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DSpace at Cambridge (227,320 recursos)
DSpace@Cambridge is the institutional repository of the University of Cambridge. The repository was established in 2003 to facilitate the deposit of digital content of a scholarly or heritage nature, allowing academics and their departments at the University to share and preserve this content in a managed environment.

Theses

Mostrando recursos 1 - 20 de 30

1. Magnetic Detection of Microstructure Change in Power Plant Steels - Yardley, Victoria Anne
Power plant components are expected to withstand service at high temperature and pressure for thirty years or more. One of the main failure mechanisms under these conditions is creep. The steel compositions and heat treatments for this application are chosen to confer microstructural stability and creep resistance. Nevertheless, gradual microstructural changes, which eventually degrade the creep properties, occur during the long service life. Conservative design lives are used in power plant, and it is often found that components can be used safely beyond the original design life. However, to bene t from this requires reliable monitoring methods. One such technique involves relating the microstructural state to measurable magnetic properties. Magnetic domain...

2. Fabrication and Characterization of Titanium-doped Hydroxyapatite Thin Films - Desai, Amit Y
Hydroxyapatite [Ca10(PO4)6(OH)2, HA] is used in many biomedical applications including bone grafts and joint replacements. Due to its structural and chemical similarities to human bone mineral, HA promotes growth of bone tissue directly on its surface. Substitution of other elements has shown the potential to improve the bioactivity of HA. Magnetron co-sputtering is a physical vapour deposition technique which can be used to create thin coatings with controlled levels of a substituting element. Thin films of titanium-doped hydroxyapatite (HA-Ti) have been deposited onto silicon substrates at three different compositions. With direct current (dc) power to the Ti target of 5,...

3. Deposition and characterisation of bismuth layer-structured ferroelectric films - Hu, Xiaobing
Bismuth layer-structured ferroelectrics have been recognised as promising film materials for ferroelectric random access memory application due to their excellent fatigue resistance and other electrical properties. This work deals with the deposition and characterisation of epitaxial and polycrystalline W-doped SrBi2Ta2O9 (SBT) and lanthanide-doped bismuth titanate (BiT) films. SBT and W-doped SBT films were fabricated by pulsed laser deposition (PLD) on platinised silicon substrates. The effects of fabrication temperature and W-doping level on film properties were studied. The crystallinity of SBTW films improved with increasing fabrication temperatures, resulting in enhanced ferroelectric properties and dielectric properties above the fabrication temperature of 750 ºC. Dense ceramic samples of Nd- and Sm-doped BiT...

4. Normal state properties of high-angle grain Y1-xCaxBa2Cu3O7-delta - Mennema, Sibe
This dissertation describes the investigation of the normal-state properties of high-angle grain boundaries in YBa2Cu3O7-d (YBCO) and Y1-xCaxBa2Cu3O7-d (calcium-doped YBCO). YBCO is a high-temperature superconducting material with a superconducting transition temperature up to 93 K. Grain boundaries are interfaces between two crystals or grains, and severely reduce the attainable currents in practical, polycrystalline material. A grain boundary is characterised by the misorientation angle between the two adjacent crystals, which determines the atomic structure of the interface. The structure of low-angle grain boundaries (misorientation angles < ~7°) is well understood; it consists of a regular array of dislocations. For higher misorientation angles the dislocations merge and form a continuously...

5. Growth and characterisation of Niobium/Gadolinium superconductor-ferromagnet nanocomposites - Parvaneh, Hamed
Superconductivity and ferromagnetism are two antagonistic physical phenomena which their coexistence in a uniform material can be resolved only under extraordinary conditions. The reason for that is the phonon-mediated attraction energy between electrons which results in the formation of the so-called Cooper pairs, is usually smaller that the exchange (Zeeman) interaction between electrons which tend to align the electron spins. However, non-zero total momentum Cooper pairs can be accomplished even in the presence of an exchange field as surprisingly! predicted first by Fulde and Ferrel [1] and independently by Larkin and Ovchinikov [2] nearly 50 years ago. This coexistence has already been observed experimentally in both bulk...

6. Focused Ion Beam Fabricated Non-equilibrium Superconducting Devices - Moseley, Richard William
The developments over the last decade in Focused Ion Beam (FIB) instrument technology have reached a point where there is sufficient control of an ion beam to make cuts, trenches, and other shapes in a sample on a scale of tens of nanometers. This work concentrates on the use of an FIB instrument for making superconducting devices. It is shown for the first time that planar-bridge (Nb/Cu/Nb) Superconductor/Normalmetal/ Superconductor (SNS) junctions can be reliably fabricated using a standard FIB instrument. This is demonstrated by the responses of junctions to microwaves and magnetic fields; the junctions display the appropriate Josephson behaviour demanded by current technological applications. In addition, the...

7. Active control of superconductivity by means of the ferromagnetic exchange interaction - Kinsey, Robert
Recent theoretical studies have suggested that the observed suppression of superconductivity in superconductor/ferromagnet (S/F) heterostructures could be modulated by controlling the ferromagnetic exchange interaction in the superconductor. The exchange interaction in the superconductor is the sum of the exchange interaction from the ferromagnetic regions, which has a phase and magnitude that depends upon the direction that the ferromagnet is magnetised and the distance. As the exchange interaction has a phase it is possible that the contribution from two regions will cancel out. The exchange interaction, which can be viewed as an imbalance in the spin populations, suppresses superconductivity so any...

8. Spin Polarised Tunnel Junctions Based on Half-Metallic Manganites - Jo, Moon-Ho

9. Critical Currents in YBaCuO Thin Films - Herzog, Robert

10. Josephson Junctions Fabricated by Focussed Ion Beam - Hadfield, Robert Hugh
This thesis details recent work on an innovative new approach to Josephson junction fabrication. These junctions are created in low TC superconductor-normal metal bilayer tracks on a deep submicron scale using a Focused Ion Beam Microscope (FIB). The FIB is used to mill away a trench 50_nm wide in the upper layer of niobium superconductor (125 nm thick), weakening the superconducting coupling and resulting in a Josephson junction. With the aid of a newly developed in situ resistance measurement technique it is possible to determine the cut depth to a high degree of accuracy and hence gain insight into how...

11. Electromagnetic Modelling of Superconducting Sensor Designs - Gerra, Guido
Masters Thesis MPhil in Materials Modelling

12. Development of (Re)BaCuO Coated Conductors by Liquid Phase Epitaxy - Cheng, Yee Siau
Since the discovery of high-temperature superconductors, there has been a worldwide effort towards the development of processes for fabricating coated conductors for power applications. Most of these processes are based on vapour phase deposition techniques that have relatively low growth rates. A high-rate processing route was proposed based on the observation of high growth rate of (RE)Ba2Cu3O7- δ superconducting compounds (RE = rareearth element) from a flux supersaturated with one or more RE elements by liquid phase epitaxy (LPE). LPE has been successfully used to grow YBCO thick films with both c- and a,b-orientations on (110) NdGaO3 substrates and pure c-oriented films on YBCO seeded (100) MgO and...

13. Josephson Junctions and Devices fabricated by Focused Electron Beam Irradiation - Booij, Wilfred Edwin
The irradiation of high Tc superconducting thin films with a focused electron beam, such as that obtained in a scanning transmission electron microscope (STEM), can result in the formation of a Josephson junction. The conditions required for the formation of these Josephson junctions in YBa2Cu3O7-d and related compounds are discussed as well as the physical properties of the irradiated material. From electrical transport measurements of individual Josephson junctions it was found that these junctions have a Superconductor/Normal/Superconductor (SNS) nature. Low temperature anneal studies indicate that Josephson junctions with optimum properties can be obtained by a combination of a high electron dose and subsequent low temperature anneal. Extremely high electron...

14. Numerical modelling of current transfer in nonlinear anisotropic conductive media - Baranowski, Robert Paul
Current transfer behaviour in anisotropic superconducting bodies is the central topic of this thesis and focuses on the effect that the nonlinearity of the electric field dependence upon the local current density value and anisotropy have on the nature of current transport. The main motivation for this work was the desire for a better understanding of the conceptually difficult behaviour of current transport in superconducting bodies and examines current transfer quantitatively for a number of important problems on the macroscopic and microscopic scale. This behaviour is examined both experimentally and using computer models. The successful development of a powerful, robust and adaptable numerical model for analysing the complex...

15. The Electronic Properties of Thin Film YBa2Cu3O7 Low Angle Grain Boundaries - Hogg, Michael
Critical currents in the latest biaxially textured ‘coated conductors’ are now limited by 2D networks of low angle grain boundaries (LAGBs) with misorientation θm = 1 - 10°. In order to understand and optimise current transfer in these materials it is essential to elucidate the electromagnetic behaviour of the LAGB. This work presents an investigation into the transport properties of [001]-tilt LAGBs formed by the thin film deposition of YBa2Cu3O7 onto bicrystalline substrates. Through the use of a precision two-axis goniometer, measurements of the V-I characteristic and critical current density were made as magnetic field was rotated in angles θ...

16. A thermogravimetric study of oxygen diffusion in YBa2Cu3O7-d - Vazquez-Navarro, Maria Dolores
YBa2Cu3O7-d (YBCO) was one of the first high temperature superconductors discovered, and its superconducting properties are strongly dependent on oxygen stoichiometry. A large amount of work has been done on the variation of stoichiometry and its effect on the superconducting properties of the material. However, in spite of all the work done, the results published in the literature are very scattered. This thesis presents a thermogravimetric study of oxygen diffusion in YBCO under isothermal and non-isothermal conditions and tries to reconcile the data available based on the results obtained and taking into account the factors that may have affected the data presented by other groups, such as the...

17. In-situ magnetoresistance measurements during patterning of spin valve devices - Morecroft, Deborah
This dissertation describes an experimental study on the patterning of thin films and spin valve devices. Initially the change in the magnetisation reversal of ferromagnetic Ni80Fe15Mo5 thin films was investigated as the shape anisotropy was increased using optical lithography to pattern wire arrays. These structures show a progressive increase in coercivity and a transition between single and two-stage reversal with increasing milling depth. A similar patterning technique was applied to unpinned (Ni80Fe20/Cu/Ni80Fe20) pseudo spin valve (PSV) structures in order to enhance the coercivity of one of the ferromagnetic layers. The increased coercivity induced by micropatterning changed the natural similarity of the magnetic layers and the structure exhibited a...

18. Phase Coexistence in Manganites - Chapman, James Christopher
The doped perovskite manganite La1-xCaxMnO3 (0
19. The development of magnetic tunnel junction fabrication techniques - Elwell, Clifford Alastair
The discovery of large, room temperature magnetoresistance (MR) in magnetic tunnel junctions in 1995 sparked great interest in these devices. Their potential applications include hard disk read head sensors and magnetic random access memory (MRAM). However, the fabrication of repeatable, high quality magnetic tunnel junctions is still problematic. This thesis investigates methods to improve and quantify the quality of tunnel junction fabrication. Superconductor-insulator-superconductor (SIS) and superconductor-insulatorferromagnet (SIF) tunnel junctions were used to develop the fabrication route, due to the ease of identifying their faults. The effect on SIF device quality of interchanging the top and bottom electrodes was monitored. The relationship between the superconducting and normal state characteristics of SIS...

20. Control of Superconductivity in Cuprate/Manganite Heterostructures - Pang, Brian SiewHan
Research has shown that the spin alignment in an adjacent ferromagnet is capable of suppressing superconductivity. In this project, devices incorporating cuprate/manganite heterostuctres were successfully fabricated to study the effects of spin transport on the high temperature superconductor, YBCO. Deposition of such oxide ferromagnet/superconductor(F/S) multilayers using the ‘eclipse’ pulsed laser deposition(PLD) technique was also examined. Reproducible multilayers with ultrathin repeats were deposited, which exhibited superconducting and magnetic properties to minimum thicknesses of 3nm for both YBCO and LSMO. Using spin injection, via a ferromagnet, to create a spin imbalance in the superconductor, a suppression of superconducting critical current was observed with increasing injection current. However, the exact cause of...

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