<p> Magnetic nanostructures have been subject of intense research as a result of their unique magnetic properties such as superparamagnetism, enhanced magnetic moment, and high magnetic density storage due to shape anisotropy. A highly reproducible and affordable method to fabricate cobalt nanostructures on silicon substrates was devised in this thesis using nanosphere lithography. The surface morphology and magnetic properties of the nanopatterned cobalt thin film were characterized using an optical microscope, scanning electron microscope, atomic force microscope, and a magneto optical Kerr effect (MOKE) magnetometer. Modification to the surface of cobalt thin film was found to extensively alter its magnetic coercivity. Continuous cobalt thin film at 10 nm thick has a coercivity of 102&plusmn;2 Oe, whereas nanostructured cobalt thin films at the same thickness have a coercivity of 167&plusmn;16 Oe. The magnetic coercivity increases by 65&plusmn;18 Oe for an array of close packed cobalt nanostructures using nanosphere templates with diameters ranging from 203 nm to 600 nm. The cobalt nanostructured sample using a 930 nm diameter nanosphere template has a coercivity comparable to a continuous cobalt thin film at 108&plusmn;7 Oe. In addition, these nanostructures exhibit unusual magnetic properties such as multistep behavior and pinching/crossing-over of the magnetization curves with regards to the MOKE signal. These features become more prominent as the diameter of the nanosphere template decreases.</p>

abstract: In this dissertation I studied the anomalous Hall effect in MgO/Permalloy/Nonmagnetic Metal(NM) based structure, spin polarized current in YIG/Pt based thin films and the origin of the perpendicular magnetic anisotropy(PMA) in the Ru/Co/Ru based structures. The anomalous Hall effect is the observation of a nonzero voltage difference across a magnetic material transverse to the current that flows through the material and the external magnetic field. Unlike the ordinary Hall effect which is observed in nonmagnetic metals, the anomalous Hall effect is only observed in magnetic materials and is orders of magnitude larger than the ordinary Hall effect. Unlike quantum anomalous Hall effect which only works in low temperature and extremely large magnetic field, anomalous Hall effect can be measured at room temperature under a relatively small magnetic field. This allows the anomalous Hall effect to have great potential applications in spintronics and be a good characterization tool for ferromagnetic materials especially materials that have perpendicular magnetic anisotropy(PMA). In my research, it is observed that a polarity change of the Hall resistance in the MgO/Permalloy/NM structure can be obtained when certain nonmagnetic metal is used as the capping layer while no polarity change is observed when some other metal is used as the capping layer. This allows us to tune the polarity of the anomalous Hall effect by changing the thickness of a component of the structure. My conclusion is that an intrinsic mechanism from Berry curvature plays an important role in the sign of anomalous Hall resistivity in the MgO/Py/HM structures. Surface and interfacial scattering also make substantial contribution to the measured Hall resistivity. Spin polarization(P) is one of the key concepts in spintronics and is defined as the difference in the spin up and spin down electron population near the Fermi level of a conductor. It has great applications in the spintronics field such as the creation of spin transfer torques, magnetic tunnel junction(MTJ), spintronic logic devices. In my research, spin polarization is measured on platinum layers grown on a YIG layer. Platinum is a nonmagnetic metal with strong spin orbit coupling which intrinsically has zero spin polarization. Nontrivial spin polarization measured by ARS is observed in the Pt layer when it is grown on YIG ferromagnetic insulator. This result is contrary to the zero spin polarization in the Pt layer when it is grown directly on SiO2 substrate. Magnetic proximity effect and spin current pumping from YIG into Pt is proposed as the reason of the nontrivial spin polarization induced in Pt. An even higher spin polarization in the Pt layer is observed when an ultrathin NiO layer or Cu layer is inserted between Pt and YIG which blocks the proximity effect. The spin polarization in the NiO inserted sample shows temperature dependence. This demonstrates that the spin current transmission is further enhanced in ultrathin NiO layers through magnon and spin fluctuations. Perpendicular Magnetic Anisotropy(PMA) has important applications in spintronics and magnetic storage. In the last chapter, I study the origin of PMA in one of the structures that shows PMA: Ru/Co/Ru. By measuring the ARS curve while changing the magnetic field orientation, the origin of the PMA in this structure is determined to be the strain induced by lattice mismatch. / Dissertation/Thesis / Doctoral Dissertation Physics 2018

abstract: A driving force for studies of water, alcohols, and amides is the determination of the role of hydrogen bonding. Hydrogen bonds can break and reform, consequently creating supramolecular structures. Understanding the role supramolecular structures play in the dynamics of monohydroxyl alcohols is important to understanding hydrogen bonding in more complex systems such as proteins. Since many monohydroxyl alcohols are good glass formers, dielectric spectroscopy in the supercooled regime is used to gather information about the dynamics of these liquids. Application of high external fields will reversibly alter the polarization responses of the material from the linear response. This results in nonlinear dielectric effects (NDE) such as field induced suppression (saturation) and enhancement of amplitudes (chemical effects) as well as shifts in the time constants toward slower (entropy) and faster (energy absorption) dynamics. The first part of this thesis describes the nonlinear dielectric experiments on monohydroxyl alcohols, with an emphasis on the time dependence of NDEs. For the first time, time-dependent experiments on monoalcohols were done, the results showed that NDEs occur on the Debye time scale. Furthermore, physical vapor deposition (PVD) is used to modify the supramolecular structure of 4-methyl-3-heptanol. Upon deposition the film cannot form the ring like structures, which are preferred in the bulk material. The as deposited film shows an enhancement of the dielectric peak by a factor of approximately 11 when compared to the bulk material. The conversion from the as deposited material back to the near bulk material was found to occur on the Debye timescale. The second part of this thesis focuses on the question of what is governing the field induced changes seen in the liquids studied. Here a complete set of high field experiments on highly polar propylene carbonate derivatives were performed. It was demonstrated that these materials exhibit a Debye-like peak and using a combination of Adam-Gibbs and Fröhlich’s definition of entropy, proposed by Johari [G.P. Johari, J. Chem. Phys 138, 154503 (2013)], cannot solely be used to describe a frustration of dynamics. It is important to note that although these material exhibit a Debye like peak, the behavior is much different than monoalcohols. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2019

Photoelasticity is an optical technique that measures the difference of the principal stresses plus the principal stress direction. A complementary technique is thermoelasticity which measures the sum of the principal stresses. Combining these two full-field, non-contact nondestructive evaluation techniques allows the individual stress components to be measured. One of the main difficulties in merging these two measurement systems is in identifying an appropriate surface coating. Thermoelasticity demands a highly emissive surface, while photoelasticity requires a thick, stress-birefringent, transparent coating with a retro-reflective backing. Two coatings have been identified that can be used for combined thermoelastic and photoelastic stress measurements: PMMA and polycarbonate.;An anisotropic electromagnetic boundary value model was developed to understand more fully the mechanisms through which photoelastic stress patterns are produced. This model produced intensity contour maps which matched the fringe patterns observed in the laboratory, and allowed the effect of measurement errors on the calculated stress tensor to be quantified. One significant source of error was the retro-reflective backing, which depolarized the light and degraded the resulting photoelastic fringes. A quantitative analysis of the degraded fringes, to be used as a rating scheme for reflective backing materials, showed that the isoclinic lines shift position as a result of the backing roughness and oblique incidence. This is a concern when calculating the stress components through the combination of photoelasticity and thermoelasticity because the data maps are integrated at the pixel level. Small shifts in the photoelastic fringes result in incorrect information being assigned to some pixels and hence lead to uncertainties in the stress tensor components. Progress in the understanding of the depolarization at the reflective backing allows the specification of new materials that will minimize this effect, as well as the development of robust computer algorithms to correct for any remaining depolarization.

Over the last several decades, two methods have emerged as the standard tools for the calculation of electronic band structures. These methods, the Car-Parinello plane wave method and the linear augmented plane wave method (LAPW), each have strengths and weaknesses in different regimes of physical problems. The Car-Parinello algorithm is ideal for calculations with soft pseudopotentials and large numbers of atoms. The LAPW method, on the other hand, easily handles all-electron and hard-core pseudopotential calculations with a small number of atoms. The projector basis method, presented here, is a hybrid mixed basis method which allows the calculation of moderately large ({dollar}\sim{dollar}200) numbers of atoms represented by hard pseudopotentials. This method will then be used to calculate two members of a relatively new mass of materials, called electrides, in which the anion has been replaced with a localized electron.

The motivation for this study was to further characterize a relatively new high-temperature polymer for application as a matrix resin for carbon fiber composites. The two primary questions addressed in this study dealt with the structural and chemical changes occurring in these polymers on exposure to high temperature.;To investigate the structural changes in the heat-treated samples, a positron annihilation lifetime spectrometer was designed, built and optimized. Because the lifetime of a positron in a material reflects the electronic structure of the material in which it annihilates, measurements by positron annihilation lifetime spectroscopy can be used to investigate changes in a material at the Angstrom level. The results of applying this technique indicate that positron annihilation lifetime spectroscopy is able to detect microstructural changes in these heat-treated polymers.;In order to study the chemical changes occurring in these polymers during heat treatment, an array of solid state analytical techniques was applied. These techniques included Differential Scanning Calorimetry, Elemental Analysis, Thermo-gravimetric and Thermo-mechanical analyses, Fourier Transform Infrared, {dollar}\sp{lcub}13{rcub}{dollar}C Cross Polarization Nuclear Magnetic Resonance, Electron Spin Resonance, and Dynamic Nuclear Polarization. The results of applying these techniques provide previously unavailable data into the chemical structures of these polymers before and after heat treatment. Additionally, the presence of stable free radicals in the polymer samples both before and after heat treatment was confirmed and the origin and location of these free radicals is proposed.

<p> It is known that for a topologically ordered state the area law for the entanglement entropy shows a negative universal additive constant contribution, &ndash;&gamma;, called the topological entanglement entropy. We theoretically study the entanglement entropy of the two-dimensional Shastry-Sutherland quantum antiferromagnet using exact diagonalization on clusters of 16 and 24 spins. By utilizing the Kitaev-Preskill construction, we extract a finite topological term, &ndash;&gamma; , in the region of bond-strength parameter space corresponding to high geometrical frustration. Thus, we provide strong evidence for the existence of an exotic topologically ordered state and shed light on the nature of this model's strongly frustrated, and long controversial, intermediate phase.</p>

<p> To determine the mechanical and optical response of diamond crystals at high stresses and to evaluate anisotropy effects, single crystals (Type IIa) were shock compressed along the [100], [110], and [111] orientations to ~120 GPa peak elastic stresses. Particle velocity histories and shock velocities, measured using laser interferometry, were used to examine nonlinear elasticity, refractive indices, and Hugoniot elastic limits of shocked diamond. Time-resolved Raman spectroscopy was used to measure the shock compression induced frequency shifts of the triply degenerate 1332.5 cm^-1 Raman line. </p><p> Longitudinal stress-density states for elastic compression along different orientations were determined from the measured particle velocity histories and elastic shock wave velocities. The complete set of third-order elastic constants was determined from the stress-density states and published acoustic data. Several of these constants differed significantly from those calculated using theoretical models. </p><p> The refractive index of diamond shocked along [100] and [111] was determined from changes in the optical path length along the direction of uniaxial strain. Linear photoelasticity theory predicted the measured refractive index along [111]. In contrast, the refractive index along [100] was nonlinear. The refractive indices for [110] compression were not determined, but the data showed evidence of birefringence. </p><p> The splitting and frequency shifts of the diamond Raman line were measured for shock compression along [111] and were in good agreement with predictions from prior shock work. Frequency shifts were also measured along [100] and [110] up to ~60 GPa, extending previous measurements. The anharmonic force constants determined from all shock compression measurements agree with the previous shock compression determinations. </p><p> Hugoniot elastic limits for diamond shock compressed along different orientations were determined from the measured wave profiles. The elastic limits for the three orientations were highest at ~90 GPa peak elastic stress, but decreased at the higher peak elastic stress. Shear strengths were determined from the measured elastic limits: shocked diamond was strongest for compression along [110] and weakest for compression along [111]. The shear strength dependence on shock propagation direction was correlated with the stress magnitude normal to the slip plane, which appeared to inhibit the onset of inelastic deformation. </p>

<p> Gyromagnetic phenomena have been of interest since the dawn of modern electromagnetic theory. While rotation-induced magnetization in electronic systems has been known for over 100 years, the phenomenon remains largely unexplored in nuclear degrees of freedom. This thesis explores the influence of external angular momentum on nuclear polarization, utilizing optical fields endowed with orbital angular momentum (OAM). To that end, I employ novel holographic methods to project light fields with programmable OAM content into fluid samples. To quantify the OAM in such fields, I introduce new techniques of holographic video microscopy to characterize optical forces. These optical manipulation and detection schemes are combined with standard NMR spectroscopy to reveal the effects of optical forces on the nuclear hyperpolatization of both absorbing and non-absorbing samples. These experiments provide evidence of a non-resonant coupling between the orbital angular momentum of light and nuclear spins.</p>

<p>Electron systems in the quantum Hall regime change from a liquid state to a Wigner crystal state as the filling factor is lowered below approximately 1/5. This phase transition can be studied with theoretical methods by comparing the ground-state energies of the quantum liquid and the quantum Wigner crystal. Past studies have not included realistic effects such as finite thickness, Landau-level mixing, and disorder on the electron system. We expand upon the classic work by Maki and Zotos to calculate Wigner crystal energies that include a finite thickness of the two-dimensional electron lattice. </p>