A new technique for fabricating magnetic nanowire transport devices is developed by exposing an ordered and microphase separated P(S-b-MMA) diblock copolymer film with an electron beam. The volume fraction of the PS (polystyrene) block of the diblock copolymer is chosen to be 0.7 to get cylindrical microphase separation, where the PMMA (polymethyl methacrylate) cylinders are aligned perpendicular to the substrate by an external electric field. Electron beam exposure of the aligned diblock copolymer crosslinks the PS matrix and degrades the PMMA inside the cylindrical regions, creating a laterally patterned hexagonal array of nanopores (12 run in diameter with a lattice spacing of 24 nm). A unique magnetotransport device is fabricated, consisting of an array of electrodeposited Co nanowires standing atop a thin gold film patterned into a four-probe resistor configuration. Magnetoresistance measurements are performed at various temperatures and magnetic field orientations. This technique is further extended to a multi-level lithographic processing which allowed to fabricate and study the magnetotransport properties through a few vertical Co nanowires in electrical contact. Magnetization reversal of Co nanowires is studied via anisotropic magnetoresistance (AMR) measurements.

Both magnetic and superconducting systems exhibit long range order physics. In the magnetic case, long range order is described by the exchange length of the magnetic material while superconducting long range order is defined by its coherence length. If the dimensional length scales of the physical systems become comparable to the long range length scale of the parent material then interesting physical phenomena are observed. This thesis is mainly about the study of physical systems whose dimensional length scales are comparable to the long range length scales of the parent materials. It is mainly divided into three groups: ferromagnetic rings, superconducting honeycomb films and ultrasmall rings on thin Nb film. Chapter 1 and 2 provide an introduction and overview. Chapter 3 discusses the fabrication techniques used for this research. It is relevant to note that a new fabrication technique is revealed, which results in the fabrication of metal rings of unprecedented small size and also small symmetric and asymmetric rings. Chapters 4 discuss the experiments on ferromagnetic ultrasmall rings (diameter 13 nm) and asymmetric small rings (diameter 150 nm). Magnetic measurements and analytical investigations indicate that magnetic transition process is simple and occurs via the formation of vortex state in the case of ultra-small rings. It is also shown that by creating asymmetry in the ring's width, the direction of vortex magnetic circulation can be controlled by simple application of magnetic field. A related structure is a honeycomb film where hole sizes are of the order of 13 nm and center-to-center distance between neighboring holes are 28 nm. Chapters 5 discuss the physics of honeycomb films made of superconducting niobium. It is found that as the dimension scale becomes comparable to the coherence length of the superconductor, interesting physical behavior is observed, which includes: Unusual suppression of transition temperature for thin honeycomb films as compared to plain films, change in dimensionality of the films and a jump in resistivity for a sample etched for a particular amount of time. Chapter 6 discusses the electrical transport measurements of ultrasmall magnetic rings on a metal film.

We present a decay scenario of superfluid turbulence (ST)—structured or non-structured tangle of quantized vortex lines—in the fundamental limit of very low temperatures, when the friction with normal component is negligible. At length scales smaller than the typical interline separation, Kelvin waves (kelvons)—the distortion waves on quantized vortex lines—play a key part in relaxation. We develop a kinetic theory of kelvons, which allows us to prove the existence of a cascade of Kelvin waves and find its spectrum. Being the only dissipative channel of vortex dynamics in this regime, radiation of sound by kelvons is responsible for the short-wavelength cutoff of the cascade. We derive a general Hamiltonian of vortex-phonon interaction starting with the hydrodynamic action. On the basis of this formalism, we calculate the rate of sound radiation by ST and estimate the value of the short-wavelength cutoff. Incidentally, the vortex-phonon interaction theory allows us to resolve apparent paradoxes of the Kosterlitz-Thouless theory of U(1) phase transitions in two dimensions. As long as vorticity quantization remains irrelevant for the long-wave physics, superfluid turbulence supports a regime macroscopically identical to the Kolmogorov cascade of a normal liquid. We analyze the transformation of the Kolmogorov cascade into the Kelvin-wave cascade, revealing a chain of three distinct intermediate cascades, supported by local-induction motion of the vortex lines, and distinguished by specific reconnection mechanisms. The most prominent qualitative feature predicted is unavoidable production of vortex rings of the size of the order of inter-vortex distance.

We present results on experiments performed to study the dynamics of melting and freezing in a model colloidal system with short-range tunable attraction mediated by depletion forces. In particular, we have created a two-dimensional system which forms crystallites and undergoes melting or freezing when a small temperature change is applied. To our knowledge, this is the first system that uses a tunable depletion potential and that allows tracking of individual particles at sufficiently short time scales. We have discovered a cross-over in the melting kinetics of two-dimensional crystallites. Crystallites first sublimate until they reach a cross-over size, after which they quickly vaporize into the gas phase. This cross-over has not been predicted by any theory of which we are aware. In freezing experiments, we have found two qualitatively different mechanisms of nucleation. In one mechanism, highly ordered crystallites form directly from the fluid phase, as in classical nucleation theory. In the other, a meta- or unstable amorphous cluster is first formed, after which the crystallite nucleates from within. These results are likely to be generally applicable to systems with short-range centro-symmetric attractive interactions. The enhancement of kinetics by meta- or unstable phases may play a major role in the melting, freezing and annealing of crystals. In particular, our results may have profound consequences in the area of protein crystallization where the interactions are believed to be similar to ours.

The purpose of this work is to demonstrate the usability of irreversible thermodynamics and kinetic theory in describing slow steady state evaporation and condensation, analyze the statistical rate theory (SRT) approach, and investigate the physical phenomena involved. Recently large interface temperature jumps have been observed during steady state evaporation and condensation experiments; the vapor interface temperature was greater than the liquid interface temperature for condensation and evaporation. To predict the temperature jump, the SRT mass flux was introduced as an alternative to the established approaches of irreversible thermodynamics and kinetic theory of gases. Simple one dimensional planar and spherical models were developed for slow evaporation and condensation based on the experiments. We considered pure liquid water evaporation and condensation to, and from its own vapor. Expressions for the mass and energy fluxes across the interface were found using irreversible thermodynamics, kinetic theory, and SRT. The SRT theory does not have an energy flux expression, as a substitute we use the irreversible thermodynamics energy flux in the SRT model. The equations were then solved to yield the mass and energy fluxes, and the liquid and vapor temperature profiles. We find the interface temperature jump is dependant on the energy flux expression. The irreversible thermodynamics energy flux closely predicts the measured temperature jump and direction. Kinetic theory models do not predict the jump, however with incorporation of a velocity dependant condensation coefficient, kinetic theory can predict the correct temperature jump direction, and vapor interface temperature. All the models predict mass fluxes that agree with the measured data. We suggest the temperature jump direction is established based on the direction of the vapor conductive energy flux, and not the direction of the mass flux (condensation or evaporation). We conclude that irreversible thermodynamics, kinetic theoiy, and SRT can all be used to model steady state evaporation and condensation.

Thesis (Ph.D.)--University of Delaware, 2007. / Principal faculty advisor: Ismat S. Shah, Dept. of Materials Science. Includes bibliographical references.

Reproduction de : Thèse de doctorat : Chimie de l'état solide : Le Mans : 2006. Reproduction de : Thèse de doctorat : Chimie de l'état solide : Faculté des sciences de Bizerte (Tunisie) : 2006. / Thèse soutenue en co-tutelle. Titre provenant de l'écran-titre. Bibliogr. p. 151-157.

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Thesis (Ph. D.)--Johns Hopkins University. / Biographical.

Thesis (Ph. D.)--Johns Hopkins University. / Biographical.