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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Advanced gradient coil design for MRI

Green, Daniel January 2003 (has links)
No description available.

An investigation of some of the oscillations of D.C. magnetic suspension systems

Elson, Michael P. January 1978 (has links)
No description available.

Synthesis, characterisation and magnetic studies of substituted lanthanide (Bis) phthalocyanine single molecule magnets

Waters, Michael January 2013 (has links)
Single molecule magnets (SMMs) are an area of significant research due to their potential applications for data storage, magnetic refrigeration and quantum computing. SMMs are molecules that have the properties of bulk magnets but also show quantum effects such as quantum tunnelling of the magnetisation. One class of molecules that have been found to be SMMs are lanthanide (bis) phthalocyanines, also known as double deckers. These complexes contain a single lanthanide ion which is 'sandwiched' between two phthalocyanine rings. This class of SMMs have been found to have large energy barriers to magnetic relaxation making them suitable for magnetic data storage applications. In this Thesis four new substituted lanthanide double deckers have been successfully synthesised and characterised. The substituents added to the phthalocyanine rings include cyano groups, esters, carboxylic acids and imides. The double deckers magnetic properties were investigated using a superconducting quantum interference device (SQUID) magnetometer and some complexes were found to have their magnetic properties changed by the addition of the substituents, in some cases the magnetic properties are improved. All complexes have been found to show magnetic hysteresis and slow relaxation of the magnetisation in an oscillating field, indicating that the complexes are SMMs. A range of other measurements were carried out to determine their magnetic and electronic properties, including far infrared (FIR) spectroscopy and magnetic circular dichroism (MeD) spectroscopy. The internal structure of the double deckers was probed with extended X-ray absorption fine structure (EXAFS) spectroscopy to investigate the effect that the substituents have on the structure

Microstructure and magnetic properties of doped Mg0-based soft ferrites

Arshad, Mohammad January 2000 (has links)
No description available.

Magnetocaloric properties of La(Fe,Si)13 compounds

Lovell, Edmund January 2016 (has links)
La(Fe,Si)13-based compounds are extremely promising for use in magnetocaloric cooling applications. This thesis aims to address multiple challenges to their use in cooling systems, as well as contribute to the fundamental understanding of their first-order magnetic phase transition. A significant field rate dependence in the field-induced paramagnetic (PM) to ferromagnetic (FM) transition in the first-order composition is determined to be a result of non-isothermal conditions from the inherent heating or cooling from the magnetocaloric effect (MCE). Reduction of the magnetic hysteresis has been shown with improved thermal linkage with the thermal bath. Metastability between the phases is demonstrated by magnetic temporal evolution (relaxation). The completion of this evolution is arrested by physical cracks and the sample shape (demagnetising field effects). The onset of the transition is shown to be dominated by the demagnetising fields, and as such a method to reduce magnetic hysteresis via sample shape is demonstrated. These effects are shown to not dominate for samples which show a second-order transition. Mn substitution on the Fe site decreases the strength of the first-order character which is correlated with the relaxation rate, the latent heat decrease and the heat capacity peak increase. Similar correlations are observed with increasing temperature and both the relaxation rate and latent heat go to zero at the critical point, defined by the temperature at which the first-order character (latent heat and hysteresis) disappears. The electronic and spin wave contributions to the low temperature heat capacity are investigated and significant enhancement of the former is shown. This is further enhanced with increased Mn-doping but to a much lesser extent when interstitial hydrogen is introduced to the compound. Suppression of this further enhancement in magnetic field suggest that there are two enhancement mechanisms: a base enhancement likely from electron-phonon interactions, and a second which is magnetic in origin.

Experimental study, mathematical modelling and dynamical analysis of magnetorheological elastomer materials and structures for vibration control

Zhu, Guanghong January 2015 (has links)
As a smart material, magnetorheological elastomer (MRE) is composed of magnetisable particles dispersed in a non-magnetic medium. Because the mechanical properties of MRE can be continuously, rapidly and reversibly controlled by adjusting magnetic field in a pre-yield regime, there has been increasing research on MRE for mitigation of unwanted vibrations, and yet the application and commercialisation in varies fields are still on a very early stage. Considering the dependence of mechanical properties on strain, frequency and magnetic field the current research on mathematical modelling for MRE is still insufficient to provide guidelines for engineering applications. In this study, the dynamical properties of MRE were studied by means of shear tests under different driving frequencies (1-80Hz), strain amplitudes (0-6.0%) and magnetic fields (0-500mT). The experimental results have shown that the storage modulus of MRE increases as the frequency increases, but the loss modulus initially increases with frequency (<10Hz) up to a maximum value and then decreases with further increasing frequencies; both the storage modulus and loss modulus decrease with an increase of strain, and they increase with increasing magnetic flux densities until the magnetic saturation occurs. With the full use of gathered information on mechanical property characterisation of MRE, a nonlinear mathematical model is established to describe the complex behaviour of MRE for the dynamical analysis of vibration systems, and a methodology of modelling is proposed for materials to continuously describe the dynamic behaviour in certain region of strain and frequency with a benefit of low requirement for the calculation on parameter identification. A structure of MRE is developed with a high bearing capacity and a good controllability of stiffness to benefit vibration control systems. The dynamical properties of this structure are predicted with the dynamic design and the mathematical modelling, and the results are examined through dynamic tests to validate that the extension of this mathematical model in MRE structures. Furthermore, dynamical analysis is presented for a two-stage vibration isolation system, a vibration absorption system and an isolation system consists of a continuous beam and an MRE isolator to examine the efficiency of MRE absorbers and isolators. Results show that a reduction of the vibration amplitude, the force transmissibility or the power flow transmissibility can be achieved by properly designing dynamical systems and considering the excitation frequency ranges. Comparing with conventional absorbers and isolators, MRE devices can locally and globally improve the performance of vibration control significantly from the perspective of dynamical behaviour, transmissibility or vibratory energy transmission.

Computer simulation studies of complex magnetic materials

Wang, Weiwei January 2015 (has links)
With the development of both computing power and software engineering, computer simulation of the micromagnetic model or atomistic spin model, has become an important tool for studying a wide range of different complex phenomena in magnetic materials. Meanwhile, the rapid improvement of advanced measurement techniques has allowed the probing of ultrafast magnetization dynamics, as well as the magnetic phenomena involving charge current, heat and light. The simulation of magnetism is now moving towards a multiphysics method. Therefore, fast, user-friendly, and extensible codes with accurate algorithms are helpful in understanding the physics and designing novel magnetic devices on the nanoscale. In the preparation of this thesis we have developed Fidimag, which is a Python/C simulation tool supporting both micromagnetic and atomistic spin models. The software has also been extended to support the Landau-Lifshitz-Baryakhtar (LLBar) equation. Using Fidimag, we have performed simulations to study the domain-wall motion and spin-wave decay with the LLBar equation. We also explain the exchange damping in the LLBar equation as the phenomenological nonlocal damping by linking it to spin pumping, therefore, LLBar equation can be considered as a phenomenological equation of the nonlocal damping. We studied magnon-induced domain-wall motion in the presence of Dzyaloshinskii-Moriya interaction (DMI) numerically and theoretically. We find that the presence of DMI and easy-plane anisotropy can drive the domain wall very effectively and that the domain-wall velocity depends on the sign of DMI constant. While the negative velocity is considered as a result of angular momentum conservation, we attribute this fast domain-wall motion to linear momentum transfer between magnons and the domain wall. By numerically solving the Landau-Lifshitz-Gilbert equation with a classical spin model on a two-dimensional system, we show that both magnetic skyrmions and skyrmion lattices can be moved with microwave magnetic fields. The mechanism is enabled by breaking the axial symmetry of the skyrmion with a static in-plane external field.

Terfenol-D magnetostrictive linear motor

Halkyard, Peter January 2011 (has links)
Actuators and linear motors are a common way to provide single axis motion and force. Rack and pinion, chain, belt, cable, screw, hydraulic and electric solenoid are a few of the many types of actuators available. All have inherent strengths and weakness such as power consumption, mechanical failure and usually a trade off with speed and resolution. The thesis investigation is to develop a linear motion actuator based on the magnetic material, Terfenol-D which alters its shape when subjected to a magnetic field. This phenomenon is known as magnetostriction which can be defined as a changing of a material's physical dimensions in response to its magnetization. Terfenol-D has the largest known magnetostriction, 2000ppm at ambient temperatures [1], of all commercially available materials. Because of these large strains it is given the term Giant Magnetostriction (GM). In Terfenol-D the GM is dependent on the direction of the applied magnetic field with respect to the direction in the crystal: the material elongates in the direction of the field and, orthogonal to this, the material contracts within the principal of constant volume. This property has been exploited by other researchers in the past resulting in Terfenol-D being used as a short stroke conventional actuator in applications requiring large force at both high and low frequencies, generating a range of applications including surgical instruments, ultrasonic transducers and many others. It is this property of Terfenol-D that will be exploited in this project in the development of the magnetostrictive linear motor. Stepping motors based upon Giant Magnetostrictive (GM) materials are more complex actuators than those based on a more conventional design. To get a long stroke, they use the addition of several small steps, each produced by quasi static deformation of the GM materials. In this project the Kiesewetter Elastic Wave Motor using Terfenol-D, as referenced in the abstract, was used as the starting point for the design of a new Terfenol-D based motor. In particular, the project investigates different topologies of Terfenol-D in the design and construction of a Magnetostrictive Linear Motor

Measurement of shielding in electrically large metallic enclosures

Armstrong, Robert January 2013 (has links)
The measurement techniques employed to obtain the shielding effectiveness (SE) of enclosures using the current version of IEEE 299.1 only apply to enclosures of side dimension greater than 0.1m, and are also lacking in a full analysis of the field distributions within the enclosure. The work presented here uses the IEEE 299.1 draft standard as a base and investigates different methods of obtaining the SE, as well as looking at making the measurement more applicable to physically small and electrically large enclosures, and hoping to inform future versions of shielding effectiveness standards for small enclosures. The first part of this thesis investigates the use of a comb generator as a source in an enclosure under test (EUT), which provides a statistically uniform electric field inside the EUT when combined with a small mechanical stirrer. The EUT used here is an equivalent size to a 19 inch rack unit used in many equipment rigs; therefore investigations using it are of relevance to the real world. It becomes apparent that it is important to be sure that statistical field uniformity is achieved within the EUT as well as in the test chamber. The chamber at the University of York is compared with the chamber used in Ancona, Italy. Meanwhile, it is found that the presence of a direct path or unstirred component distribution in an enclosure or chamber can change the measured SE. A study of aperture dominated EUTs reveals that it is possible to obtain an indication of the SE of an enclosure using the Q−factor. This test method has the advantage that it can be applied to enclosures that have a low SE or have many apertures, as is the case in some real enclosures. Continuing the development into testing physically small enclosures that are outside the scope of IEEE 299:1997, it is shown that a physically small enclosure can be represented by an electrically equivalent larger enclosure. This is also of use when considering IEEE 299.1.

Orientation and thermal stability of advanced recording media

Chureemart, Jessada January 2013 (has links)
This study is focused on the characterisation of advanced perpendicular recording media. In particular, the distribution of the effective easy axis orientation and the effect of silica (SiO2) content on the magnetic properties have been investigated via magnetic measurements. The variation of coercivity (HC) as a function of angle has been used to determine the easy axis distribution. Due to the efficiency of the SiO2 segregation at the grain boundaries leading to intergranular exchange decoupling, the magnetisation reversal of each single domain particle can be described by a numerical simulation using Stoner Wohlfarth theory. The standard deviation of a Gaussian distribution of easy axis orientations in the calculations replicates the magnetic easy axis distribution in thin film media. Three types of media; granular, exchange coupled composite (ECC) and the combination of exchange coupled composite and a coupled granular continuous (CGC) film have been studied. The crystallographic orientation was also measured via X-ray rocking curves for a comparison to be made. It was found that the ECC media which have their grains almost completely segregated by SiO2 give an excellent fit to the simulation with σ = 5 degree. The results for granular and ECC/CGC media do not fit to the same level of accuracy. In-plane and cross-section TEM analysis of both samples show far from perfect segregation leading to intergranular exchange coupling. The activation volume has been used to study the effect of SiO2 content in ECC media. Vact and the physical grain volume have been investigated for identical structures with three different levels of SiO2. Time dependence measurements have been used to determine Vact. TEM analysis has been also carried out to investigate the grain size distribution and to examine the grain boundaries. Vact and the single grain volume are in excellent agreement for the sample with the highest SiO2 content, indicating complete exchange decoupling.

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