Hall probe magnetometry measurements have been used to investigate the magnetic state of various Pb microcrystals grown by electrodeposition, where careful control of deposition parameters has allowed the tuning of their architectures. The internal flux distribution when in the superconducting state is seen to be highly dependent on the size and geometry of the investigated samples. In nanowires single flux vortex states are seen to form, showing typical type II behaviour. As the diameter of the nanowires increases these vortex states are seen to bunch together, behaviour that begins to approach that of a type I superconductor. Measurements of Pb triangles with sloped sides show the formation of giant vortex states typical of type I behaviour. These results also highlight the importance of geometry on surface barriers and the corresponding effect these have on flux distribution. Investigation of an icosahedron shows how symmetry effects can determine the stability of different flux states. Experimental results are in good agreement with Ginzburg-Landau calculations.

The detailed observational analysis of three massive B- and O-type stars was carried out in this study in order to characterize their fundamental, magnetic, and variability properties. The bulk of the data acquired were obtained with the high-resolution ESPaDOnS spectropolarimeter at the Canada-France-Hawaii telescope, within the context of the Magnetism in Massive Stars (MiMeS) project. Two of these stars (HR5907 and HD57682) are newly detected magnetic stars, discovered from observations acquired as part of the broader survey component of the MiMeS program, while the last star, ω Ori, was previously reported as magnetic in the literature. The rotation periods of HR5907 and HD57682 were inferred from photometric, Hα emission and longitudinal field variations. A period of 0.508276 was inferred for HR5907, making this the shortest period, non-degenerate, magnetic massive star identified to date. Furthermore, ultraviolet and optical spectroscopy were combined to infer the fundamental properties of HR5907 and HD57682. Direct modelling of the Least-Squares Deconvolved line profiles and the longitudinal magnetic field measurements were used to infer the magnetic properties of HR5907, HD57682, and ω Ori. A detailed investigation of the newly obtained and archival polarimetric data of ω Ori revealed no convincing evidence for a magnetic field, despite evidence of variability in some emission quantities in this dataset, which had been previously attributed to a large-scale magnetic field. The strength and variability of the optical photospheric helium lines of HR5907 suggests that this star is He-rich, with a non-uniform distribution of its surface chemistry. Lastly, the emission variations in the hydrogen lines of HR5907 suggest this star hosts a highly-structured, rigidly-rotating, centrifugally supported magnetosphere. Similarly, line profile variations throughout the optical spectrum of HD57682 are attributed to emission variations caused by a rotationally-modulated, dynamical magnetosphere. As magnetism in massive stars is a relatively rare and poorly-studied phenomenon, these studies comprise the bulk of the detailed investigations of magnetic massive stars carried out to date. The results of these investigations are also discussed in the context of addressing the current outstanding issues related to magnetism in massive stars. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2012-09-28 17:22:35.115

Thesis (Ph. D.)--University of Texas at El Paso, 2008. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

The magnetic and physical properties of oxide-free, ligand passivated, iron nanoparticles were studied using superconducting quantum interference device (SQUID) magnetometry and synchrotron based X-ray radiation. Particles used for this study ranged in diameter between 2 and 10 nm, which made it possible to distinguish between bulk and surface effects in the nanoparticles’ properties. Additionally, the effects of two different weakly interacting ligands (2,4-pentanedione and hexaethylene glycol monododecylether) on the nanoparticles’ behavior were studied. The results of this study were compared to theoretical predictions of magnetic transition metal behavior in both thin films and nanoparticles, as well as experimental results from measurements of transition metal clusters formed in an inert carrier gas and measured with a Stern-Gerlach magnet. Magnetometry revealed that the iron nanoparticles have a magnetocrystalline anisotropy an order of magnitude greater than bulk iron. At the same time, these particles exhibit a saturation mass magnetization up to 209 Am2/kg, which is only slightly lower than bulk iron. The structural properties of these particles were characterized using high energy X-ray diffraction analyzed using the atomic pair distribution function method (PDF). The PDF analysis indicates that the Fe particles have a distorted and expanded form of the bcc lattice, which could, at least in part, explain the magnetocrystalline anisotropy of these nanoparticles. X-ray absorption fine structure (XAFS) was used to study the surface properties of the iron nanoparticles and further characterize their structural properties. XAFS showed that oxidized species of iron exist at the nanoparticles’ surface and can be attributed to iron/ligand interactions. The percentage of oxidized species scales with the surface to volume ratio of the nanoparticles, and therefore appears limited to the nanoparticle surface. The layer of Fe(II) species present at the nanoparticles’ surface accounts for the reduction in saturation mass magnetization values (when compared to bulk iron) observed in these particles. XAFS analysis also provided further confirmation of the nanoparticles’ expanded crystalline lattice. / text

Thesis (M.A.)--Portland State University, 2002. / Computer-produced typeface. Includes bibliographical references (leaves 119-123).

Oriented archaeomagnetic samples were collected from 158 in situ features at 33 sites in the southwestern U.S. Seventy-three independently dated features were used for analysis of secular variation. A moving-window smoothing technique with outlier rejection was developed to objectively compute a virtual geomagnetic pole (VGP) path. Weighted Fisher statistics, accounting for errors in both dating and archaeomagnetic direction, were used to compute a mean VGP and standard error (A95) within each window of time. The VGP moves from 86°N, 83°E at A.D. 750 to 74°N, 192°E at A.D. 1075, and then to 85°N, 236°E at A.D. 1425. Secular variation of the VGP ranges from .00423-.350°/yr with a median of .0359°/yr. The A95s for the VGP curve range from 1.33-5.28°. Archaeomagnetic declinations and inclinations in Tucson range from 346-359°E and 47-60°. The close agreement with a similarly constructed VGP curve for Arkansas implies a small nondipole field in North America between A.D. 1150-1450. The VGP curve is significantly different from that of R. L. DuBois. Three case histories of archaeomagnetic dating suggest the validity of the new curve. The Thellier-Thellier paleointensity experiment was performed on 187 specimens from 77 potsherds. A significant magnetic fabric anisotropy, typically 30%, was found. Using an easy-plane model of magnetization, the anisotropy correction systematically increased the paleointensities by 5%. Paleointensities were interpreted for 127 samples from 54 sherds. All ceramics were independently dated and used along with other results to construct a virtual axial dipole moment (VADM) curve. A moving-window smoother using weighted statistics within each interval of time and incorporating outlier analysis was used. The Southwestern VADM at 300 B.C. is 14 x 10('22)Am('2), decreases to 8.6 x 10²²Am² at A.D. 800, and increases to 12.9 x 10²²Am² at A.D. 1500. Paleointensities from Hohokam ceramics compare favorably with paleointensities from other Southwestern ceramics, regardless of whether the long Hohokam chronology beginning at 300 B.C. or the short chronology beginning at A.D. 500 are used. There is also reasonable agreement between the Southwest and Mesoamerican VADM records.

No description available.

The existence of the matter in the universe is still an unsolved puzzle. After the Big Bang, both matter and antimatter should have been created in equal amounts, and subsequently annihilated. The leading theories to explain the existence of matter require an imbalance in the production of matter and antimatter in the early universe. This in turn requires CP violation, an asymmetry of the laws of physics between matter and antimatter. cryoEDM is designed to explore the total amount of CP violation and resolve this issue. cryoEDM is a next-generation neutron electric dipole moment search in a commissioning phase of development at the Institut Laue-Langevin, Grenoble. A critical requirement of EDM searches is knowledge of the magnetic environment. This work is concerned with the development, implementation, and performance of the currently operating magnetometry system based on SQUID magnetometers. An analysis scheme to provide magnetometry data over the volume occupied by the neutrons, from measurements using the available magnetometers, is developed. An updated method to calibrate the magnetometers using internal sources of magnetic fields is presented, and found to give good agreement with independent measurements. A new method of calibration using the neutrons as a reference is discussed, and tests on an example arrangement are shown to be promising. Algorithms for detecting and correcting for hardware induced artefacts in the data are produced, and demonstrated to reconstruct the field with good agreement in all but the noisiest environments. A software framework is developed to combine these into a real-time analysis that provides feedback and diagnostics to the experiment. Using this new system the resolution of the magnetometers installed in cryoEDM is found to be limited by the environmental noise, and would give a false EDM signal that is greater than the statistical uncertainty in neutron counting. However, the resolution has been somewhat artificially limited to reduce the susceptibility to the RF interference present. This still allows the magnetometry to act as a useful diagnostic tool on any issues in the current magnetic environment, even if in a sub-optimal configuration. For example, investigation of the magnetic shielding of the experiment finds a reduction in the shielding relative to the design, a situation which is being addressed with the design of additional shielding. Once this shielding is installed the resolution of the magnetometers will improve as well as the slew rate of the SQUIDs, which is found to be lower than the $47,mu extup{Ts}^{-1}$ required to measure AC fields applied during a measurement. The current system can also determine sources of magnetic perturbations created within the experiment, which will require addressing before a full EDM run can be performed. For example, cryogenic effects are observed to occur approximately hourly causing large shifts in the magnetic field. Also operation of valves controlling the flow of neutrons around the experiment are found to produce both AC magnetic fields from the driving motors, and shifts in the field from their movement. Situations which can be resolved by reexamination of installation and operational procedures.

Among several hundred impurities and defects that can occur in diamond, the nitrogenvacancy (NV) center is one of the most interesting for quantum technologies at room temperature. Several properties make it an excellent platform for many applications, from nanosensor to quantum information processing. This thesis presents the first explorations of NV-based technologies in our laboratory, investigating three sets of studies aimed at learning the know-how of this field of research and developing the necessary infrastructure to explore them in future quantum technologies. The first set of studies focus on magnetometry and how to improve NV-based magnetic sensing. We show that, using engineered ensembles of NV centers in ultrapure synthetic diamonds, one can build a relatively simple apparatus to do magnetic imaging of relatively large areas while determining the full vector field with high spatial resolution and very good sensitivity. We also show that these measurements can be used to reconstruct the current density distribution of nearby sources, opening exciting possibilities to study two-dimensional materials. Another set of studies involve spin coherence of an ensemble of NV centers. For that, we developed a method based on a CCD camera and an imaging protocol that allows implementing pulse sequences like Rabi, Ramsey and Hahn echo, performing Electron Spin Resonance (ESR) spectroscopy over extended areas. Using this method we extract parameters from our sample, including measurements for T1, T2 and T*2. The method was also used to observe Electron Spin Echo Envelope Modulation (ESEEM), due to hyperfine interaction with nearby 15N nucleus, resulting in improved frequency sensitivity. The third set of studies explore how to use femtosecond lasers to produce NV centers in diamond and investigated the nonlinear index of refraction (n2) of the diamond (type IIa) in a broad spectral region, from the infrared (1500 nm) to the ultraviolet (260 nm). / Entre várias centenas de impurezas e defeitos que podem ocorrer no diamante, o centro Nitrogênio-Vacância (NV) é um dos mais interessantes para tecnologias quânticas em temperatura ambiente. Diversas propriedades fazem dele uma excelente plataforma para muitas aplicações, desde nanosensores até o processamento de informações quânticas. Esta tese apresenta a primeira exploração de tecnologias baseadas em NV no nosso laboratório, investigando três conjuntos de estudos com objetivo de aprender o know-how desta área de pesquisa e desenvolver a infra-estrutura necessária para explorá-los em futuras tecnologias quânticas. O primeiro conjunto de estudos enfoca a magnetometria e como melhorar o sensores magnético baseado em centros NV. Mostramos que um aparato relativamente simples pode ser usado para produzir imagens magnéticas do campo vetorial, usando ensembles de centros NV em diamantes sintéticos ultrapuros. Também mostramos que essas medidas podem ser usadas para reconstruir a distribuição de densidade de corrente de fontes próximas, abrindo possibilidades interessantes para o estudo de materiais bidimensionais. Outro conjunto de estudos envolve a coerência de spin de um ensemble de centros NV. Para isso, desenvolvemos um método baseado em uma câmera CCD e um protocolo de imagem que permite implementar sequências de pulsos como Rabi, Ramsey e eco Hahn, realizando espectroscopia de ressonância de spin eletrônico (ESR) sobre áreas estendidas. Usando esse método, extraímos parâmetros de nossa amostra, incluindo medidas para T1, T2 e T*2. O método também foi usado para observar o efeito ESEEM (Envelope de Modulação de Eco de Spin Eletrônico), devido à interação hiperfina com o núcleo próximo de 15N, resultando numa melhor sensibilidade de freqüência. Finalmente, o terceiro conjunto de estudos explorou como usar lasers de femtossegundos para produzir centros NV em diamante e investigou o índice de refração não-linear (n2) do diamante (tipo IIa) em uma ampla região espectral, desde o infravermelho (1500 nm) até o ultravioleta (260 nm).

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.