Lake sediments can play an important role in understanding and reconstructing temporal characteristics of the geomagnetic field, as they potentially offer near continuous high-resolution archives of magnetic information extending throughout the Holocene. To date lake sediment geomagnetic data has typically been excluded from British archaeomagnetic secular variation curves (SVC) due to uncertainties with the acquisition of magnetisation by lake sediments. This paper presents the argument that, with regards to British datasets, the real problem lies with poor chronological control and sets out to illustrate that with British archaeomagnetic data some progress has been made. The results indicate that it is not currently possible to resolve secular variation on a time scale of ¿100 years from published British lake sediment data but more success has been made with data from archaeological sediments. This level of detail has been considered necessary for the incorporation of lake sediment data into the British archaeomagnetic dataset, as the ability to resolve short-term geomagnetic changes is critical for the integration of any dataset into the British SVC. As the latter is predominantly employed to date archaeological architecture and artefacts requires that it has the ability to resolve changes over timescales relevant to human lifetimes. Using currently available data this retrospective critique confirms that, in archaeological sediments, depositional and thermoremanent magnetisation can record the same direction over the same time interval.

Synchronous generators used for hydro power and nuclear power is a well known topology but there is a vast amount of intricate technologies and methods to making them function properly. This masters thesis covers the development, implementation and verification of a magnetisation system for a synchronous generator. The software implementation is made in the LabVIEW programming environment and uses a high performance CompactRIO with an FPGA for measurements, calculation and output control signals. Together with several peripheral devices, the CompactRIO forms an excitation system and most importantly, an automatic voltage regulator. This system keeps the output voltage of the generator stable and has a variety of safety features such as over excitation limits, under excitation limits and a V/Hz limiter. The resulting system successfully monitors and controls the generator characteristics and the controllers, based on PI controllers, have short rise times, low overshoot and no significant static error. This magnetisation system was verified on a 185 kW synchronous machine and all functions showed satisfying results with the exception of the implemented power system stabiliser which need to be re-tuned.

Nous présentons une étude par Résonance Magnétique Nucléaire (RMN) de l'azurite, Cu3(CO3)2(OH)2, un système de spins quantiques. Ce composé peut être modélisé comme une chaine quasi-unidimensionnelle, frustrée, ‘de type diamant', de spins électroniques S=1/2 portés par les ions de cuivre Cu2+. Il présente dans sa courbe de l'aimantation en fonction du champ magnétique, entre 11 et 30 T et à très basse température, un plateau à 1/3 de l'aimantation à saturation. Nous avons effectué des mesures RMN du cuivre dans l'azurite à T=1.5 K afin de déterminer sa structure magnétique microscopique. Les résultats obtenus dans le plateau démontrent que le ‘dimère' des deux spins qui sont plus fortement couplés est approximativement dans l'état singulet, tandis que le troisième spin (le ‘monomère') est presque complètement polarisé. Cela confirme que la configuration électronique du plateau à 1/3 est un nouvel état quantique qui n'a pas d'équivalent classique [F. Aimo et al., Phys. Rev. Lett. 102 127205, (2009)]. Par RMN du proton à très haut champ magnétique, entre 31 et 34 T à T=0.6 K, nous avons aussi étudié le passage depuis le plateau à 1/3 vers la polarisation complète du système, afin de confirmer ou infirmer l'existence éventuelle d'un plateau à 2/3. Ce plateau est attendu dans le cas exceptionnel où les corrélations longitudinales de spins sont dominantes et stabilisent un ordre incommensurable longitudinal. L'analyse détaillée du dédoublement très symétrique des spectres RMN nous amène à conclure que c'est un ordre antiferromagnétique transverse et non longitudinal qui est établi, ce qui est incompatible avec l'existence du plateau à 2/3. / We present a Nuclear Magnetic Resonance (NMR) study of azurite, Cu3(CO3)2(OH)2, a quantum spin system. This compound has been recognised as a model system for a quasi-1D, frustrated, ‘diamond' chain of S=1/2 spins beared by Cu2+ ions. In the magnetisation curve as a function of magnetic field it presents, between 11 and 30 T and at very low temperatures, a plateau at 1/3 of the saturation magnetisation. We performed Cu NMR measurements in azurite at T=1.5 K in order to determine its microscopic magnetic structure. The obtained results show that the ‘dimer' of two more strongly coupled spins is approximately in a singlet state while the third spin (the ‘monomer') is almost fully polarised. This confirms that the electronic configuration of the 1/3 plateau is a new quantum state without classical analogue [F. Aimo et al., Phys. Rev. Lett. 102, 127205, (2009)]. By very high magnetic field proton NMR, between 31 and 34 T and at T=0.6 K, we have also studied the transition region between the 1/3 plateau and the full polarisation of the system in order to test for the possible existence of a 2/3 plateau. This plateau is expected in rather exceptional case when longitudinal spin correlations are dominant and stabilise an incommensurable longitudinal order. However, our analysis showed that the symmetric splitting of NMR spectra corresponds to an antiferromagnetic transverse and not longitudinal order, which is incompatible with the existence of a 2/3 plateau.

We solved the Landau-Lifshitz equations numerically to examine the time development of a system of magnetic particles. Constant or periodical external magnetic field has been applied. First, the system has been studied without dissipation. Local energy excitations (breathers) and chaotic transients have been found. The behaviour of the system and the final configurations can strongly depend on the initial conditions, and the strength of the external field at an earlier time. We observed some sudden switching between two remarkably different states. Series of bifurcations have been found. When a weak Gilbert-damping has been taken into account, interesting behaviour has been found even in the case of one particle as well: bifurcation series and period multiplication leading to chaos. For a system of antiferromagnetically coupled particles, highly nontrivial hysteresis loops have been produced. The dynamics of the magnetization reversal has been investigated and the characteristic time-scale of the reversal has been estimated. For more particles, the energy spectrum and the magnetization of the system exhibits fractal characteristics for increasing system size. Finally, energy have been pumped into the system in addition to the dissipation. For constant field, complicated phase diagrams have been produced. For microwave field, it has been found that the chaotic behaviour crucially depends on the parity of the number of the particles.

The mechanisms by which magnetisation changes occur in magnetic materials may be investigated by a variety of hysteresis measurements. During this study both alternating and rotational hysteresis measurements were used to characterise the mechanisms of magnetisation change in a number of permanent magnet materials. Studies of the time dependence of magnetisation, remanent magnetisations and the dependence of the reversible magnetisation on the irreversible magnetisation were undertaken. These studies revealed that in sintered rare-earth iron magnets the magnetisation change is predominately controlled by domain nucleation, with a lesser contribution from domain wall pinning within the boundary regions of the grains. Similar mechanisms control the magnetisation change in the larger grains of melt-quenched rare-earth iron magnets. In the single domain grains of the melt-quenched materials incoherent rotation mechanisms control the changes of magnetisation. Magnetisation change in MnAlC and sintered AlNiCo was found to be controlled by domain wall pinning within the interior of the grains of the materials. Two devices were constructed for the measurement of rotational hysteresis. The first measures the angular acceleration of a sample set spinning in a magnetic field, from which the rotational hysteresis loss may be determined. The second employs rotating search coils to make direct measurements of the component of magnetisation that contributes to rotational hysteresis loss during the rotation of a sample in a field. Both devices were found to produce data consistent with that in the literature and to be useful for the characterisation of rotational hysteresis in permanent magnet materials. A simple model was used to examine the dependence of rotational hysteresis loss on various material parameters. It was found that the value of the rotational hysteresis integral is dependent on interactions and to a lesser extent distributions in anisotropy. This is contrary to assumptions commonly made in the literature but consistent with published experimental data, which has been reinterpreted. Measurements of rotational hysteresis losses in the materials studied were found to be effected by geometric demagnetisation effects. A method by which such data may be corrected for these effects is proposed. Following correction and consideration of the interactions within the materials, the rotational hysteresis data was found to be consistent with the characterisations performed in linearly alternating fields.

Through our whole work we study the XY model with all its entirety, a particular spin model where spins are confined in a plane. We try to bring out a good understanding of this model with all different types of phases and phase transition, it undergoes in critical situations. We conceive of these external conditions from very different physical models like High Tc Superconductor, Ultracold atoms in optical lattice which are in focus of recent research. Firstly we model high Tc Superconductors with very simple 2D XY model to get an idea about the diamagnetic response exhibited by these materials when kept in a external magnetic field. This modeling is reasonable because most of the physics of cuprate High Tc Superconductors are governed by their 2D copper oxide planes which insists us to consider 2D models. Later we shifted to a more realistic 3D anisotropic XY model , as the coupling between cuprates plane may have a considerable role in devising physics of those materials. We particularly focus on the 2D to 3D crossover effect on magnetisation showed by these models, with keeping an eye on how all these can be relate to the experimentally acquired magnetisation profile of High Tc Supercondutors. On the second project we investigate on the phase diagram of a fully frustrated 2-leg ladder Bose Hubbard model. After mapping it properly to a classical model, a bi-layer Fully Frustrated XY model on square lattice, we found that the frustration leads to the emergence of a new phase "Chiral Mott insulator(CMI)" sandwiched between "Chiral Superfluid(CSF)" and "regular Mott insulator(MI)" phase. We divide the whole report into four parts. The first chapter is basically contain-ing introductory part comprising the motivation. In the second chapter we discuss various types of phases and phase transitions of the 2D & 3D XY models. We try to address their critical behaviors. In the third chapter and onwards we consider our model in external magnetic field and observe magnetisation in these systems. Here we specially focus on 2D to 3D crossover effect on magtisation measurement. Lastly in the fourth chapter we bring out a correspondence of XY model with the 2 leg ladder fully frustrated Bose Hubbard Model. There we report the emergence of a new phase, Chiral Mott Insulator(CMI) due to frustration in system.

A way to obtain materials that show novel phenomena is to explore the interplay between geometry and interactions. When it is not geometrically possible to satisfy all the interactions by a given configuration, then to find the ground state becomes very complicated. This interplay between geometry and interactions defines geometrical frustration. One of the most popular examples of geometrical frustration in magnetism is spin ice. In this system, nearest neighbour ferromagnetic interactions between Ising spins in a pyrochlore structure emulate water ice by showing the same degree of frustration. This is manifested by the same ground state residual entropy. Although the clearest example of spin ice among magnets is shown by Dy₂Ti₂O₇, the behaviour of this material is richer than that of pure spin ice. The large magnetic moments of the rare earth Dy form a spin ice that also interacts via dipolar interactions. These long range interactions give rise to monopolar excitations which dramatically affect the dynamics of the system with respect to the pure spin ice case. In this thesis magnetisation experiments and numerical methods are used to explore the properties of the magnetic insulator Dy₂Ti₂O₇. We study its excitations at low temperature and describe the out-of-equilibrium characteristics of the magnetisation processes, below a temperature where the system freezes out. For temperatures above the freezing temperature, we describe and measure a 3D Kasteleyn transition and the concomitant Dirac strings associated to it, for the field in the [100] crystallographic direction. For temperatures below the freezing temperature, we find new out-of-equilibrium phenomena. Magnetic jumps are measured and their sweep rate dependence analysed. A deflagration theory is proposed and supported by simultaneous magnetisation and sample temperature measurements obtained by a new design of a Faraday magnetometer.

The paper reports on a composite based on a polydimethylsiloxane matrix filled with liquid metal and magnetic hard microparticles. The effect of the concentration of such a complex filler on the elasticity and relative permittivity of the composite has been investigated. Inclusions of liquid metal provide improved permittivity compared to the unfilled matrix, while the stiffening effect is not as high as when the matrix is filled with solid particles alone. On the other hand, magnetic hard fillers allow the functionalisation of the composite in terms of its magnetic properties. It is also shown that changing the residual magnetisation of the composite has no significant effect on the elasticity and relative permittivity.

The effect of repetitive quasi-static magnetisation of a magnetorheological elastomer on its magnetic response is discussed. Typical components of this material, namely soft silicone rubber and carbonyl iron powder, are used to produce magnetically sensitive composite samples. The composite specimens are examined by vibrating sample magnetometry. The influence of the elasticity of the composite matrix on the change in magnetic differential susceptibility of the material as a function of the number of repeated magnetisation cycles is evaluated. The soft matrix elastomers are characterised by the presence of maxima in the initial part of the differential susceptibility curves. The sample magnetisation values corresponding to such maxima vary with the number of magnetisation cycles. In addition, the initial magnetic susceptibility curves of soft samples obtained at different polarity of the applied field are not symmetrical. Symmetry is obtained by performing several subsequent magnetisation cycles. The differential susceptibility curves are affected by both reversible and irreversible processes related to particle mobility in the matrix. The magnetic response of composite samples with rigid matrices is more similar to that of classical soft magnetic materials.

Micro- or nanometer sized magnetic particles (beads) currently have a vast range of life science applications in, for example, bioseparation techniques, cancer therapy, development of contrast agents and biosensing techniques. In the latter field, magnetic beads offer several unique advantages, including minimal background signals, physical and chemical stability and low manufacturing costs. Because of these properties, magnetic biosensing techniques are potential candidates for low-cost, easy-to-use molecular diagnostic devices. This doctoral thesis focuses mainly on the proof of principle and further development of a new magnetic biosensor platform for detection of DNA targets, a potential candidate for a new generation of low-cost, easy-to-use diagnostic devices: the Volume-Amplified Magnetic Nanobead Detection Assay (VAM-NDA). The VAM-NDA principle combines target recognition by padlock probe ligation followed by rolling circle amplification (RCA) of the reacted probes with changes in Brownian relaxation behaviour of magnetic nanobeads (typically ~100 nm in diameter) induced by a change in hydrodynamic bead volume. More specifically, the RCA products (coils, typically ~1 μm in diameter) are detected magnetically by adding magnetic beads tagged with detection probes complementary to part of the repeating RCA-coil sequence. Thus, depending on the target concentration, a certain quantity of beads binds to the coils by base-pair hybridisation (bead immobilisation), resulting in a dramatic bead volume increase, which is then detected by measuring the complex magnetisation spectrum. Use of a commercial SQUID magnetometer for measuring complex magnetisation resulted in a detection limit in the low pM range for DNA targets with excellent quantification accuracy. Simultaneous multiplexing was also evaluated. The stability and aging of typical commercial ferrofluids (suspensions of magnetic beads) were investigated by measuring the complex magnetisation of and interbead interactions in oligonucleotide-functionalised ferrofluids. In summary, the bead surface characteristics were found to have a strong impact on the measured dynamic magnetic properties.