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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.
81

Hall resistivity and torque magnetometry studies of the ferromagnetic superconductors UGe2 and URhGe

Lithgow, Calum Thomas January 2016 (has links)
Ferromagnetism (FM) and superconductivity (SC) are traditionally thought of as competing states of matter, since the opposite-spin electron pairing mechanism required for conventional SC is rendered impossible by FM spin alignment. However, recently discovered heavy-fermion compounds UGe2 and URhGe are examples where SC and FM are cooperative, and rather than antagonistic the presence of FM is actually necessary for the occurrence of the SC phase. A cooperative state of FM and SC is a topic of interest because it presents a possible solution to one of the two main problems with present superconductors: technology inhibiting limits on the highest temperature and highest magnetic field to which the SC phase can exist. Although both UGe2 and URhGe cease to be superconductors before even reaching 1 K, unlike the various `high temperature' superconductors currently known that easily surpass 100 K, it is their magnetic properties that are interesting, the inherent FM ordering allowing them to exceed conventional limits on the maximum magnetic field that SC can withstand. For example, URhGe remains superconducting above 35 T and the upper limit is so high that it is still experimentally undetermined. How exactly the FM SC phase arises in these compounds is as yet unknown. The necessary opposite-spin pairing mechanism is theoretically provided by magnetic fluctuations in an easily polarizable system right on the edge of a magnetic phase transition, and indeed SC emerges in UGe2 and URhGe around a first-order quantum critical point (QCP) where the temperature of the transition to an FM phase is reduced to absolute zero, by application of pressure in the case of UGe2 and by application of a magnetic field for URhGe. The aim of the research detailed in this thesis is to probe the FM phase transition and the associated QCP related to the emergence of SC in these compounds, to gather more information about the precise nature of the phases either side of the transition and exactly what changes occur in the system crossing the QCP. Specifically, the main objective is to characterise the magnetic fluctuations at the phase boundary and determine whether, by current FM SC theory, these fluctuations could be responsible for SC or if instead other, modified, unconventional theories are required to explain the unconventional electron pairing. The probes of choice for this PhD were Hall effect and magnetoresistance measurements of UGe2, and capacitive torque magnetometry and simultaneous magnetoresistance measurements of URhGe. The main result of the UGe2 project is an observed order-of-magnitude change in the Hall coefficient crossing the FM transition as a function of temperature and a dramatic change, similar in magnitude but also accompanied by a sign reversal, crossing the QCP as a function of pressure. Furthermore, the sign reversal at the critical pressure persists up to roughly 12 K, far beyond the 7 K critical end point of the phase transition, suggesting that in fact three different phases converge at the QCP where fluctuations between them presumably lead to the emergence of SC. Further investigation of the Fermi surface, either by deeper analysis of the Hall effect results or by other experimental methods, will be required to complete the main objective and determine exactly what the differences are between these newly identified phases. The main result of the URhGe project is actually the successful development of the capacitive torque magnetometry technique itself and the proof of operation for simultaneous measurement of all the individual components of both the magnetization and differential susceptibility tensors in a high magnetic field, which is currently not possible by any other technique. Completing the main objective was hampered by the extremely high susceptibility components encountered in the vicinity of the QCP, which in itself could be considered evidence for the theoretical relationship between strong FM fluctuations and the emergence of SC in URhGe. A number of results incidental to the main aim of the URhGe project are also summarised in this thesis, including the characterisation of quantum oscillations frequencies not previously reported in scientific literature and a variety of subtle features in resistivity measurements, which could, in conjunction with evidence from the susceptibility measurements, suggest the presence of another superconducting state such as surface or domain wall SC.
82

Magnetic Field Dependent Electroluminescence and Charge Transport in Organic Semiconductors

Shao, Ming 01 August 2011 (has links)
It has been found that a small magnetic field (<300 mT) can substantial change the electroluminescence, photoluminescence, photocurrent, electrical injection current in nonmagnetic organic semiconductors. It is generally believed that these magnetic field effects (MFE) are related to the spin dependent processes in organic semiconductor. However, the origin of MFE is still not well understood. In this dissertation, we investigate the underlying mechanism for magnetic field effects on electroluminescence (MFEEL) and magnetoresistance (MR) and demonstrate the complete tuning of MFEEL and MR based on our theoretical understanding. We consider MFE arising from magnetic field sensitive intersystem crossing (ISC) and triplet charge reaction. Magnetic field can increase the singlet ratios through ISC, accounting for positive MFEEL. Magnetic field modulated ISC strongly depends on the electron-hole pair separation distance. MFE can be enhanced by increasing the electron hole pair distance through material mixing and interplaying the electric dipole-dipole interaction. Meanwhile, two possible mechanisms corresponding for negative MFEEL: triplet-triplet annihilation and triplet charge reaction are also discussed. The negative MFEEL is achieved through adjusting triplet density charge confinement and exciton/charge ratio, which indicates that triplet charge reaction is a dominate process accountable for negative MFEEL. Significant MR and MFEEL are observed in strong spin orbital coupling iridium complex based OLED device after introducing the non-magnetic insulating blocking PVA layer. A possible mechanism for this new interface induced MR and MFEEL is proposed based on magnetic field perturbed spin-spin interaction at short capture distance of inter-charge carriers. The comparative study of two strong spin orbital coupling materials Ir(ppy)3 and Ir(ppy)2(acac) with different electrical dipole moments indicate the electric dipole-dipole interaction can change MR and MFEEL from short distance capture based regime to long distance intersystem-crossing regime. At last, we demonstrate the fully tuning sign of magnetic field effect on the fluorescence (MFEFEL) and phosphorescence (MFEPEL) by using the ISC, energy transfer and spin-spin interaction. In addition, we demonstrate a giant MFEEL (400%) in electrochemical cells and attribute this giant MFEEL to Lorentz force driven ion transport and Lorentz force dependent diffusion layer thickness through convection.
83

The Enhancement of Magnetoresistance in La1-xAxMnO3(A=Sr,Ca) films

Ou, Min-Nan 27 July 2000 (has links)
Abstract It is know that the crystal structure and the magnetic ordering in La1-xAxMnO3(A=Ca,Sr,Ba) may disrupted by introducing various defect. This disorder weakens the Hund coupling and, thus, the double-exchange interaction between Mn3+ and Mn4+. Combining with John-Teller distortion enhanced by the defects, the magnetoresistance (MR) effect is enhanced. Up to date, the generated defects were mostly columnar defects or chemical substitution defects. In this study we generated different type of defect, point defects, and inrestigate its effect on MR effect. La0.7Sr0.3MnO3 and La0.7A0.3MnO3 films were deposit on LaAiO3 (001) and SrTiO3 (001) substrates by pulse laser deposition technique. Films were patterned with a standard photolithography. Point defects were introduced by irradiated high (1.7 and 3MeV) and low (10KeV) energy protons. We found that, the low dose sample exhibits both conductivity and the low field MR enhancement. Which were believed due to introduce acceptor level and the magnetic structure defects. When the dose was high, the structure defects leaded strong scattering effect that lowed the conductivity. The MR in high dose sample was also enhanced near by transition temperature.
84

The Study of Microstructure and Magnetoresistance of La0.67Ca0.33MnO3

Li, Hsiu-Chuan 01 July 2002 (has links)
Abstract Recent progress in oxide perovskite thin-film technology has led to the discovery of a large negative magnetoresistance in doped manganate perovskite thin films. These films may have potentials for magnetic random access memory (MRAM) and magnetic sensors. Therefore, the research of magnetoresistance has been attracted a lot of attentions. The magnetoresistance is directly related to the microstructure. In an application point of view, the ulta-thin film may be more appropriate compares with those utilizing with thicker films. In this paper, we report the detail results of electrical property of La0.67Ca0.33MnO3 (LCMO) films related with their microstructure. The La0.67Ca0.33MnO3 (LCMO) films were deposited on (001) STO substrate with RF sputtering technique. The working pressure was maintained at 100m torr and the growth temperature was kept at 750¢J. After growth the films was annealed at 850¢J for 1 hour in a 500 torr O2 annealing environment. The growth time was 3mins.¡B6mins. and 12mins. respectively. The crystal structure of LCMO films were characterized with X-ray diffraction (XRD). The surface morphology of LCMO films were observed by scanning electron microscope (SEM) and the interface of microstructure between LCMO films and STO substrate were investigated by transmission electron microscope (TEM). Finally the M-I transition temperature were evaluated with 4-point probe at the temperature range from 300K down to 77K. The results show that the LCMO films were amorphous when the growth time was in 3 mins. The microstructure of the film gradually became poly-crystal and had a (001) prefer orientation after the growth time increasing to 6 mins. The grain size of the 12 mins growth film was at 40-50 nm scale. The Curie temperature and magnetoresistance change of these films were increased as the degree of crystallization of these films became better.
85

Growth, characterization, and properties of Co/Re superlattices

Charlton, Timothy R. January 2001 (has links)
Thesis (Ph. D.)--West Virginia University, 2001. / Title from document title page. Document formatted into pages; contains x, 73 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 69-72).
86

Functionality Tuning in Vertically Aligned Nanocomposite Thin Films

Chen, Aiping 03 October 2013 (has links)
Vertically aligned nanocomposite (VAN) oxide thin films are unique nanostructures with two-phase self-assembled, heteroepitaxially grown on single-crystal substrates. Both phases tend to grow vertically and simultaneously on a given substrate with lattice matching in the system. The nanostructured thin film system could form different in-plane morphologies including nano-checkerboard, nanopillar in matrix and nanomaze structures. The VAN thin films with tunable vertical lattice strain and novel microstructures provide fascinating approaches to achieve enhanced functionalities. In this dissertation, the microstructure and vertical strain effect on low-field magnetoresistance (LFMR) have been investigated in heteroepitaxial La0.7Sr0.3MnO3 (LSMO):CeO2 and LSMO:ZnO VAN thin films with a vertical strain of 0.13 % and 0.5 %, respectively. We demonstrate that LFMR can be tuned by column width and vertical strain in these VAN systems, i.e., smaller column width and larger vertical strain could result in a larger LFMR in the vertical nanocomposite heteroepitaxial thin films. The physical mechanism of enhanced LFMR in LSMO-based VAN has been explored. Single-phase LSMO and LSMO-based VANs have been grown on different substrates with different secondary phase compositions. Substrate effect in single-phase LSMO films shows that LFMR tends to increase with grain misorientation factor because the cross-section of electron conduction paths reduces as grain misorientation factor increases. (LSMO)1-x:(ZnO)x VAN heteroepitaxial films without large angle grain boundary (GB) have been used to study the pure phase boundary (PB) effect on the LFMR. It shows that increased PBs tends to reduce the cross-section of the conducting path and thus favor the spin-dependent tunneling in nanomaze structures with ferromagnetic/insulating/ferromagnetic vertical sandwiches. Tilted aligned LSMO nanostructured films with artificial GBs have been designed to investigate pure GBs influence on LFMR. The results indicate that decoupling of neighboring ferromagnetic (FM) domains by artificial GBs is necessary to achieve enhanced LFMR properties; and the strength of the GBs can be controlled by post-annealing to tune the LFMR effect. The VAN heteroepitaxial films display excellent microstructure compatibility and strain tuning. Perovskite oxides can be combined with many other oxide materials to form VAN architectures. The microstructure and lattice strain in the unique heteroepitaxial VANs can be used to engineer and tune the existing/new functionalities.
87

Numerical investigation of micro-macro coupling in magneto-impedance sensors for weak field measurements

Eason, Kwaku 25 August 2008 (has links)
There is strong interest in the use of small low-cost highly sensitive magnetic field sensors for applications (such as small memory and biomedical devices) requiring weak field measurements. Among weak-field sensors, the magneto-impedance (MI) sensor has demonstrated an absolute resolution on the order of 10-11 T. The MI effect is a sensitive realignment of a periodic magnetization in response to an external magnetic field within small ferromagnetic structures. However, design of MI sensors has relied primarily on trial and error experimental methods along with decoupled models that separate the micromagnetic and classical electromagnetic equations describing the MI effect. To offer a basis for more cost-effective designs, this thesis research presentation begins with a general formulation describing MI sensors, which relaxes assumptions commonly made leading to decoupling. The coupled set of nonlinear equations is solved numerically using an efficient meshless method in a point collocation formulation. For the problem considered, the chosen method is shown to offer advantages over alternative methods including the finite element method. In the case of time, projection methods are used to stabilize the time discretization algorithm while quasi-Newton methods (nonlinear solver) are shown to be more computationally efficient, as well. Specifically, solutions for two MI sensor element geometries are presented, which were validated against published experimental data. While the examples illustrated here are for MI sensors, the approach presented can also be extended to other weak-field sensors like fluxgate and Hall effect sensors.
88

Studies on the synthesis, characterization and properties of colossal magnetoresistive (CMR) materials

Gao, Feng. January 2004 (has links)
Thesis (Ph.D.)--University of Wollongong, 2004. / Typescript. Includes bibliographical references.
89

A study of the magnetoresistance effect in Bi-2212 for the purposes of utilisation in magnetic field sensors

Winton, Brad. January 2005 (has links)
Thesis (M.Sc.)--University of Wollongong, 2005. / Typescript. Includes bibliographical references: leaf 146-148.
90

Magnetoresistive phenomena in nanoscale magnetic systems

Burton, John D. January 1900 (has links)
Thesis (Ph.D.)--University of Nebraska-Lincoln, 2008. / Title from title screen (site viewed Aug. 12, 2008). PDF text: vii, 123 p. : ill. ; 2 Mb. UMI publication number: AAT 3297588. Includes bibliographical references. Also available in microfilm and microfiche formats.

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