh bilayer was probed using PNR over a range of temperatures and applied magnetic fields. In this system, the FePt coercivity is reduced when the FeRh is heated through the magnetic phase transition via an exchange spring mechanism. A characteristic length scale over which this mechanism is effective is determined. Finally, strain-mediated magnetoelectric coupling in a PMN-PT/FeRh artificial multiferroic structure was investigated. The relationship between the complex ferroelectric properties of the PMN-PT substrate and the resulting magnetoelectric effect is explored using a combination of magnetometry, X-ray scattering, and PNR measurements with in situ applied electric fields. These results demonstrate that magnetoelectric coupling can be observed, and highlight the challenges inherent to optimising the interfacial coupling in this system.

This thesis presents experimental results that assess the suitability of a ferromagnetic cobalt-iron-boron alloy (CoFeB) for further research and application in 'superconducting spintronics' - the field that aims to utilise the distinct physics arising at interfaces between superconducting and magnetic thin films for the development of functional memory and logic devices. Following a review of the current field and an initial materials characterisation, CoFeB is assessed alongside the superconductor niobium (Nb) in two archetypal superconducting spintronic devices. The first of these is the superconducting spin-valve, in which a modification of the superconducting critical temperature of up to 23 mK is observed concurrent with infinite magnetoresistance. This is the first demonstration of complete switching between the resistive and superconducting states of Nb depending on relative ferromagnet magnetisation directions in a Nb-3d ferromagnet system. The potential for further optimisation of these devices is demonstrated by study of devices grown at elevated temperatures. Device optimisation via thermal treatment is also investigated in Nb/CoFeB/Nb ferromagnetic Josephson junctions - the second device type studied in this work. The key parameters of this particular materials system are characterised for the first time, and an increase in the Josephson critical current following thermal annealing up to a maximum of ≈ 670% is demonstrated, which is attributed to an increase in the electron mean free path. The results show CoFeB to be a promising material candidate for the development of more complex superconducting spintronic devices that may have tunable properties via annealing.

This thesis focusses on the application of materials informatics to the study and discovery of inorganic compounds that exhibit magnetism and superconductivity. In particular, the materials discovery process is viewed through the lens of data-mining and natural language processing, by which large databases of chemical properties and structures can be auto- generated from the scientific literature. Application of machine learning to these data enables exploration of structure-property trends and thereby enriches the materials discovery process. Chapter 1 reviews the current literature on materials informatics and materials discovery. This includes the introductory principles of magnetism and superconductivity as well as existing work on applying materials informatics to these domains. In addition, the numerous challenges that pose barriers to further progress in computer-aided materials design are discussed. This motivates the need to extract chemical information from scientific doc- uments for materials discovery purposes. Chapter 2 outlines the various methodologies used throughout this thesis, focussing particularly on information extraction from scientific documents, semi-supervised machine learning and generative deep-learning models. The results chapters of this work present the main stages of a materials discovery process driven by data-mining, natural language processing and machine learning. In Chapter 3 a novel probabilistic relationship extraction algorithm is presented and applied to the extraction of Curie and Néel phase transition temperature relationships. Following on from these developments, Chapter 4 presents a completely new workflow for the extraction of scientific quantities from text and tables to auto-populate hierarchical chemical ontologies. This work creates the first fully auto-generated database of crystal structures. Chapter 5 shows the results of applying the aforementioned techniques to the analysis of magnetic and superconducting phase-diagrams. This demonstrates the efficacy of using automatically extracted data for property prediction and visualisation. Chapter 6 uses generative deep- learning models to create novel 3D inorganic crystal structures and perform prediction of their associated properties. Chapter 7 brings together all of these techniques to generate novel ferromagnetic materials from the Heusler alloy family. Finally, Chapter 8 outlines the progress made in this thesis and the opportunities for further work.

High pressure transport and magnetic measurements have been undertaken on the full Heusler alloys Pd2MnIn, Fe2MnSi and Fe2VSi. In Pd2MnIn a change in the resistivity of the sample is observed, as gradually a peak appears under pressure. The phenomenon has been analyzed through a high temperature variable range hopping conduction. Alternatively, following some magnetic measurements that showed a difference between the ZFC and FC magnetization curves, the resistivity results were interpreted as a competition between the Kondo effect and the RKKY (Ruderman-Kittel-Kasuya-Yosida) interactions in the system, a phenomenon observed in some spin glass systems. In Fe2MnSi, both the high pressure resistivity and the high pressure magnetic measurements point to a weak increase in the two magnetic transition temperatures. In addition, at low temperatures we observe a minimum in resistivity. The magnitude and the temperature of the minimum increases with pressure. In Fe2VSi we provide some evidence, based on high pressure electric and thermoelectric transport measurements, of a possible pressure induced suppression of the structural transition that is present in the system at ambient pressure. The results are discussed in relation to an earlier theoretical study on the system.

Lorentz transmission electron microscopy (LTEM) has proved itself to be an invaluable technique when investigating in-situ micromagnetic behaviour of magnetic thin films. The nano-scale visualisation of the magnetic structure allows for not only the mapping of hysteresis behaviour, but quantitative characterisation of the materials micromagnetic properties. In this thesis, much of the previous work of experimental magnetisation ripple characterisation is reviewed. A newly developed methodology for characterisation of large sets of Fresnel images displaying magnetisation ripple properties is presented which is utilised throughout the rest of the body of work. Ni45Fe55 has long been a high moment alternative for the more commonly used permalloy composition of Ni80Fe20 in hard disk (HDD) read-write head design. In this work a conventional TEM, and LTEM study was undertaken to investigate the effect of ultra-thin Ni79Fe21 seed layers on the physical and magnetic properties of Ni45Fe55. The dramatic effect of seed layer addition resulted in grain size and texture reduction, an increase in uniaxial anisotropy and a reduction in magnetisation ripple properties. This suggests that the film is a good candidate for controlling properties of Ni45Fe55 thin films, whilst maintaining a high magnetic moment density. Micromagnetic modelling of magnetisation ripple using MuMax3 software has been presented using two different methods. Both models produced visually representative simulated Fresnel images, with work specifically focusing on magnetisation reversal processes and the quantitative analysis of magnetisation ripple properties. Models produced insights into the effect of parameters such as grain size, inter-granular exchange and magnetocrystalline anisotropy directionality. Lastly, we can quantitatively assess the physical and magnetic variations between permalloy magnetostrictive samples with varying signs, without the need for external straining, to represent normal operating conditions in HDD devices. Analysis revealed near identical physical properties, with a subtle variation in bulk and magnetic ripple properties. However, it is not possible to determine if these variations are exclusively due to differences in magnetostrictive properties or if it is more likely due to compositional variations.

Magnetic particles offer an interesting platform for self-assembly, as their dipolar interactions and response to magnetic fields can be used to drive and manipulate assembly. Many magnetic particles used so far in these applications are quite simple in their properties, such as iron oxide particles or magnetic colloids. To grow the potential of magnetic self-assembly, more complex particles are being developed to increase the complexity and functionality of the assemblies. In this vein, this work has created magnetic particles based on thin films of CoFeB/Pt with perpendicular magnetic anisotropy (PMA) to use for self-assembly. PMA films have been long developed for magnetic recording and computing applications, so they are well understood and optimized. Their high moment and strong anisotropy make them good candidates for a highly directional equilibrium self-assembly system. The particle construction was first optimized in films, looking at both the magnetic and structural properties needed for particles. The particles are then patterned from the film using top-down lithographic methods. Using top down methods, as opposed to the chemical synthesis methods used for most magnetic particles, is what allows the tuneable magnetic film properties to be transferred to particles. Once in liquid, the individual field response and interactions of the particles were analysed, to understand how they would best assemble. Then, the particles were assembled into 1D and 2D structures, which were then manipulated with magnetic fields to understand how these assemblies could be controlled and actuated. Assemblies of 3D structures on chip were also created and observed. These results demonstrate the fundamental assembly capabilities of the system and begin to show how it might be applied in microrobotics or micromachinery.

This thesis reports the results of spectroscopic techniques including terahertz time-domain spectroscopy (THz-TDS) that were used to characterise and develop the understanding of a variety of thin film materials, including few-layer graphene and spintronic metallic heterostructures. The complex transmission of few-layer graphene at terahertz frequencies was measured using THz-TDS, and was used to determine figures of merit including frequency-dependent conductivities, DC conductivities, and mean scattering times. DC conductivities ranging from 0.2-2.8 mS, and scattering times ranging from 3-130 fs were extracted by fitting to the Drude model of conductivity, and were found to be correlated to the number of layers of each sample, demonstrating the practical uses of THz-TDS for characterising these materials. Supporting measurements using Raman and ultraviolet-visible (UV-VIS) spectroscopy were compared and contrasted against that of THz-TDS. These methods suggest that characterisation of graphene requires a range of different spectroscopic techniques to be used in parallel to fully describe its properties. A suite of thin metallic film heterostructures including Co20Fe60B20/Pt and Ni80Fe20/Pt were investigated, which were used as alternative sources of terahertz emission. These heterostructures consisted of nanometre-thick layers of ferromagnetic and non-magnetic metals, and are thought to utilise the spin of the electron and the inverse spin-Hall effect (ISHE) to generate broadband terahertz radiation, which was measured at frequencies of up to 15 THz. The magnetic field dependence of terahertz emission of these heterostructures was studied to determine the role that the magnetic structure had on their emission process. It was found that the terahertz emission closely followed the magnetisation properties of the samples found via vibrating sample magnetometry (VSM) measurements, displaying a uniaxial magnetic anisotropy depending on the direction of the externally applied magnetic field. The effects of the heterostructure design on the emission were also investigated, as well as the consequences of other external conditions such as temperatures as low as 5 K and laser pump fluences from 0.05-50 mJ/cm^2. These findings show promise towards further development of this new generation of low-cost terahertz emitters with tunable emission properties.

Magnetic particles have been widely implemented across research areas in liquid and biological environments, such as cancer therapy, drug delivery and cell sorting. Perpendicularly magnetised (PM) synthetic antiferromagnetic (SAF) particles exhibit a range of desirable properties that make them strong candidates for applications in fluid. These include a zero remanence state, sharp and tunable switching, and a high and variable saturation magnetisation. Abstract This thesis will first continue to explore the flexibility of the SAF particle design. Through engineering of the SAF thin film, the basis of the particles, this work demonstrates the ability to tune the SAFs towards a suite of applications. In addition, a novel take on the fabrication of thin film based magnetic nanoparticles is presented. This method is capable of efficient and effective production of particularly high yields of well-defined nanodiscs with robust magnetic properties. Abstract The magnetic behaviour of the PM SAF particles, and the thin films they are created from, is analysed with particular focus on the characteristics displayed in a fluidic environment. This leads to the discovery of novel magneto-mechanical transitions of SAF particles in liquid and continues to demonstrate the applicability of SAFs across a spectrum of applications. Abstract Application of the PM SAF particles is examined in the context of cancer therapy. Previous studies into SAFs in cancer therapy utilised them in the magneto-mechanical destruction of tumour cells. Leading on from this work, the concept of integrating SAF microdiscs with iron oxide nanoparticles (IONPs) in a ‘magnetic combination therapy’ is explored. This preliminary study reveals interesting inter-particle interactions between the SAFs and IONPs and shows the potential for a synergistic effect in the combined therapy. Abstract This work provides a robust toolbox for the fabrication of tailored nanodiscs for use in a range of fluidic and biological applications.

This thesis presents work exploring the use of pressure as a tuning parameter for exploring the phase diagrams and properties of magnetically ordered insulators, to add understanding to several areas of current interest in condensed matter research. It shows the versatility of pressure as an experimental technique for exploring material properties free from complicating factors which arise with similar techniques such as chemical doping. The properties of low-dimensional magnetic materials, and how these systems respond as they are pushed toward a more three-dimensional nature is explored through studies of both the crystal and magnetic structures of the family of quasi-two-dimensional magnetic insulators MPS3 (M = Fe, Ni, Mn). With previous work largely being specific to individual compounds, this thesis contributes to a more unified understanding of their properties. It shows that Ni and MnPS3 undergo similar structural transitions under pressure to those previous observed in FePS3, the highest pressure of which is linked to an insulator-to-metal transition in that system. Through record high-pressure neutron diffraction measurements, the evolution of the antiferromagnetic order in FePS3 through this metallisation is studied for the first time. In contradiction to previous indirect measurements, it is seen that magnetism persists into the metallic phase, with long range antiferromagnetism giving way to a previously unobserved short-range order. This work is relevant on a broader scale for numerous layered magnetic materials such as cuprate high temperature superconductors. Secondly, pressure is used to explore the magnetocaloric properties of the antiferromagnet EuTiO3. Recent work has shown that this compound compares favourably to many materials commonly used in magnetic refrigeration. Measurements show that these properties are suppressed by the application of pressure and point towards the potential existence of a previously undiscussed transition in the material between 0.4 GPa and 0.5 GPa.

In this Thesis, investigations on spin wave propagation in sufficiently thick, magnetic unsaturated nanostructures of various shapes have been carried out. Analytical, numerical and experimental techniques in the time and the frequency domains have been used throughout this thesis. Unsaturated magnetic states are of high interest, more specifically the magnetic vortex configuration since the remanent state is typically dominated and stabilised by the shape anisotropy of the structure, with no need of strong or any bias fields. Due to the diversity of magnetic inhomogeneities, a high degree of reconfigurability by applying external low bias fields can be obtained. On the other hand, the inhomogeneous magnetic landscape becomes, very often, difficult to model analytically and therefore, propagating spin waves can turn out to be complicate to control in practice. The aim of this work has been twofold. Firstly, to explore the most prominent magnetisation dynamics found in thick enough magnetic patches, or ‘2.5-dimensional’ nanostructures. The explored propagating modes are exchange-dominated spin waves in the range of GHz, which allows us to obtain very short wavelength spin waves that can propagate along different ‘paths’ in the unsaturated landscape. The ‘thickness and shape-induced’ enhancing of these spin waves in structures in a flux closure configuration, suggests their use as highly tuneable spin wave emitters. Secondly, analytical and mathematical models are proposed for controlling the spin wave propagation in multidomain structures of various shapes and in domain walls. Their magnetic configuration is dominated by the shape anisotropy of the patch, which allows us to design particularized shapes to control the spin wave wavenumber (or equivalently, wavelength) while it propagates. Also, the modes are shown to be sensitive to bias fields, which further enhances their tuneability and reconfigurability. All this previous work is joint in the last part of the Thesis, more focused on potential applications for Magnonics, where spin waves are not necessarily seen as undesirable energy loss mechanisms in magnetic structures but as information carriers or the base of novel computing paradigms. Single, or interconnected unsaturated elements of various shapes are proposed to be the base of interesting highly tuneable spin wave devices.

Spintronic technology utilises the spin of electrons as an extra degree of freedom to store and process information in microelectronic devices. Of special interest among all spintronic materials are topological insulators (TIs), which have insulating bulk but conductive surface states. In this thesis, a comprehensive study of various TI and non-TI spintronic systems is presented, with an emphasis on the magneto-optic Kerr effect (MOKE) characterisation. The design and construction of a MOKE microscope is described, for simultaneous magneto-optical and magneto-transport measurements in an applied field of up to 9 T at temperatures from 1.5 to 300 K. This microscope is used to study Cr:Sb$_2$Te$_3$, (Cr,V):Sb$_2$Te$_3$, EuS/Bi$_2$Se$_3$, and [Cr:Sb$_2$Te$_3$/Dy:Bi$_2$Te$_3$]$ magnetic TI thin films. A good agreement is established between the electrical transport and MOKE results of the Cr:Sb$_2$Te$_3$ films. However, in (Cr,V):Sb$_2$Te$_3$, a discrepancy is found between the two measurement techniques, indicating the presence of the Cr$_2$Te$_3$ secondary phase in the sample. For EuS/Bi$_2$Se$_3,$ its interface-induced perpendicular magnetisation is successfully verified using the MOKE microscope. Regarding the ferromagnet/paramagnet [Cr:Sb$_2$Te$_3$/Dy:Bi$_2$Te$_3$]$_0$ superlattice film, unexpected exchange bias is observed in the material with an exchange field of up to 0.15 T at 10 K. This finding is supported by a density-functional-theory calculation. Apart from TIs, several ferromagnet/semiconductor heterostructures are investigated for spin transport experiments, including $L1_0$-FePt/MgO/GaAs and [Co/Pt]$_n$/GaAs. None of them show promising magnetisation features for the further pursuit of spin injection into semiconductors. Finally, the surface science of a few magnetic and non-magnetic materials is investigated using low energy electron microscopy (LEEM). A new approach is established to de-oxidise the surface of W(110) substrates at 1800 K, a temperature considerably lower than in the standard W(110) cleaning procedure (2100 K). LEEM also reveals the high-temperature dynamics of the Pt-Si alloy on Si(100) substrates. Following the temperature increment, the Pt-Si alloy goes through the phases of nanowires, islands, and eutectic droplets. This research paves the way towards the future energy-efficient spintronic devices.

We are reaching capacity limits in data storage devices, due to restrictions on further miniaturization. There are also sustainability concerns regarding long-term data storage solutions, specifically associated with the use of critical raw elements. The use of novel materials must be investigated to address both sustainability and capacity concerns in the field of spin transport electronics (spintronics). This thesis investigates thin films of Fe/Mn/Al based Heusler alloys and antiferromagnetic Mn₃X(X = Sn, Ga) as new materials for spintronic devices. Heusler alloys offer a very promising platform to tailor physical properties of materials through element compositional changes and substitution. In the first part of this thesis, both polycrystalline and epitaxial Fe/Mn/Al Heusler alloy thin films are investigated as potential sustainable electrodes for spin valves. 200-nm-thick Fe₂₋ₓMn₁₊ₓAl (x = -0.25, 0, 0.25) polycrystalline films (deposited on thermally oxidized Si-substrates) with intermixed antiferromagnetic and ferromagnetic phases were investigated as a single layer exchange biased system. The variation of Mn concentration determines the magnitude of the exchange bias effect, which can be either enhanced (in Fe₁.₇₅Mn₁.₂₅Al) or suppressed (in Fe₂.₂₅Mn₀.₇₅Al). An Fe-rich phase embedded in an Mn-rich microstructure was revealed to be associated with a ferromagnetic L2₁ phase and an antiferromagnetic B2 phase, respectively. Therefore, revealing that exchange coupling between these two phases is the cause of the exchange-bias effect. 20 nm - thick Fe₂₋ₓMn₁₊ₓAl (x = 0, 0.25, 0.5, 0.75, 1) epitaxial films are also investigated in this work. Films deposited on TiN buffer-layers show higher quality than those deposited directly onto MgO substrates, with L2₁ order verified by X-ray diffraction. SQUID magnetometry and X-ray magnetic dichroism have been utilized and show deviations from their predicted magnetic properties, which could possibly diminish their predicted half-metallic properties. In the second part of this thesis, the use of antiferromagnets, substituting conventional ferromagnets, for spintronic devices is explored. Antiferromagnetic materials offer higher stability than ferromagnetic materials, with higher packing density, due to minimal stray fields and insensitivity to external magnetic fields. Non-collinear antiferromagnetic D0₁₉ ε-Mn₃ X(X = Ga, Sn) thin films are investigated to this end. Structural characterization of the growth of the Ru-buffer (on cAl₂O₃ substrates) and the subsequent deposition of Mn₃ X films are detailed. Epitaxial films together with smooth surfaces have been achieved. The growth parameters have been optimized for several compositions with unobserved interfacial diffusion. The non-collinear antiferromagnetic structure has been confirmed in both Mn₃Ga and Mn₃Sn by SQUID magnetometry. A small in-plane magnetic moment of ~12 kA m⁻¹ and ~9 kA m⁻¹ is observed for Mn₃Ga and Mn₃Sn respectively, with an insignificant out of plane contribution confirming bulk-like properties and paving the way for the generation of novel antiferromagnetic devices.

Magnetic nanoparticles (MNPs) are widely used in many different sectors especially within biological applications for heat generation or as contrast agents in magnetic resonant imaging. Our research concentrates on the magnetic susceptibility of MNPs and the effect on heat generation applications for use in biomedical applications as an alternative to invasive treatments such as chemotherapy. In this thesis we have studied the response of clusters and chains of magnetic nanoparticles in solution. In particular we have focused on chains of MNPs that form through the spontaneous agglomeration of clusters. We are interested in magnetic response of MNP chains and how the response of the MNPs change with increasing chain lengths and different particle sizes. To understand the response of MNPs we have implemented the stochastic-Landau-Liftshitz-Gilbert equation in the high friction limit to study their rotational magnetic response. We have modelled chains of dipolarly coupled MNPs up to twenty particles long and two different sizes of MNP (10 nm and 20 nm radius particles). We have found that chains of MNPs show at least two resonant frequency modes and a marked deviation from the response predicted by a simple rigid body model. In particular the high frequency rotational response of the chain shows a crossover between the rigid body coherent response to an incoherent independent particle response. Implications to the specific absorption rate and heating capabilities of these chain are finally discussed. The code implemented during this doctoral project has been made available from https://mangocode.gitlab.io.

This thesis presents research on the effect of superconducting-helimagnetic heterostructures. First, the electric and magnetic properties of epitaxial Nb(20 nm)/HoDy(40 nm) bilayers are studied. The saturation magnetisation values are consistent with previously reported values for Ho and Dy. The magnetic spiral is observed up to 43% Dy content although the Dy content changes the stability of the helimagnetic state. The transition temperature (Tc) of the bilayers is obtained from resistance vs temperature curves for different external magnetic field values. Tc lowered as the HoDy alloy transitioned from the helimagnetic state to the ferromagnetic state. The change in Tc decreases with the Dy content. Highly oriented Nb/Ho/Nb Josephson junctions are also studied in this work. The electron mean-free-path in the normal state is longer in the textured junctions than in the polycrystalline junctions reported below but still below the thermal coherence length implying that the transport processes are diffusive. The decay length of the critical current (Ic) in the helimagnetic state is over 100 nm and consistent with the non-magnetic coherence length in the dirty limit. The external magnetic field (H) dependence of Ic deviates from the expected Fraunhofer pattern and shows split peaks with extra minima and concave central peak. A theoretical model was constructed to explain the Ic(H) curves. The model assumes that the magnetisation increases faster along the edges of the junction and the highly magnetised regions propagate from the outside to the centre of the Ho barrier. The model successfully reproduces the characteristic features observed in the junctions and implies that in certain junctions, regions with 0 and π phase change coexist next to each other.

Notions of topology are of considerable interest currently in physics, especially in the way topology influences the electronic properties of crystalline solids. This thesis concerns the experimental investigation of four different magnetic materials, which have been predicted to display a strong interplay between magnetism and the topology of the electronic band structure. I demonstrate that EuCd2Sb2, below TN= 7.4 K, displays an A-type anti-ferromagnetic order on the Eu sub-lattice with a magnetic propagation vector of k=(0,0,1/2). I also establish that the C3 rotational symmetry along the crystal c axis (essential for the stabilisation of band crossings called Dirac nodes) is broken by the in-plane orientation of the Eu moments. Following the initial discoveries of crystalline solids which can host topologically-protected band crossings called Weyl nodes, there is now a need for better material realisations, ideally comprising a single pair of nodes located at or very close to the Fermi level and in an energy window free from other overlapping bands. I propose that EuCd2As2, in a magnetic field of B >1.6 T along the c axis, to be such a system. This material is of general interest as it represents the simplest possible Weyl semimetal, and is therefore a model system for fundamental investigations of Weyl physics. I refute the proposition that semimetallic YbMnBi2 hosts Weyl nodes induced by the time-reversal symmetry breaking mechanism. Furthermore, the full magnetic excitation spectrum of YbMnBi2, which has been mapped for the first time, demonstrates that the magnetic order of the Mn sub-lattice is weakly coupled to the charge carriers in the Bi square net. Finally, the complex magnetic order of the Mn sub-lattice in Mn3Ge has been elucidated for the first time. This will shed light on the origin of the large anomalous Hall effect in Mn3Ge, which is unusual for an antiferromagnet.

Magnetic heterostructures may exhibit new features at the interface while leaving the bulk properties of the films unaltered, which make them ideal for technological applications. Here, I explore the interfaces of EuO(001)/LaAlO₃(001), EuO/NiO(111) and graphene/metal heterostructures. Published DFT calculations postulated the formation of a spin polarised two-dimensional electron gas (2DEG) in the EuO film at ~1.5 nm away from the EuO(001)/LaAlO₃(001) interface, due to the polar catastrophe which would also enhance the Tc of the EuO. Magnetisation depth profiles of the interface with the LaO⁺- and mixed-terminated LaAlO₃(001) substrate are investigated using low energy muon spin relaxation (μSR) and polarised neutron reflectivity (PNR). The behaviour of our EuO(001) film is unlike that reported in the literature for stoichiometric EuO and EuO₁₋ₓ films, and no increase in the Tc was detected by the μSR. Instead, a reduction of ~2.6 K in the Tc is measured by the SQUID which is attributed to the lattice mismatch between the EuO(001) and the LaAlO₃(001) substrate. However, contradictory results were obtained by the PNR for the LaO⁺-terminated sample, where a magnetic moment of 3.59 μBEu atom, which is consistent with the value reported by Barbagallo et al. for a 9% oxygen-deficient EuO, was detected at 20 nm from the EuO(001)/LaAlO₃(001) interface. Therefore, I question the accuracy of the DFT calculations. Exchange bias and the magnetisation depth profile of the polar interface of the antiferromagnet NiO(111) with EuO are investigated using PNR, and the results are compared with those of an uncoupled EuO grown on a MgO(001) substrate. A small double shifted hysteresis loop with enhanced coercivity, which arise as a result of the competition of the random NiO(111) S-domains, is measured by the SQUID magnetometer for the polar interface. However, a larger spin-split in the PNR is observed for the EuO(001)/MgO(001) sample in contrast to the EuO/NiO(111). The small spin-split in the latter is attributed to the pinning of the EuO magnetisation by the randomised NiO(111) S-domains which restrict it from aligning fully with the applied field. This postulation is verified by further PNR measurements on a field-cooled EuO/NiO(111) sample, which showed equal probability for the presence of compensated and uncompensated NiO(111) spins at the interface. A magnetic moment in graphene can be induced in the presence of a ferromagnetic layer as a result of the proximity effect. PNR measurements show that a magnetic moment of ~0.57 μB is induced in the graphene when it is grown on Ni(111) and on Co(111) films, which is about 5x higher than that reported in the literature. The origin of the induced magnetic moment is found to be due to the opening of the graphene's Dirac cones as a result of the strong Cp-3d hybridisation which is confirmed by additional measurements using a non-magnetic Ni₉Mo₁(111) substrate. The results were also validated by the Bayesian uncertainty analysis, and proved by the X-ray magnetic circular dichroism measurements.

All-metallic spintronic heterostructures have the ability to generate pure spin currents at low resistance-area products via spin-orbit coupling. Heavy metal/ferromagnet (HM/NM) bilayers are often used in this context to extract key spin-dependent parameters such as the spin Hall angle, θ, and spin diffusion length, λ, of the constituent materials. However, in the ultrathin film limit, morphology is highly sensitive to growth parameters, making consistent extraction of the spin-dependent properties challenging. Although much work has been done to study spin transport in nanodevices, a long-standing controversy in literature remains over quantifying the magnitudes of spin-dependent parameters in highly spin-orbit coupled HMs. Despite growing evidence that spin-dependent properties vary with HM resistivity, a concerted effort to understand the effect of HM microstructure morphology on spin transport in HM|FM bilayers has yet to be made. In this thesis, we investigate the role of ultrathin HM microstructure morphology on spin transport properties within HM|FM bilayers (FM = Co, CoFeB; HM = W, Ta, Ru, Pt). By seeding HM growth with thin metallic buffer layers including Ta and Ru, we are able to tune the growth mode and so morphology of the HM layer. X-ray diffractometry and transmission electron microscopy confirm the good control of HM growth modes through film wetting, continuity, texture and roughness. The subsequently altered electronic properties mediate the coupling of HM morphology to spin transport by impacting resistance and voltage readouts in common electrical spin injection/detection schemes. We probe spin reflection, transmission and accumulation at the HM|FM interface in three such measurement schemes with a systematic set of spin Hall magnetoresistance (SHMR), spin pumping and spin-orbit torque (SOT) effective field measurements on (un)buffered Pt|CoFeB bilayers. We experimentally demonstrate a large enhancement of generated spin currents with SHMR and spin pumping measurements in ultrathin buffered devices, which can be directly correlated to HM microstructure. After extending current magnetoelectronic circuit theory to include the seed layer, we find that spin transport in buffered Pt|CoFeB bilayers can only be well understood when considering HM film morphology through Elliot-Yafet-dominated spin relaxation and intrinsic spin scattering in the Pt layer. By modelling both SHMR and spin pumping data simultaneously following this methodology, the different dependence on θ in the schemes allows us to estimate a single set of θ and λ despite drastically different spin signals in buffered Pt|CoFeB layers. We further confirm the domination of intrinsic spin scattering in Pt by measuring near-constant normalised SOT effective fields across the (un)buffered bilayers. The demonstration of the significant effect of HM microstructure morphology on spin current generation in common measurement schemes indicates this work may potentially provide resolution to the widely varying values of θ and λ reported across the literature.

With there being a strong connection between the underlying structure of materials and their properties, understanding this inherent coupling will lead to more effcient materials. This thesis studies three non-centrosymmetric compounds MnSb2O6, Cu3Nb2O8 and CeRhSi3 with the goal of understanding the coupling between the structure and the underlying magnetic and electronic properties. All three materials lack an inversion centre and this thesis investigates how this constraint influences the underlying interactions and therefore properties of these compounds. MnSb2O6 is predicted to be multiferroic - a group of materials that has drawn considerable attention these days. One of the most interesting aspects of multiferroics is the so-called magnetoelectric coupling. Whereas in ferroics, one can tune magnetization with magnetic field, polarization with electric field, elasticity with stress, the idea behind multiferroics is that one can tune the electric polarization by the magnetic field (or vice versa). In terms of applications, the idea of electric-field control of magnetism is particularly exciting and it could lead to smaller and more effective devices. In the first part of this thesis, I discuss single crystal preparation based on three different methods in an attempt to obtain high quality single crystals for neutron inelastic scattering studies. Improvement in the size of single crystals was achieved using the flux method. This turned out to be an important step for inelastic experiment with the total mass of 1.3 g single crystals (approximately 150 sealed ampoules). Moreover, polycrystalline sample of MnSb2O6 has been prepared successfully with the hydrothermal method. To the best of our knowledge, this process of preparation has not been published before. The second part of the MnSb2O6 study is focused on neutron experiments and following analysis. The experiment on triple axis spectrometer RITA-II mapped behaviour of spins in horizontal magnetic field. Critical exponent β was estimated with discussion on properties of this system. Obtained results were compared with complementary experiment on BT4 spectrometer with same results. The magnetic and structural chirality was tested with polarized neutrons on instrument D3. Whereas, magnetic chirality in compound has a dominant domain, we were not able to confirm the same for the structural chirality. The last part of this section is discussing another interesting aspect of MnSb2O6 - it's noncollinear magnetic structure within triangular lattice is making it an ideal candidate to study so called multimagnon processes. The classical picture of magnon as non-interacting and long-lived excitation has been questioned by recent studies of classical systems with measured multimagnon excitations. Whereas existing spin wave theory can explain these excitations for quantum systems (S = 1/2), it doesn't provide an answer for their existence in classical systems. Cu3Nb2O8 compound belongs to type-II multiferroics, which means that electric polarization emerges at a magnetic order temperature. I prepared a polycrystalline sample that was measured on the time-of-flight instrument MARI. Experiments revealed excitations around 30 meV that it is not expected for system with spin S = 1/2. We close this study with the discussion about origin of these excitations - seems like they do result from localised clusters of spins (coupled triplet) rather than individual Cu2+ ions. Whereas for elements and some alloys, magnetism and superconductivity are in an antagonistic relation, several non-centrosymmetric heavy fermion compounds have been reported, where superconductivity and magnetism do coexist together. This is also the case of CeRhSi3 (superconducting at high pressures). For these systems, it is not unusual to see transitions between localized and itinerant (delocalized) magnetism. We describe neutron inelastic experiment focused on study of magnetic fluctuations in the normal state at ambient conditions. This has been followed by the experiment in magnetic field, in respect to define their antiferromagnetic or ferromagnetic character. The study contains also a heuristic description and therefore possible models that can be used as an interpretation of our measured data. This thesis also provides an example of two different types of magnetism - Mn atoms in MnSb2O6 are d elements with characteristic localized magnetism and LS coupling, whereas Ce atoms in CeRhSi3 are f elements which typically displays itinerant magnetism and j-j coupling. I discuss the framework on how to describe these two types of magnetism and also discuss various implications to 4d and 5d transition metal ions.

The interaction between superconductivity and non-collinear ferromagnetism has been investigated in hybrid structures. The nature of the interaction between the two can convert spin-singlet Cooper pairs from a conventional superconductor into spin-triplet pairs within the ferromagnet. These can survive over longer distances in the ferromagnetic material, which is a clear signature of a novel electronic state. This thesis primarily describes efforts to realise and study these fascinating long-range spin-triplet correlations in superconductor-ferromagnet hybrid structures composed of conventional superconducting Niobium (Nb) and ferromagnetic Permalloy (Py). A disk geometry has been used to promote a magnetisation vortex in the ferromagnet, whose naturally non-collinear magnetisation makes it ideal for spin-triplet pair generation. Firstly, two distinct superconductor-ferromagnet hybrid arrays have been fabricated in which the subsystems are either electronically or magnetically coupled, and the superconducting vortex behaviour investigated by scanning Hall probe microscopy. Distinct vortex nucleation patterns occur in different hybrid arrays and unconventional vortex behaviour near the Py disks has been observed, manifested as an apparent intervortex attraction. These interesting findings motivated the fabrication of planar superconductor-ferromagnet Josephson junctions with a disk-like geometry, which exhibited anomalous electrical magnetotransport phenomena, indicating the possible role of spin-triplet supercurrents. Secondly, a commercial second generation high-temperature superconducting (2G-HTS) tape has been investigated by scanning Hall probe microscopy in order to obtain a deeper understanding of the vortex pinning energy landscape, which is critical to underpin future developments. The magnetic flux front propagation in the tape has been mapped, showing very distinct behaviours at different temperatures. Our results have been compared to a classical critical state model, and find that this only describes the observed vortex behaviour well at temperatures closer to Tc. In addition, fishtail-like magnetisation peaks have been observed that have been linked to a 'vortex matching' effect for pinning sites near the edge of the tape.

The antiferromagnetic coupling between a rare-earth (RE) and a tran- sition metal (TM) ferromagnet can be exploited to engineer normal state and superconducting functional devices. RE/TM ferromagnetic multi- layers were previously used as spin-mixers to generate spin-triplet su- percurrents. This was possible due to magnetic inhomogeneity present in the devices, however the precise nature of the inhomogeneity was not understood. Here we present a comprehensive study of the Ni/Gd/Ni system using a powerful element-specific measurement technique: x-ray magnetic circular dichroism. In order to analyse the experimental results we present a novel model based on the Stoner-Wohlfarth model, which shows that significant inhomogeneity exists at the Ni/Gd interfaces due to the competition between the exchange energies within the system and the Zeeman energy of the applied magnetic field. The experiment and model together provide a complete overview of the Ni/Gd/Ni system due to the breadth of temperatures and thicknesses studied. The knowledge gained from this work is then applied to designing and test- ing new spin valves based on the intrinsic inhomogeneity at the RE/TM interface, and both Ni/Gd- and Gd/Ho-based devices show reversible magnetic switching behaviour which alters the superconducting critical temperature.

This thesis studies the physics of multilayer heterostructures grown from topological insulators (TIs), primarily bismuth selenide and antimony telluride, and other topologically trivial materials. This is done by extending a standard low-energy 3D TI Hamiltonian and varying its associated material parameters across the simulation domain. New results arising from the position-dependent TI interface model are found. For the first time, this method is incorporated into a density-functional theory (DFT) solver in order to study the self-consistent charge density in multilayer TI heterostructures due to the interface states. The thesis is structured as follows. The introduction (Ch. 1) presents a pedagogical review of the theory of 3D TIs and low-energy Hamiltonians used to study them, as well as typical methods in solid state physics that are made use of throughout the thesis. Chapter 2 presents the position-dependent Hamiltonian, showing new evidence for topological features of bulk states including varying degrees of band mixing and inversion; also, interface state tunnelling is shown to be affected by atomic layer orbital overlap, and incomplete localisation of surface states is demonstrated for antimony telluride. Chapter 3 presents a new DFT model of TI heterostructure interfaces and shows how conduction through TI interface states can be controlled with an electric field. Chapter 4 covers the extension of the model in Ch. 1 to 2D cross-sections of TI wires and heterostructures, showing for the first time evidence of localisation of conduction almost entirely within the inner interfaces of a 2D heterostructure wire. Chapter 5 presents our work with magnetic fields, demonstrating evolution of interface and bulk states with changing magnetic field and Landau level, as well as presenting new evidence for more complex spin structures in bismuth selenide arising from Landé factor signs. Our conclusions are presented in Chapter 6.

A major area of interest in condensed matter physics over the past decades has been the emergence of new states of matter from strongly correlated electron systems. A few limited examples would be the emergence of unconventional superconductivity in the high-T$_c$ superconductors and heavy-fermion systems, the appearance of the skyrmion magnetic vortex state in MnSi and magnetically mediated superconductivity in UGe$_2$. While detailed studies of many of the emergent phases have been made, there are still many gaps in understanding of the underlying states and mechanisms that allow them to form. This work aims to add to knowledge of the basic physics behind such states, and the changes within them as they are tuned to approach new phases. The cubic perovskite material SrTiO$_3$ has been studied for many decades and is well-documented to be an incipient ferroelectric, theorised to exist in the absence of any tuning in the proximity of a ferroelectric quantum critical point. This work presents the first high-precision dielectric measurements under hydrostatic pressure carried out on a quantum critical ferroelectric, leading to a full pressure-temperature phase diagram for SrTiO$_3$. The influence of quantum critical fluctuations is seen to diminish as the system is tuned away from the quantum critical point and a novel low temperature phase is shown to be emergent from it. The Néel Temperature of the two-dimensional antiferromagnet FePS$_3$ was found to increase linearly with applied hydrostatic pressure. Evidence of an insulator-metal transition is also presented, and an unexplained upturn in the resistivity at low temperatures in the metallic phase.

The spinel structure (general formula AB2O4) is widely occurring in natural and synthetic materials, and has a marked technological and scientific significance due to its magnetic, electric and multiferroic behaviours. The presence of transition metal cations with multiple oxidation state and the resulting charge, orbital and spin degrees of freedom of the partially occupied d-orbitals lead to uniquely ordered ground states. The coupling of all the three degrees of freedom can result in a structurally distorted ground state where the direct metal-metal interaction forms atomic clusters, or 'orbital molecules'. The Verwey phase of magnetite (Fe3O4), occurring below TV ~ 125 K, is driven by a cooperative bond distortion that forms linear Fe3+-Fe2+-Fe3+ arrangement (trimeron). The effect of non-stoichiometry and chemical modification on this complex structure has been investigated with a variety of samples through microcrystal synchrotron XRD. A mineral sample (Al, Si, Mg and Mn impurities, TV = 119 K) confirms the Verwey phase as the most complex long-range electronic order known to occur naturally; its relevance in space sciences is discussed. Moreover, the structural analysis of two synthetic magnetites (Fe3(1-δ)O4 with 3δ = 0.012 and TV = 102 K, Fe3-xZnxO4 with x = 0.03 and TV = 90 K) univocally confirmed the persistence of the transition, and its first order, at doping level > 1 %, contrary to previous reports. Moreover, the temperature evolution of the trimerons and their persistence above TV was probed through X-ray Pair Distribution Function analysis on pure Fe3O4: the data analysis between 90 K < T < 923 K show that the Verwey phase goes from long-range ordered (T < 125 K) to short-range ordered (T > 850 K). Magnetite can thus only be considered to have a regular cubic spinel structure above the Curie temperature (TC = 858 K). The pyrochlore lattice of B cations in a spinel gives the structure the potential for frustration upon antiferromagnetic ordering. Fe2GeO4 and γ-Fe2SiO4 were synthesised through conventional solid state routes, with the use of high-pressure synthesis for the latter. Magnetometry and heat capacity measurements highlighted two transitions (Tm1 = 8.6 K and Tm2 = 7.2 K, and Tm1 = 11.2 K and Tm2 = 7.5 K respectively). Powder neutron diffraction data between 2 K < T < 25 K showed that both materials stay undistorted below TN. Magnetic Rietveld refinement led to two highly unconventional magnetic structures, with incommensurate propagation vectors and modulation of the moment magnitude. γ-Fe2SiO4 also shows a spin-ice order below Tm2. The results are unique and unusual for transition metal oxides; the models are systematised by proposing a 'frustration wave' model, in which the degree of frustration is a spatial quantity that can be distributed through the structure in order to stabilise the ground state.

In paleomagnetism, practical measurements are rarely made using perfect, isolated, single-phase, ferromagnetic crystals. Experimental observations are typically made using magnetic materials formed by a variety of natural processes. In this thesis, we will look at bridging the gap between current numerical modelling capability and experimental observations. First, we work towards micromagnetic modelling of multi-phase magnetic materials, including magnetostriction, embedded in a rocky matrix, along with crystal defects. We present a derivation of the Boundary Element Method formulation used by the micromagnetics package, MERRILL, and provide an extension of this from single-phase materials to multi-phase. After discussing issues with previous approaches to modelling magnetostriction, we derive and present a more robust and flexible approach. This model of magnetostriction is suitable for non-uniformmagnetizations, for multi-phase materials, and for arbitrary boundary conditions, and can be incorporated into MERRILL.We then outline a method for extending our model to materials embedded in an infinite elastic matrix of arbitrary elasticity. Finally, we present a method for modelling the magnetic response of a material due to crystal defects, along with a concrete example of a magneto-dislocation coupling energy at a magnetite-ilmenite boundary where stress due to lattice misfit is eased by regular edge dislocations. Second, we work towards being able to verify micromagnetic models against nano-scale experimental data. To do this, we present two techniques for simulating electron holograms from micromagnetic modelling results, a technique capable of imaging magnetic structures at the nano-scale. We also present example electron holograms of commonly occurring magnetic structures in nano-scale rock and mineral magnetism, and highlight some distinguishing features, which may be useful for interpreting experimental electron holography data.

URhGe is the first ferromagnet discovered that shows superconductivity at ambient pressure. It shows a rich temperature-magnetic field phase diagram with a re-emergence of superconductivity at high magnetic field where the moments rotate. This suggests that the quantum fluctuations associated with the moment rotation may provide the pairing interaction for superconductivity. The objective of this thesis was to study these critical fluctuations with inelastic neutron scattering and heat capacity measurements, using the latter to test the bulk nature of the superconductivity and determine the types of gap nodes to help test this hypothesis. To perform the heat capacity measurements, it was necessary to develop an apparatus that measures milligram samples in the temperature range 50-1000 mK, and magnetic field range 0-12 T. The field exerts a mechanical force upon the sample, which causes it to rotate, perturbing the system destructively. The apparatus developed in this thesis overcomes this diffculty by holding the sample with tensioned kevlar wires. Testing was done by making measurements on UPt3, a well characterised superconductor. It was then used to measure URhGe in zero magnetic field. The extension to measurements in high magnetic field were not performed however, due to the structural integrity of the apparatus being weak - this was in an attempt to reduce the thermodynamic signature of the background. After many iterations of apparatus design and build, the device was proved not appropriate for high fields. A discussion of the zero-field data, as well as the design and build process, is given. The Curie temperature of URhGe is suppressed with magnetic field (applied along the b-axis), reaching zero temperature at the moment rotation transition referred to above. Small angle neutron scattering (SANS) was measured at both zero and finite fields to detect the evolution and relaxation of the critical fluctuations. The scattering is inelastic and the SANS measurement integrates over energy. Nevertheless it was possible to compare models with different dynamical dependences for the magnetic relaxation. In field, however, the magnitude of the fluctuations was strongly reduced, falling below the detection limit at half the critical field. Comparing Landau damping to various forms of non-Landau damping, a result was found that agrees with that for the ferromagnetic superconductors UGe2 and UCoGe, but the lack of critical scattering at field is found to be in contradiction with NMR measurements, which is discussed. UAu2 is a new material on the heavy fermion landscape. The crystal structure found suggests some frustrated magnetism, culminating in a Neél temperature of 43 K and a further transition at 400 mK; this suggests some new quantum criticality not seen before, and so heat capacity measurements were performed with the already-tested apparatus to see if, as the resistivity measurements suggest, a Fermi-liquid state is found. Results revealed differences between annealed and non-annealed samples in their thermodynamic signature, and the behaviour expected for antiferromagnetic spin-fluctuations is found to continue to temperatures below 150 mK, suggesting the existence of a quantum critical point. The validity of these results along with implications are discussed.

A greater understanding of precessional dynamics in magnetic systems is central to several emerging technologies. This thesis presents the design, construction and development of a Vector Network Analyser based Ferromagnetic Resonance measurement instrument (VNA-FMR), and its application in characterising dynamic material properties in hybrid anisotropy [CoPd]8-NiFe films, produced by remote plasma sputtering. Potential applications for hybrid films include Spin Torque Oscillators (STOs) or Vortex Oscillators (VO) for use as microwave emitters in, for example in Microwave Assisted Magnetic Recording (MAMR). The VNA-FMR system was first used to measure thin films of NiFe (permalloy) which allowed its capabilities to be quantified and compared to systems reported in the literature. The instrument demonstrated the capability of measuring permalloy films down to a thickness of 3 nm and was used to measure resonance and damping behaviour which agreed well with theory. The results obtained forMs were in agreement with measurement using Vibrating Sample Magnetometry. The effect of interlayer exchange on FMR was explored in hybrid films using a sample series with varying Pd spacer layer thickness, t, [Co/Pd]-Pd(t)-NiFe. As Pd spacer thickness increased, a transition was observed from near complete coupling with a single resonance mode to separate acoustic and optical branches of resonance. As spacing was further increased, the branches converged towards the resonances of the individual component layers of the hybrid films. The results suggest exchange coupling has a range of less than 2 nm, and is completely extinguished at 5 nm, in agreement with previous measurements. However, a change in damping behaviour was observed between 10 and 20 nm spacer thickness, independent of field orientation.

This thesis presents the results from investigations of ultrafast magnetisation dynamics driven by pure spin currents. Spin orbit coupling in heavy metal layers - such as tungsten, tantalum or platinum - allows for the generation of pure spin currents, whereby spin up and spin down electrons move in opposite directions. Hence, a flow of angular momentum can be controlled through the manipulation of charge current through a heavy metal layer. When a spin current is injected into a ferromagnet, a torque is exerted on its magnetisation, with the potential to induce a wide variety of ultrafast dynamics. The experimental investigation of these phenomena employed a variety of high-frequency electrical techniques and time-resolved scanning Kerr microscopy (TRSKM) methods. In addition, various simulative and analytical approaches were used to gain insight into the underlying mechanisms. Spin Hall nano-oscillators (SHNOs) have recently been shown to support a tunable GHz spin wave `bullet’ under injection of direct current (DC), making it an exciting candidate for microwave communication applications. This thesis will show how TRKSM can be used to measure the torques within these devices, revealing that radio frequency (RF) current does not possess the same distribution as the DC. The competition between self-inductance and focusing within the device geometry results in a modified distribution of spin current. Further TRSKM measurements show the modified torque landscape to promote the mobility of the `bullet' within the magnetic layer. Devices that exploit spin currents for magnetisation reversal have received interest from academia and industry for their potential use as memory elements. The perpendicular magnetic anisotropy present in Ta/CoFeB/MgO leads to lower write currents and higher thermal stability. However, ultrafast processes have not been previously observed in such devices. TRSKM measurements of Hall bar devices were compared with a macrospin model to understand the underlying torques, and to investigate the conditions required to promote switching. Square elements built from the same stack structure exhibited contrasting static and dynamic behaviour. Pulsed currents drove differing dynamics at the edge and center of the device, while enabling the realignment of magnetic domains. The domains themselves could be driven directly by the spin current leading to domain wall dynamics. Measurements with a bipolar electrical pulse demonstrated that meta-stable switching can be achieved in micron-scale elements.

The ability of a ferromagnet to maintain its magnetic state in the absence of an external magnetic field has made ferromagnetic materials an important subject of study in physics since the end of the 19th century. Moreover, ferromagnetic materials are the cornerstone for data storage systems such as magnetic tapes, magnetic disk drives and magnetic random access memory. The discovery of the Giant Magneto Resistance (GMR) in 1988 suggested that, since the magnetic state of the electrical conductor has an important effect upon the current flow, there may also be an inverse influence of the current upon the magnetization. In this effect, predicted in 1989 [1] by Slonczewski and called Spin Transfer Torque, angular momentum transferred by a spin polarized current can exert a torque on the magnetization of a ferromagnetic material, changing the local magnetization and stimulating the precession of the magnetic moments, generating microwave signals. This provides a new method of manipulating magnetization without applying an external field. Large polarized currents lead to spin transfer effects which are the driving force for the magnetic dynamics of devices known as Spin Transfer Oscillators (STO). In this new kind of nano-device the emission of microwaves is stimulated by a DC electrical current and measured as a change in the output voltage due the GMR effect. The specific characteristics of these devices such as working frequency and DC current ranges, microwave emission linewidth, and maximum emission power among others, are given by the design and size of the device,and the nature of the magnetic oscillations producing the emission. Among the multiple types of STO that now exist , I have focused my research upon three of them: Spin Transfer Vortex Oscillators (STVO), Single Layer Spin Transfer Oscillators (SL-STO) and Orthogonal Pseudo Spin Valves. Within STVOs and SL-STOs we can nucleate what is called a magnetic vortex. A magnetic vortex is a curling of the in-plane of a magnetic layer with its centre pointing out of the magnetization plane. The gyration of this vortex due to STT produces a microwave emission < 1GHz with a greater emission power than that produced by the precession of magnetic moments in STOs. The phase-locked synchronisation of multiple vortices is expected to exhibit enhanced microwaved power and phase stability compared to a single vortex device, providing a solution to the drawbacks of the STO in the low frequency regime. On the other hand, Orthogonal Pseudo Spin Valves promote the nucleation of magnetic dissipative solitons, also called magnetic droplets. This type of magnetic structure has an opposite out of plane magnetization to the layer that contains it. Compared to the microwave emission of magnetic vortices , magnetic droplets have a higher frequency range and emission power. However, their nucleation is subject to large external fields being applied to the sample. In this thesis, I electrically characterized these devices and applied magnetic imaging techniques in order to go further in the understanding of the spatial features and dynamic behaviour of these magnetic structures. It is not possible to acquire this knowledge by only using electrical characterization. Understanding the magnetization dynamics in these devices is crucial for the design of STO based devices while imaging studies are required to prove the existence of these magnetic structures, as in case of the magnetic droplet. In chapter 2 I will introduce the background concepts of magnetism that are relevant to this thesis. I will go from the basics principles of ferromagnetism, its quantum mechanical treatment, and the theory that explain the dynamics of the magnetisation. I will also present the state of the art in experimental research in the field of spin transfer oscillators. My aim is to give the basic background needed to understand the results presented in this thesis. In chapter 3 I will introduce the two main experimental techniques used for imaging the magnetisation of the devices presented: Holography with Extended Reference by Autocorrelation Linear Differential Operator (HERALDO) and Time Resolved Scanning Kerr Microscopy (TRSKM). I will revise the theoretical background concepts and the development of the techniques in order to demostrate the uniqueness of each technique and how they were used in this thesis. It is interesting to note that while MOKE is a well-known and widely-used technique, far fewer laboratories in the world area able to perform time resolved measurements using MOKE, with the University of Exeter being one of them. Furthermore, HERALDO is a novel technique that is used for the first time to image magnetic structures within multilayer systems in this thesis, which is a milestone in the development of the techinque. In chapter 4 I present an investigation of the magnetization dynamics of a SL-STO. Electrical transport measurements provided an initial characterization of the device. We then used HERALDO for the first time to investigate the magnetization dynamics in an intermediate layer of a multilayer stack. We present time averaged measurements of the magnetisation of a magnetic vortex formed underneath a nano contact (NC) positioned on top of the multilayer, using a combination of x-ray holography and x-ray magnetic circular dichroism. In chapter 5 I present the first direct measurement at the time of a magnetic dissipative droplet, using holography with extended reference autocorrelation by linear differential operator (HERALDO). I studied the out of plane magnetisation of the free layer under a NC within an orthogonal pseudo spin salve. In chapter 6 I present and study STVO devices with pairs of NCs of 100 nm diameter and centre-to-centre separation D = 200 to 1100 nm, by a combination of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM). It will be shown that the dynamic behaviour of vortices and anti vortices changes when the distances between the NCs within the devices is changed.

Magnetic domain walls (DWs) could form the basis of potential novel magnetic based memory, logic, and Lab-On-a-Chip devices, with a great potential to increase performance and enable new processes. Being DW detection and manipulation the main obstacle to overcome to achieve integration into new devices. The aim of this thesis is to investigate the manipulation and tracking of DWs through electrical measurements, and study the effect of localized magnetic moments on DW dynamics. Study of DW dynamics inside of in-plane magnetization devices is performed using L-shaped magnetic nanostructures. The L-shaped devices are used as a DW trap to study DW tracking using magnetoresistance (MR) measurements. It is shown, through magnetic force microscopy (MFM) imaging, that the L-shaped nanostructures of certain dimensions geometrically constrain the magnetization to four specific remanent states (i.e. referring to the magnetization at the corner of the L-shape: head-to-head DW, tail-to-tail DW, tail-to-head and head-to-tail magnetization), thus allowing the use of L-shaped devices as DW traps. Due to these simple magnetization states and using anisotropic magnetoresistance effect (AMR), which links magnetization with electrical resistance, we demonstrate that it is possible to track the magnetization and identify the magnetic state of the devices by performing MR measurements. To ease detecting the effect of external influences on the DW dynamics, variations of the design of the L-shaped DW traps are tested. MR measurements, in combination with in situ MFM and micromagnetic simulations, demonstrate that square corner L-shaped nanostructures with circular disks at the end of the nanowires show a larger difference between DW pinning/depinning fields, and have a more symmetric behaviour when magnetic field is applied at different angles. Magnetic bead (MB) detection experiments are performed by placing a single superparamagnetic bead near the DW pinning position on top of the L-shaped nanostructure. The placement is done using micromanipulation. The results demonstrate detectable influence of the MB for L-shaped devices with widths below 200 nm. Additionally, simulations demonstrate existence of two DW depinning mechanisms, which appear at different field orientations. In the first depinning mechanism, the DW at the corner remains pinned and is annihilated by another moving DW, hence, no influence of the MB is detected. In the second depinning mechanism, the original DW depins by moving from the corner and, hence, it is affected by the MB. To complement the detection experiments, atomic force microscopy (AFM) probes modified with a MB attached are used. Magnetic scanning gate microscopy (mSGM) is used to move the MB around the L-shaped nanostructure at different heights, and measure the sensing volume of the L-shaped devices. Planar Hall effect (PHE) measurements, perform on hybrid Py/Au junctions demonstrate larger changes in resistance than the analogous AMR-based magnetization tracking techniques. Moreover, by performing mSGM in the PHE geometry, we demonstrate that PHE type of measurements can also be used to detect single MB, with a sensing volume similar that of the L-shaped nanostructures. Study of DW dynamics in out-of-plane magnetization nanostructures is carried out on CoFeB nanostructures. We study the anomalous Hall effect (AHE) and the anomalous Nernst effect (ANE) and how these two effects can be used to track magnetization in nanostructures and potentially be implemented to detect MBs. Additionally, the experiments with CoFeB devices measuring the AHE and ANE, in combination with differential phase contrast imaging, allow to measure the stray field of the MB used for measuring the sensing volume in in-plane devices. MFM imaging complements MR measurements when studying magnetization evolution. However, when studying DW dynamics, probe-sample interaction has to be carefully analysed. In the last part of this thesis we study custom-made MFM probes with nanostructures lithographically defined onto the probe's tip and compare them with standard MFM probes. Electron holography experiments in combination with in situ MFM imaging demonstrate that the magnetization states of the custom-made probes can be controllably switched into two high magnetic moment states, and two low moment states, both exhibiting high switching fields. This enables the use of custom-made probes to study samples that exhibit appreciable probe/sample interaction, and therefore require a probe with high coercivity and low moment.

This thesis describes the application of various techniques to the study of magnetic phenomena in condensed matter systems with a particular focus given to a.c. magnetic susceptibility. This is accomplished by briefly discussing the basic theory of condensed matter magnetism before describing measurement techniques employed in this thesis. I discuss magnetometry in detail, contrasting d.c. susceptibility with a.c. susceptibility in the context of the applicability of the latter to the study of slow magnetic dynamics. In order to understand the a.c. response of materials containing slow magnetic dynamics I take a theoretical approach and show how the commonly used models for describing such behaviour arise from the more overarching linear response theory. I then show common treatments of a.c. susceptibility data before reviewing the a.c. response of various classes of material while discussing the origin of the associated magnetic sluggishness. Subsequently, I show studies of two different classes of material showing magnetic sluggishness. In the first study of single-molecule magnets two families are examined: the simpler Co3+2Ln3+ family (Ln = Gd, Ho, Tb and Er) and the more complex Ni2+4Ln3+4 family (Ln = Gd, Ho, Dy, and Tb). By beginning with the Co2Ln family in which only the central Ln3+ ion possesses a magnetic moment I use a.c. susceptibility to show the Tb and Er analogues to display field-induced slow magnetic relaxation consistent with single-molecule magnet behaviour while the Gd and Ho analogues do not. Combining these results with d.c. magnetic measurements in the context of the structure of these compounds allows the origin of this behaviour to be shown to be associated with the relevant Ln3+ ions. This sets out the process utilised for elucidating the magnetic behaviour of the more complex Ni2+4Ln3+4 family in which both transition metal ions and lanthanide ions possess a magnetic moment. As such, the possible magnetic exchanges between ions are evaluated using d.c. techniques and I propose the Dy and Tb analogues to form Ni-Ln dimers (ferromagnetic and antiferromagnetic respectively). Using a.c. susceptibility I demonstrate Ni4Dy4 and Ni4Tb4 to display field-induced slow magnetic relaxation of single-molecule magnet origin only in Ni4Dy4. The second study I present focuses on the low temperature magnetism associated with the superconducting family Li1-xFex(OH)Fe1-ySe which is comprised of alternating two-dimensional layers of Fe1-ySe and Li1-xFex(OH). The Fe2+ ions of the hydroxide layer possess a magnetic moment and by considering a random distribution of these ions in such a layer I suggest the low temperature magnetic state to likely be disordered and spin glass-like. Taking advantage of samples provided by synthetic chemists of superconducting and nonsuperconducting members of this family I show that d.c. magnetometry and heat capacity measurements support a spin glass-like state. A.c. susceptibility and μSR measurements demonstrate that the magnetic state of Li1-xFex(OH)Fe1-ySe contains slow relaxation of a spin glass type.

Meteorites provide information about the early history of our solar system and the formation and evolution of planetesimals. One of the few direct observations of internal geophysical processes within planetary bodies is the presence or absence of a dynamo-driven magnetic field. These observations provide essential constraints on the degree of differentiation, core solidification timescales and the driving forces for convection. This thesis focusses on the paleomagnetic information recorded by iron and stony-iron meteorites, providing us with a unique view-point for the generation and variability of core dynamo activity. Iron and stony-iron meteorites are primarily comprised of FeNi metal. The Widmanstätten pattern; an intergrowth of taenite and kamacite lamellae. Between these lamellae, a range of microstructures develop, dictated by the ‘M-shaped’ Ni diffusion profile. Among these microstructures is the cloudy zone, a region of tetrataenite islands in a Fe-rich matrix, formed by spinodal decomposition. The tetrataenite islands are extremely reliable paleomagnetic recorders. The direction of magnetisation and composition of FeNi microstructures was imaged using synchrotron X-rays. Magnetic contrast is generated using X-ray magnetic circular dichroism. The dimensions of tetrataenite islands within the cloudy zone directly correlate with cooling rates. Cooling rates vary from ~0.5–10,000°C/Myr and correspond to island sizes of ~500–10nm, respectively. The slowest cooled group of iron meteorites reveal multidomain magnetic behaviour within the cloudy zone, whereas in faster-cooled meteorites islands are vortex state. This demonstrates that cooling rate influences the magnetic properties of the cloudy zone. The subtly different cooling rates between different pallasite meteorites means that each meteorite provides a ‘snapshot’ of the parent body magnetic field at a different point during its thermal evolution. Paleointensity results provide the first observations of a quiescent period in dynamo activity preceding core solidification. This also helps to constrain the paleomagnetic signals associated with core nucleation, which, in turn, constrains the mechanism of solidification. Paleomagnetic studies of meteoritic metal were complemented with measurements of magnetic inclusions in olivines. Alternating-field and thermal demagnetisation experiments were carried out using both 2G SQUID and WSGI small-bore SQUID magnetometers. Results suggest that pallasite silicates are unreliable paleomagnetic recorders, and a planetary-strength paleointensity cannot be recovered. Paleomagnetic fidelity was also investigated for a dusty olivine grain from the Semarkona chondrite. Lorentz microscopy, transmission X-ray microscopy, nanotomography and micromagnetic simulations were used to rigorously test the behaviour of Fe-nanoparticles. The final study in this thesis focusses on the IAB iron meteorites. These meteorites have an unusual and complex history. Paleomagnetic results are accompanied by a detailed microstructural study using X-PEEM and electron backscatter diffraction to constrain the formation of two microstructures: pearlitic and spheroidised plessite. Paleomagnetic results suggest the IAB parent body did not have an active core dynamo. Meteorites represent the oldest material in our solar system, and their complex histories and susceptibility to alteration make them some of the most challenging samples to extract reliable paleointensity estimates from. Advanced electron microscopy and synchrotron techniques are now making it possible to extract reliable paleomagnetic information, with profound implications for the formation and evolution of the solar system.

Chapter 1 of this thesis contains an extensive literature review of magneto-structural correlations (MSCs) of a range of transition metal dimers. Different structural parameters are considered and a critical assessment is given of the principal factors influencing the exchange interactions in each case. Following this, two different synthetic strategies for the synthesis of polymetallic systems are compared and contrasted. Their role in the formation of paramagnetic frameworks is also considered. Chapter 2 describes the synthesis of a family of MnIII dimers and highlights the importance of chelating ligands in their formation and magnetic properties. A family of MnIII dimers of general formula of [MnIII2(R-sao)2(dpa)2](ClO4)2 (1-5) has been synthesised using derivatised phenolic oximes (R-saoH2, where R=H, Me, Et, Ph) in combination with di-(2-picolyl)-amine (dpa). The structures reveal a double-oxime bridged [MnIII(NO)]2 magnetic core in which the Jahn-Teller axes lie perpendicular to the bridging plane, in contrast to two previously reported family members (6 and 7). The switch in the orientation of the Jahn-Teller axes is enforced through the use of the chelating ligand which is present in 1-5 and absent in 6-7. Dc magnetic susceptibility measurements reveal that the exchange interactions between the MnIII metal centres in 1-5 are antiferromagnetic in contrast to that observed for 6 and 7 which are ferromagnetic. DFT calculations have also been undertaken in order to confirm these findings. Chapter 3 describes the synthesis of a family of novel cages from the metalloligand [FeIIIL3], where HL = 1-(4-pyridyl)butane-1,3-dione and metal salts CuBr2, Cu(NO3)2, NiCl2, CoCl2 Co(SCN)2, ZnBr2 and Pd(OTf)2 (where OTf = triflate anion). The flexibility in the design of these structures offers huge potential to tune their physical properties as the constituent parts of the cage can be altered without any change to the structure. Due to the large size of the internal cavity, it is possible that the cage could play host to different species, such as magnetic guests, which would then exert control over the exchange interaction between the metal ions in the host framework and between the host and guest(s). Chapter 4 presents the synthesis and crystallographic studies of two novel metal organic framework (MOF) [CuIIL2]n and HL = 1-(4-pyridyl)butane-1,3-dione. An in-depth high-pressure crystallographic study of structure shows that the framework undergoes a single-crystal phase transition as well as a switching of the orientation of the Jahn-Teller axis, which is dependent on the hydrostatic media and also the presence of negative linear compressibility (NLC) behaviour.