Keepered media refers to traditional magnetic media with the addition of a soft magnetic "keeper" layer adjacent to the recording layer. Numerous recent reports have shown performance improvements associated with keepered media read with inductive heads. This report examines performance changes associated with keepered media read with magneto-resistive heads. This report presents a simple model for MR head response based on imaging, and experimental data in qualitative agreement with this model. Signal and noise spectra for keepered media are also discussed at length, and an optimized keepered media is shown to screen low spatial frequencies, narrowing isolated read pulses. Lastly, substantial noise increases at low spatial frequencies for keepered media are shown, and a possible cause is briefly discussed.

Propagation of single-cycle terahertz (THz) pulses through a random medium leads to dramatic amplitude and phase variations of the electric field because of multiple scattering. We present the first set of experiments that investigate the propagation of THz pulses through scattering media. The scattering of short pulses is a relevant subject to many communities in science and engineering, because the properties of multiply scattered or diffuse waves provide insights into the characteristics of the random medium. For example, the depolarization of diffuse waves has been used to form images of objects embedded in inhomogeneous media. Most of the previous scattering experiments have used narrowband optical radiation where measurements are limited to time averaged intensities or autocorrelation quantities, which contain no phase information of the pulses. In the experiments presented here, a terahertz time-domain spectrometer (THz-TDS) is used. A THz-TDS propagates single-cycle sub-picosecond pulses with bandwidths of over 1 THz into free space. The THz-TDS is a unique tool to study such phenomena, because it provides access to both the intensity and phase of those pulses through direct measurement of the temporal electric field. Because of the broad bandwidth and linear phase of the pulses, it is possible to simultaneously study Rayleigh scattering and the short wavelength limit in a single measurement. We study the diffusion of broadband single-cycle THz pulses by propagating the pulses through a highly scattering medium. Using the THz-TDS, time-domain measurements provide information on the statistics of both the amplitude and phase of the diffusive waves. We develop a theoretical description, suitable for broadband radiation, which accurately describes the experimental results. We measure the time evolution of the degree of polarization, and directly correlate it with the single-scattering regime in the time domain. Measurements of the evolution of the temporal phase of the radiation demonstrate that the average spectral content depends on the state of polarization. In the case of broadband radiation, this effect distinguishes photons that have been scattered only a few times from those that are propagating diffusively.

Scanning Electron Microscopy with Polarization Analysis (SEMPA) provides a novel tool with which to image surface magnetic structure. In SEMPA, a tightly focussed electron beam is directed at the sample surface and the polarization of the ejected low-energy secondary electrons, which mirrors the local surface magnetization, is measured. A magnetic image is then built up by scanning the incident electron beam point-by-point over the sample surface. Here, a new SEMPA instrument is described that makes use of a high-efficiency retarding-potential Mott polarimeter. The incident electron beam is provided by an ISI-SX40 SEM column. Low-energy secondary electrons ejected from the sample surface are collected and transported to the Mott polarimeter by a series of electrostatic lenses. A Wein spin rotator is also included in the electron transport optics to allow measurement of the full vector polarization of the ejected electrons, and thus the surface vector magnetization. Tests show that the performance of the present SEMPA instrument is superior to that of earlier designs. The apparatus has been used to image the domain structure on the surface of an Fe 3% Si sample, and to image tracks recorded in-plane on a cobalt alloy medium.

This dissertation presents one of the most comprehensive computer simulations to date of a geomagnetic storm. During the geomagnetic storm of June 4--5, 1991, five spacecraft took measurements of conditions within the inner magnetosphere, including measurements of the electric and magnetic fields and the particle distribution at several different locations. These data are used to test the theoretical understanding of magnetospheric physics by comparisons to the Rice Convection Model (RCM). The RCM is a first-principles model of the inner magnetosphere that calculates the movement of magnetospheric particles, the currents into and out of the magnetosphere, and the magnetospheric electric field patterns. Furthermore, these comparisons provide answers to some of the pressing questions in magnetospheric physics: How strong does the shielding electric field become during a magnetic storm? Can a self-consistently calculated electric field inject plasma sheet particles to within 3 Earth radii of the Earth? How much of the increase in the ring current is due to the injection of plasma sheet particles and how much is due to the energization of pre-existing particles? The comparisons between the RCM electric field results and the corresponding measurements show very good agreement between the observations and the models, and indicate that a fairly strong electric field develops during the storm. Furthermore, comparisons of RCM calculated particle fluxes and the observations indicate that a self-consistently calculated electric field is able to inject plasma sheet particles deep into the inner magnetosphere, and that this injection is the dominant cause of the increased ring current.

This doctoral thesis documents my research on novel devices and systems for terahertz (THz) spectroscopy and imaging. The research is particularly focused on the manipulation of THz radiation, including subwavelength concentration and low-loss wave guiding. One of the major obstacles for THz imaging is the poor spatial resolution due to the diffraction of the long-wavelength light source. To break this restriction, we build a THz near-field microscopy system by combining apertureless near-field scanning optical microscopy (ANSOM) with terahertz time-domain spectroscopy (THz-TDS). The experimental result indicates a sub-wavelength spatial resolution of about 10 micron. Abnormal frequency response of the ANSOM probe tip is observed, and a dipole antenna model is developed to explain the bandwidth reduction of the detected THz pulses. We also observe and characterize the THz wave propagation on the near-field probe in ANSOM. These studies not only demonstrate the feasibility of ANSOM in the THz frequency range, but also provide fundamental insights into the near-field microscopy in general, such as the broadband compatibility, the propagation effects and the antenna effects. Motivated by our study of the propagation effects in THz ANSOM, we characterize the guided mode of THz pulses on a bare metal wire by directly measuring the spatial profile of electric field of the mode, and find that the wire structure can be used to guide THz waves with outstanding performance. This new broadband THz waveguide exhibits very small dispersion, extremely low attenuation and remarkable structural simplicity. These features make it especially suitable for use in THz sensing and imaging systems. The first THz endoscope is demonstrated based on metal wire waveguides. To improve the input coupling efficiency of such waveguides, we develop a photoconductive antenna with radial symmetry which can generate radially polarized THz radiation matching the waveguide mode. Through THz-TDS measurements and theoretical calculations, we study the dispersion relation of the surface waves on metal wires, which indicates the increasing importance of skin effects for surface waves in the THz frequency range.

Bakalauro darbą „ Fizikos eksperimentas ir jo loginė analizė vidurinės mokyklos elektros ir magnetizmo kurse“ sudaro įvadas, 3 skyriai, išvados ir 20 literatūros šaltiniai. Darbo apimtis 57 puslapiai. Darbe nagrinėjami XI - XII klasės fizikos demonstraciniai bandymai, jų eiga, pateikiamos samprotavimų schemos, logiškai įprasminančios bandymo esmę. Pirmajame skyriuje yra apžvelgiama pedagogikos ir psichologijos literatūros šaltinių analizė. Antrajame skyriuje yra aprašomi XI - XII klasės fizikos kurso mokomieji bandymai ir jų loginės samprotavimo schemos iš: elektrostatikos (5 eksperimentai), nuolatinės elektros srovės (6 eksperimentai), magnetinio lauko (3 eksperimentai), elektromagnetinės indukcijos (2 eksperimentai). Trečiajame skyriuje yra aprašomas demonstracinių eksperimentų ir jų loginių schemų panaudojimas praktikoje. Pateikiami 2 eksperimentai, kurie buvo atlikti praktikos metu. Jie yra iš temos – elektros srovė įvairiose terpėse. Loginės schemos padeda mokiniams suprasti demonstracinio eksperimento esmę ir lengviau įsiminti naujai dėstomą medžiagą. / Bachelor's thesis ,,Experiment of physics and it's logical analysis in the course of electricity and magnetism of secondary school" contains an introduction, 3 chapters, a conclusion and 20 literature resources. It's volume is 57 pages. The educational experiments of physics of XI-XII grade are examined, their logical schemes and their explanations are presented. Analysis of pedagogy and psychology is reviewed in the first chapter. Educational experiments and logical schemes of XI-XII grade are being reviewed in the second chapter as follows: electrostatics (5 experiments), direct current (6 experiments), magnetic field (3 experiments), electromagnetic induction (2 experiments). Demonstrational experiments and their logical schemes application is reviewed in the third chapter. Two experiments are presented, that were carried out during practice. They are from the course of electrical current in different mediums.Logical schemes help the students to better understand the demonstrational experiment and to remember the newly taught materia.

STACEE (Solar Tower Atmospheric Cerenkov Effect Experiment) is a ground-based gamma-ray telescope. Using a large array of mirrors from a solar heliostat facility to capture Cerenkov light from gamma-ray showers in the atmosphere, STACEE plans to lower the threshold available to ground-based gamma-ray telescopes. Determining the threshold of STACEE is reliant upon understanding the components of the telescope. The photomultiplier tubes used to convert the Cerenkov light to electronic signals need to be carefully characterized as a part of this process. We describe a method to determine the gain of the photomultiplier tubes using an uncalibrated light source. Using a statistical model, the output distribution of a photomultiplier tube with a fixed-amplitude, pulsed light source can be used to determine the gain.

<p> The use of magnetic nanoparticles (mNPs) to induce local hyperthermia has been emerging in recent years as a promising cancer therapy, in both a stand-alone and combination treatment setting, including surgery radiation and chemotherapy. The mNP solution can be injected either directly into the tumor, or administered intravenously. Studies have shown that some cancer cells associate with, internalize, and aggregate mNPs more preferentially than normal cells, with and without antibody targeting. Once the mNPs are delivered inside the cells, a low frequency (30-300kHz) alternating electromagnetic field is used to activate the mNPs. The nanoparticles absorb the applied field and provide localized heat generation at nano-micron scales. </p><p> Treatment planning models have been shown to improve treatment efficacy in radiation therapy by limiting normal tissue damage while maximizing dose to the tumor. To date, there does not exist a clinical treatment planning model for magnetic nanoparticle hyperthermia which is robust, validated, and commercially available. The focus of this research is on the development and experimental validation of a treatment planning model, consisting of a coupled electromagnetic and thermal model that predicts dynamic thermal distributions during treatment. </p><p> When allowed to incubate, the mNPs are often sequestered by cancer cells and packed into endosomes. The proximity of the mNPs has a strong influence on their ability to heat due to interparticle magnetic interaction effects. A model of mNP heating which takes into account the effects of magnetic interaction was developed, and validated against experimental data. An animal study in mice was conducted to determine the effects of mNP solution injection duration and PEGylation on macroscale mNP distribution within the tumor, in order to further inform the treatment planning model and future experimental technique. In clinical applications, a critical limiting factor for the maximum applied field is the heating caused by eddy currents, which are induced in the noncancerous tissue. Phantom studies were conducted to validate the ability of the model to accurately predict eddy current heating in the case of zero blood perfusion, and preliminary data was collected to show the validity of the model in live mice to incorporate blood perfusion.</p>

Order-by-disorder is the phenomenon of the selection of a long-ranged ordered state by fluctuations in a many-body system. This mechanism, at first sight, seems paradoxical, since fluctuations (disorder) intuitively tend to suppress the order. However, when ObD happens, disorder works in favour of a particular ordered phase. Order-by-disorder can happen where an accidental degeneracy occurs in classical or mean field treatment of a system. These degeneracies, which are not due to exact symmetries of the system, can then be lifted by quantum or thermal fluctuations. The ObD phenomenon is ubiquitous in condensed matter systems with competing or frustrated interactions. Traditionally, the ObD is studied at $T = 0^+$ where the ground state of the system can be selected by quantum fluctuations. The study of ObD at temperature regimes near criticality, $T \lesssim Tc$ where transition happens from the paramagnetic phase to an ordered phase, however, have not received as much attention. In this thesis, we study the ObD phenomenon in three dimensional frustrated sys- tems close to criticality. We consider 3-component classical Heisenberg spins on pyrochlore lattice and FCC lattice. In the former, the spins interact via a Hamiltonian that can include the most general nearest-neighbour symmetry allowed bilinear interactions, long- range magnetostatic dipole-dipole interaction and second and/or third nearest-neighbour exchange interactions. However, in the latter, the Hamiltonian only consists of long-range magnetostatic dipole-dipole interactions. These two systems, correspond to insulating rare- earth pyrochlore oxides and rare-earth FCC salts. The mean field treatment shows, that accidental $O(4)$ and $U(1)$ symmetries emerge in two different regions of the parameter space of the Hamiltonian of pyrochlore system. While in the FCC system, an accidental $O(3)$ symmetry emerges at the mean field level. We show that fluctuations break these symmetries by respectively introducing cubic (in 4-vector and 3-vector models) and hexag- onal anisotropies to the free-energy of the system. To study these system beyond mean field approximation, we use Monte Carlo simulations, spin wave theory and we develop the E-TAP method which is an extended version of the method originally proposed by Thouless, Anderson and Palmer to study fluctuations in spin glasses.

<p> Locating hydrocarbon reservoirs has become more challenging with smaller, deeper or shallower targets in complicated environments. Controlled-source electromagnetics (CSEM), is a geophysical electromagnetic method used to detect and derisk hydrocarbon reservoirs in marine settings, but it is limited by the size of the target, low-spatial resolution, and depth of the reservoir. To reduce the impact of complicated settings and improve the detecting capabilities of CSEM, I apply synthetic aperture to CSEM responses, which virtually increases the length and width of the CSEM source by combining the responses from multiple individual sources. Applying a weight to each source steers or focuses the synthetic aperture source array in the inline and crossline directions. To evaluate the benefits of a 2D source distribution, I test steered synthetic aperture on 3D diffusive fields and view the changes with a new visualization technique. Then I apply 2D steered synthetic aperture to 3D noisy synthetic CSEM fields, which increases the detectability of the reservoir significantly. With more general weighting, I develop an optimization method to find the optimal weights for synthetic aperture arrays that adapts to the information in the CSEM data. The application of optimally weighted synthetic aperture to noisy, simulated electromagnetic fields reduces the presence of noise, increases detectability, and better defines the lateral extent of the target. I then modify the optimization method to include a term that minimizes the variance of random, independent noise. With the application of the modified optimization method, the weighted synthetic aperture responses amplifies the anomaly from the reservoir, lowers the noise floor, and reduces noise streaks in noisy CSEM responses from sources offset kilometers from the receivers. Even with changes to the location of the reservoir and perturbations to the physical properties, synthetic aperture is still able to highlight targets correctly, which allows use of the method in locations where the subsurface models are built from only estimates. In addition to the technical work in this thesis, I explore the interface between science, government, and society by examining the controversy over hydraulic fracturing and by suggesting a process to aid the debate and possibly other future controversies.</p>