Theses - Device Materials Group

PhD theses and associated research data from the Device Materials Group.


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  • ItemOpen Access
    Fabrication and Characterization of Titanium-doped Hydroxyapatite Thin Films
    (2007-08) 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, 10, and 15W films with compositions of 0.7, 1.7 and 2.0 at.% titanium were achieved. As-deposited films, 1.2 μm thick, were amorphous but transformed into a crystalline film after heat-treatment at 700C. Raman spectra of the PO4 band suggests the titanium does not substitute for phosphorous. X-ray diffraction revealed the c lattice parameter increases with additional titanium content. XRD traces also showed titanium may be phase separating into TiO2, a result which is supported by analysis of the Oxygen 1s XPS spectrum. In-vitro observations show good adhesion and proliferation of human osteoblast (HOB) cells on the surface of HA-Ti coatings. Electron microscopy shows many processes (i.e. filopodia) extended from cells after day one in-vitro and a confluent, multi-layer of HOB cells after day three. These finding indicate that there may be potential for HA-Ti films as a novel implant coating to improve upon the bioactivity of existing coatings.
  • ItemOpen Access
    Deposition and characterisation of bismuth layer-structured ferroelectric films
    (2006-02) 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 (BNdT and BSmT) were successfully fabricated for PLD targets by solid state processing. Highly epitaxially (001)-, (118)-, and (104)-oriented Nd-doped bismuth titanate (BNdT) films were grown by PLD on (001)-, (011)-, and (111)-oriented SrTiO3 (STO) single crystal substrates, respectively. A three-dimensional orientation relationship between films and substrates was derived as: BNdT(001)//STO(001), BNdT[ 110 ]//STO[100]. Films showed strong dependence of structural and ferroelectric properties on the crystal orientation. PLD-grown BSmT films on platinised silicon substrates were studied as a function of fabrication temperature, effects of Pt bottom layer orientation, Sm doping level, and LaNiO3 buffer layer. An alkoxide-salt chemical solution deposition (CSD) method was adopted to prepare the precursors for BSmT (BNdT) film fabrication. Precursors of Bi-Sm(Nd)-Ti which were stable for at least eight months in air ambient were successfully developed. In-situ FT-IR studies suggest that acetic acid serves as chelating agent to improve the homogeneity of the precursor solution by generating a dense and homogeneous Ti-O-Ti polymeric network. The electrical properties of the films fabricated in this study (dielectric and ferroelectric properties, leakage current characteristics and electrical fatigue properties), are comparable or superior to these previously reported for similar films developed by other techniques or with other doping elements. Low temperature electrical properties of BSmT films suggest that the films are very promising for extremely low temperature nonvolatile memory applications. The results of BNdT films annealed at different oxygen partial pressure (O2, air, N2) showed that oxygen ambience affected structural properties of the films by enhancing the growth of perovskite phase (phase formation), increasing grain size (grain growth), and assisting the growth of (117)-oriented grains (crystallographic orientations). Piezoresponse force microscopy (PFM) was adopted to characterise BSmT films. Domain structures were clearly observed in a PLD-grown BSmT film, which were closely related to the grain structures. Domain manipulation was carried out in a CSD-derived BSmT film, showing that the film can be nearly uniformly polarised, which can be used in nanoscale device fabrication. Clear hysteresis loops were measured by PFM, which was an important proof of ferroelectricity. Large spatial variations of piezoelectric hysteresis loops of a CSD-derived BSmT film were observed across the film surface. Effective electrostriction coefficient (Qeff) of a PLD-grown BSmT film was measured, showing that BSmT films had better piezoelectric properties (higher Qeff, higher dzz) than SBT films, un-doped BiT ceramics and films. It suggests that BSmT films are promising piezoelectric materials for MEMS use.
  • ItemOpen Access
    Normal state properties of high-angle grain boundaries in Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-}$$_\delta$
    (2006-05) Mennema, Sibe
    This dissertation describes the investigation of the normal-state properties of high-angle grain boundaries in YBa$_{2}$Cu$_{3}$O$_{7-}$$_\delta$ (YBCO) and Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-}$$_\delta$ (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 distorted zone. The structure of these high-angle grain boundaries, and, hence, the mechanism for charge transport across the interface, is less well understood. The current – voltage behaviour of grain boundaries below the transition temperature of the YBCO has been investigated extensively, but less data is available of the resistive behaviour of the grain boundary in the normal state above the transition temperature. The doping of YBCO with calcium is known to decrease its transition temperature, but it can simultaneously improve the charge transport properties of grain boundaries in polycrystalline material. YBCO and calcium-doped YBCO thin films were fabricated on bicrystalline substrates. The grain boundaries had misorientation angles between 18° and 45°. The films were processed in order to obtain microscopic devices that made it possible to determine the resistance of the grain boundary below and above the transition temperature. A measurement system was used with which the voltage across the grain boundary can be measured as a function of applied current between 5 K and room temperature. A detailed model for charge transport by tunnelling across a grain boundary was used to interpret the results of the measurements of grain boundaries. An algorithm based on this model was formulated that made it possible to calculate a shape for the potential barrier at the grain boundary from the temperature dependence of its resistance. The microstructure of the grain boundary was investigated using Transmission Electron Microscopy. It was found that the grain boundary can show considerable deviations from a straight path in line with the substrate grain boundary, but there are also locations where such deviations are not observed. Extensive measurements showed that the resistance of the grain boundary decreases with increasing temperature above the transition temperature, and that resistance and the extent of resistance variation increases with misorientation angle. The resistance below the transition temperature was for certain misorientation angles observed to be independent of temperature at voltages sufficiently high to exclude the influence of the superconducting behaviour of YBCO. A reduction of the oxygen content of YBCO (higher value of $\delta$) increases the resistance and the temperature dependence of the resistance. The doping of YBCO with calcium decreases the resistance of the grain boundary below and above the transition temperature. The shape of the potential barrier at the grain boundary was calculated on the basis of most resistance – temperature measurements. The shape and size of potential barriers are used to explain the variation of the grain boundary resistance with misorientation angle, oxygen content and calcium doping percentage. The model shows little validity for grain boundaries with a higher misorientation angle and resistance, which indicates that charge transport across the grain boundary does not necessarily take place according the tunnelling with the assumptions made.
  • ItemOpen Access
    Growth and characterisation of Niobium/Gadolinium superconductor-ferromagnet nanocomposites
    (2006-08-01T14:37:18Z) 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 samples [3, 4] and in thin films [5-7] which result from a different type of electron-pairing mechanism which electrons with spin pointing in the same direction team up to form Cooper pairs with one unit of spin, resulting in the so-called triplet superconductivity. Apart from this so-called ferromagnetsuperconductors which both superconducting and ferromagnetism order parameters are present in a uniform material, hybrid systems [8] are made form materials with different or even mutually exclusive properties. Therefore the overall property can be strongly affected by the interaction between constituent materials. The present work, concerns such a hybrid system where Nb, a superconducting metal having transition temperature below 9.5K, is placed in contact with a ferromagnetic metal, Gd with bulk Curie temperature of around 290 K in a form of a nanocomposite. The mutual immiscibility of these two elements gives us the opportunity to take advantage of both the superconduction and ferromagnetism properties of the constituents and further study the transport and magnetic behavior of the system and their effects on each other specially on the critical current of the superconductor which is expected to be modified by the proximity of the ferromagnetic metal.
  • ItemOpen Access
    Focused Ion Beam Fabricated Non-equilibrium Superconducting Devices
    (2000-04) 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 reproducibility in junction behaviour (the variation of critical current is approximately 10%) is the best so far observed for this type of junction. The SNS junction fabrication method has been successfully extended for making high-density SNS junction arrays, dc-SQUIDs, and related devices. A simple model is devised to explain the normal-state resistance and critical current of a junction. The model is based on the geometry of a junction as defined by the FIB instrument and the film deposition. The model is mostly successful in qualitatively explaining many of the geometrical factors that affect the electrical properties of the junction. Nb/Cu/Nb junction series arrays, made using an FIB instrument, are also successfully fabricated. The yield of the junctions forming small arrays is found to be similar to the yield of single junctions. For the series arrays studied here, new observations have been made: the electrical properties of an array have been found to be dependent on the spacing of the junctions and the number of junctions in the array. This work also investigates the thermal properties of SNS and micron-scale superconductor/insulator/normal-metal junction based devices for use in bolometer device based applications. It is shown that self-heating raises the temperature of the junctions significantly above their operating temperatures. For a device sitting on a low thermally conductive membrane, it is found that the effects of heating, or cooling, in the junctions are exaggerated.
  • ItemOpen Access
    Active control of superconductivity by means of the ferromagnetic exchange interaction
    (2001-04) 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 reduction in the exchange interaction will increase the superconducting transition temperature (Tc) of the heterostructure. Thus by changing the magnetisation of the ferromagnetic regions it is possible to control the exchange interaction in the superconductor and so the superconducting properties of the heterostructure. I have measured the superconducting properties of niobium/cobalt bilayers as a function of the applied magnetic field. I have observed that one component of the superconducting properties is controlled by the net magnetisation rather than the magnitude of the applied field. I have been able to show that this component of the observed change in the superconducting properties is not simply due to stray magnetic field but that the superconductivity is being actively controlled by means of the exchange interaction. This is the first experimental evidence that the superconducting properties of a S/F heterostructure can be controlled in this way, which opens up the possibilities for the construction of future devices.
  • ItemOpen Access
    Critical Currents in YBaCuO Thin Films
    (1997-12) Herzog, Robert
  • ItemOpen Access
    Josephson Junctions Fabricated by Focussed Ion Beam
    (2002-10) 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 this affects the resulting device parameters. Devices fabricated over a wide range of cut depths and copper normal metal layer thicknesses (0-175 nm) have been thoroughly characterized at 4.2 K in terms of current-voltage (I-V) characteristics, magnetic field- and microwave-response. In selected cases I-V characteristics have been studied over the full temperature range from TC down to 300 mK. Devices with resistively-shunted (RSJ) I-V characteristics and ICRN products above 50 µV at 4.2 K have been fabricated reproducibly. This work has been complemented by Transmission Electron Microscopy (TEM) studies that have allowed the microstructure of the individual devices to be inspected and confirm the validity of the in situ resistance measurement. The individual junction devices are promising candidates for use in the next generation of Josephson voltage standards. In collaboration with Dr. Sam Benz at the National Institute of Standards and Technology (NIST) in the U.S., series arrays of junctions have been fabricated and characterized. Phase-locking behaviour has been observed in arrays of 10 junctions of spacings 0.2 to 1.6 µm between 4.2 K and TC in spite of the relatively large spread in individual critical currents. Strategies for minimizing junction parameter spread and producing large-scale arrays are discussed. The opportunities offered by the FIB for the creation of novel device structures has not been overlooked. By milling a circular trench in the Nb Cu bilayer a Corbino geometry SNS junction is created. In this unique device the junction barrier is enclosed in a superconducting loop, implying that magnetic flux can only enter the barrier as quantized vorticies. This gives rise to a startling magnetic field response – with the entry of a vortex the critical current is suppressed from its maximum value to zero. Experimental results and theoretical modeling are reported. Possible future applications of this novel device geometry (which may be of relevance to Quantum Computing and to studies of Berry’s phase effects) are considered.
  • ItemOpen Access
    Electromagnetic Modelling of Superconducting Sensor Designs
    (2003-08) Gerra, Guido
    The problem of design optimisation of thin film direct current Superconducting QUantum Interference Device (SQUID) magnetometers made of YBCO (YBa2Cu3O7-x) was considered. The inductances and effective areas were calculated using the software package 3D-MLSI. Resolution and reliability issues were first tested on simple superconducting systems, showing good agreement with analytical formulae and experimental results, and demonstrating that a remarkable precision can be obtained though at the expense of CPU time and memory. The software was then used to simulate a SQUID magnetometer fabricated in the Device Materials Group of the Department of Materials Science and Metallurgy, proving that 3D-MLSI can be used to predict the parameters of real systems with acceptable accuracy.
  • ItemOpen Access
    Development of (Re)BaCuO Coated Conductors by Liquid Phase Epitaxy
    (2002-01) 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 (100) SrTiO3 under carefully controlled growth temperature and undercooling. The film growth mode (c- or a,b-oriented) is determined by the growth rate, which is directly related to the level of RE supersaturation that could be controlled by the undercooling used along with the amount of total RE solubility in the solution. The LPE grown films were highly epitaxial and biaxially aligned with good in-plane and out-of-plane textures. YBCO thick films grown on NdGaO3 by LPE showed high Tc of ~92 K and zero-field Jc at 77 K of 2.5×105 A/cm2. The initial growth of YBCO was found to be a multi-nucleation process. However, above a critical film thickness, dislocations started to form as a lattice-misfit stress relieving mechanism that led to step formation and spiral growth around dislocation cores. The growth kinetics from an unstirred solution was found to obey a $t$ law, whereas the growth rate from a stirred solution (substrate rotation) was found to have two growth stages with initialtransient and steady-state regimes. The transient regime extended to ~180 s with $t growth kinetics. In the steady-state regime, diffusion across an established diffusion boundary layer led to a linear increase of film thickness with time. Detailed study of the film growth confirmed the presence of interface kinetics that limited the growth on a competitive basis with volume diffusion in the solution. Furthermore, film growth on seeded substrates was found to be dependent on the stability of the seed layers where the mechanism of the dissolution of seed films was investigated. The major obstacle towards continuous deposition of coated conductor in long lengths by LPE is the lack of non-vacuum techniques capable of producing long lengths of suitably buffered substrate or a closely-matched substrate where YBCO can be deposited directly. However, despite the lack of suitably buffered and/or seeded substrates in long lengths, continuous metallic tape processing had been tested at a preliminary level showing scalability of the process.
  • ItemOpen Access
    Josephson Junctions and Devices fabricated by Focused Electron Beam Irradiation
    (1997-12) 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 doses resulted in the formation of a purely resistive region. The electrical transport in such regions with a dimension of 200 nm in the direction of current transport is shown to be compatible with variable range hopping (VRH). Barriers with the same length but a finite superconducting transition temperature showed a low bias resistance that is significantly lowered due to proximity coupling. Using purely resistive regions in combination with Josephson junctions, devices consisting of two closely spaced Josephson junctions with a third terminal connected to the shared electrode were fabricated and characterised (minimum separation 20 nm). The distinct behaviour of the Josephson critical current with applied magnetic field (Ic(B)) of these devices was found to be well described by a newly developed model, which incorporates the effect of the static redistribution current in the shared electrode on the phase distribution of the Josephson junctions. An important finding is that the behaviour of the high critical current with applied magnetic field of two closely spaced junctions was found to be consistent with a model system consisting of a closely spaced Josephson junction and a resistive barrier. A three terminal device with Josephson junctions at small separations was found to have a significantly increased transresistance when compared with the individual resistance of the Josephson junctions it constituted of. A number of illustrative examples of device structures realised with the focused electron beam irradiation technique are also included.
  • ItemOpen Access
    Numerical modelling of current transfer in nonlinear anisotropic conductive media
    (1999-04) 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 current transfer behaviour has been the primary aim of this work. The range of parameters appropriate to macroscopic models of the Bi-2212 CRT system has been experimentally examined using a specifically constructed apparatus for the measurement of current transport characteristics. A study of the self-field properties of the Bi-2212 CRT material using a new experimental technique and mathematical analysis is presented and has allowed the importance of the self-field effect in the numerical model to be assessed. An essential requirement for the practical application of high current superconducting devices is the development of low resistance current contacts. The research presented examines this macroscopic current transfer problem and aims to explain experimentally observed current transfer characteristics at high applied currents. Existing models cannot explain these characteristics. Current transfer on the microscopic scale is also examined. Models of current transfer have been developed from descriptions of specific microstructures that are thought to characterise the microstructure of Bi-2223 and Bi-2212 silver-sheathed tapes. This thesis specifically presents modelling of current transfer between c-axis, low-angle c-axis and edge-on c-axis tilt oriented grain interfaces; the principal current transfer paths between individual current elements of the microstructural models of current flow in polycrystalline HTSs.
  • ItemOpen Access
    The Electronic Properties of Thin Film YBa2Cu3O7 Low Angle Grain Boundaries
    (2002-07) 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 and φ relative to the LAGB defect. It is found that for fields applied parallel to the LAGB defect plane, dissipation is dominated by the viscous flux flow of vortices along the boundary. Clear evidence for this is found in the V-I characteristic, which displays an increased linearity indicative of the viscous regime. It is shown that the number of vortex rows involved in the flow process can discontinuously switch, leading to a V-I characteristic made up of straight segments of different gradient. For fields applied away from the LAGB defect plane a kinked vortex structure develops and the boundary critical current density, JcGB, is determined by the channelling of vortex segments still lying within the LAGB. The channelling regime is seen in angular measurements as a marked decrease in JcGB(θ,φ) as field becomes aligned to within critical angles φK or θK of the LAGB. The aligned vortices experience a reduction in dimensionality that is manifest in a reduced temperature dependence of JcGB(T). For fields applied at a sufficiently large angle from the defect plane the deleterious effects of the LAGB on current transport are mitigated considerably. In this regime a collinear vortex structure is regained and transport is controlled by the intragranular (IG) sections of the LAGB track; JcGB = δJcIG where δ ~ 0.8, a result that is independent of field, temperature and angle for a 4.9° LAGB. For rotation of the applied field within the LAGB defect plane, the presence of a pinning peak in JcGB for field aligned to the dislocation array is confirmed and modelled. The peak is found to be absent in IG track measurements and increases relative to the intrinsic peak with both increasing field and temperature. In addition, a pronounced angular hysteresis is presented, which is directly linked to a corresponding ‘static’ hysteresis in JcGB(B) with field. Magneto-optic measurements confirm that this effect is controlled by the flux density profile in the IG regions of the LAGB track. Finally, above a temperature, angle and sample dependent merging field, B*, the LAGB is found to be effectively transparent, as δ ~ 1. This is due to the irreversibility line, above which dissipation occurs across the whole LAGB track.
  • ItemOpen Access
    A thermogravimetric study of oxygen diffusion in YBa2Cu3O7-d
    (1998-08) 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 effects of the microstructure and the different diffusion coefficients measured with the techniques used. An Arrhenius expression for the chemical diffusion of oxygen has been calculated from the analysis of isothermal oxygenation data, and it has been corroborated by a study of the nonisothermal experiments carried out. This work includes the development of a macroscopic model for oxygen diffusion in YBCO based on the diffusion coefficient calculated from experimental data. The model is used to simulate for the first time oxygenations under both isothermal and non-isothermal conditions. The study of non-isothermal oxygenations has led directly to the design of novel cooling procedures that can be introduced at the end of the processing stage of YBCO samples, producing highly oxygenated specimens in shorter times than for conventional isothermal and ramped oxygenation procedures. The final section of this dissertation presents a study of the Direct Current Zoning effect. The generation of a mobile hot zone in a polycrystalline YBCO bar when passing a current across it is directly linked to the diffusion of oxygen ions in the material. A mechanism for the motion of the zone along the sample has been suggested. A computer model has been developed to reproduce this process taking into account the motion of ions due to chemical diffusion and the potential difference established. The results from this model have corroborated the mechanism suggested and give for the first time the opportunity to study this phenomenon in more detail.
  • ItemOpen Access
    In situ magnetoresistance measurements during patterning of spin valve devices
    (2003-08) Morecroft, Deborah; Blamire, Mark; Blamire, Mark [0000-0002-3888-4476]
    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 Ni$_{80}$Fe$_{15}$Mo$_{5}$ 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 (Ni$_{80}$Fe$_{20}$/Cu/Ni$_{80}$Fe$_{20}$) 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 small spin valve response. These initial measurements were carried out with separate milling and electrical characterisation steps. However, it was decided that it would be ideal to design a technique to do in-situ magnetoresistance measurements during milling. This meant that the samples could be milled and characterised in the same step, leading to a much cleaner and more efficient process. In-situ magnetoresistance measurements were carried out during micropatterning of PSV devices, and the measurements showed the evolution in the electrical response as wire structures were gradually milled through the thickness. Contrary to what was expected, the structures showed a maximum spin valve response when fully milled through. The effect of further increasing the shape anisotropy by reducing the wire width, and changing the material properties in the PSV structure has also been investigated. MR measurements were taken as the temperature was increased from 291K to 493K, and the results show that the patterned PSV structures have a better thermal stability than exchange biased spin valves with an IrMn pinning layer. The experiment was extended to the nanoscale, and the results show that a significant increase in MR is not observed despite the fact that the magnetic configuration tends more towards single domain. This is thought to be due to an increase in the initial resistance of the structures. A small increase in MR was observed as the wire width was decreased from 730 to 470nm, although the spin valve response is heavily dependent on the gallium dosage density during patterning in the Focused Ion Beam (FIB). Micromagnetic simulations were carried out, which agree with the experimental results and showed the change in the magnetisation reversal from rotation to switching as the dimensions were reduced on the nanoscale.
  • ItemOpen Access
    Phase Coexistence in Manganites
    (2005-04) Chapman, James Christopher
    The doped perovskite manganite La1-xCaxMnO3 (0
  • ItemOpen Access
    The development of magnetic tunnel junction fabrication techniques
    (2002-07) 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 junctions was investigated. Criteria were formulated to identify devices in which tunneling is not the principal conduction mechanism in normal metal-insulator-normal metal junctions. Magnetic tunnel junctions (MTJs) were produced on the basis of the fabrication route developed with SIS and SIF devices. MTJs in which tunneling is the principal conduction mechanism do not necessarily demonstrate high MR, due to effects such as magnetic coupling between the electrodes and spin scattering. Transmission electron microscope images were used to study magnetic tunnel junction structure, revealing an amorphous barrier and crystalline electrodes. The decoration of pinholes and weak-links by copper electrodeposition was investigated. A new technique is presented to identify the number of copper deposits present in a thin insulating film. The effect of roughness, aluminium thickness and voltage on the number of pinholes and weak-links per unit area was studied. High frequency testing of read heads at wafer level was performed with a network analyser. Design implications for read head geometry were investigated, independent of magnetic performance. This technique has great potential to aid the rapid development of read and write heads whilst improving understanding of the system.
  • ItemOpen Access
    Control of Superconductivity in Cuprate/Manganite Heterostructures
    (2004-06) 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 this suppression could not be solely attributed to spin-induced nonequilibrium effects, as it proved difficult to eliminate the effects of localized heating, current summation and magnetic field. Interfacial studies of the device junction provided evidence of an alternative currnent path at the interface. The control of superconductivity was also examined using F/S proximity effects, which improves the understanding of how magnetic and superconducting materials coexist. We observed that oxide F/S samples deposited by high O2 sputtering [1] and ‘eclipse’ PLD were similar, and that Tc was clearly more suppressed in F/S compared to N(normal metal)/S systems. However, the magnetic moment and exchange coupling, two magnetic properties of significance in ferromagnets, did not, individually, have a major influence on the increased Tc suppression. The Curie temperatures of the multilayers were suppressed with increasing manganite thickness because of structural effects, and also with increasing thickness of the YBCO layer which reduced the coupling between manganite layers. To study the use of the spin-valve effect as a means to control high temperature superconductors, we fabricated an LSMO/YBCO/LC(0.3)MO pseudo spin-valve structure, which is equivalent to a superconductor sandwiched within a spin valve where both parallel and antiparallel configurations of the F layers can be achieved within a single magnetic field sweep. Previous research involving a metallic F/S/F/AF structure, showed that the superconductivity was suppressed when the ferromagnets were in the parallel configuration [2]. From the onset of superconductivity, when the normal metallic behaviour of YBCO switches to superconductivity, a magnetoresistance(MR) peak was observed when the F layers were antiparallel. The MR effect increased with decreasing bias current and temperature, characteristic of a pseudo-spin valve. The result is suggestive of spin transport across the YBCO spacer layer.
  • ItemOpen Access
    Growth and Characterization of Epitaxial Oxide Thin Films
    (2001-12) Garg, Ashish
    Epitaxial oxide thin films are used in many technologically important device applications. This work deals with the deposition and characterization of epitaxial WO3 and SrBi2Ta2O9 (SBT) thin films on single crystal oxide substrates. WO3 thin films were chosen as a subject of study because of recent findings of superconductivity at surfaces and twin boundaries in the bulk form of this oxide. Highly epitaxial thin films would be desirable in order to be able to create a device within a film without patterning it, by locally creating superconducting regions (e.g. twins) within an otherwise defect free film by reducing or doping the film with Na. Films were deposited by reactive magnetron sputtering at various temperatures on single crystal SrTiO3 (100) and R-sapphire substrates. X-ray diffraction studies showed that the optimised films were highly (001) oriented, quality of epitaxy improving with decreasing deposition temperature. AFM studies revealed columnar growth of these films. Films were heat treated with Na vapour in order to reduce or dope them with Na. Low temperature measurements of the reduced films did not show existence of any superconductivity. SBT is a ferroelectric oxide and its thin films are attractive candidates for non-volatile ferroelectric random access memory (FRAM) applications. High structural anisotropy leads to a high degree of anisotropy in its ferroelectric properties which makes it essential to study epitaxial SBT films of different orientations. In this study, SBT films of different orientations were deposited on different single crystal substrates by pulsed laser ablation. Highly epitaxial c-axis oriented and smooth SBT films were deposited on SrTiO3 (100) substrates. AFM studies revealed the growth of these films by 3-D Stranski-Krastanov mode. However, these films did not exhibit any ferroelectric activity. Highly epitaxial (116)-oriented films were deposited on SrTiO3 (110) substrates. These films were also very smooth with root mean square (RMS) roughness of 15-20 Å. Films deposited on TiO2 (110) were partially a-/b-axis oriented and showed the formation of c-axis oriented SBT and many impurities. Completely a-/b-axis oriented SBT films were deposited on LaSrAlO4 (110) substrates. Films deposited at non-optimal growth temperatures showed the formation of many impurities. Attempts were also made towards depositing Sr2RuO4 films on LaSrAlO4 (110) substrates, which can act as a bottom electrode for ferroelectric SBT films.
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