<p> Magnetic property of the symmetric Double Exchange Spring System, soft(S)/hard(H)/soft(S) ferromagnetic layers NiFe (Py)/SmFe/NiFe (Py), was investigated using Magneto Optical Kerr Effect (MOKE) measurement. Exchange spring magnet, H/S bilayer, shows a unique magnetic hysteresis curve due to the non-collinear magnetization developed by magnetic coupling of the two layers. In order to produce a symmetric non-collinearity in magnetization, the thicknesses of the two soft layers are controlled to be the same during the deposition. Due to the finite skin depth of MOKE measurement magnetic hysteresis loop for each soft layer could be measured separately by adjusting the right thickness of the layers. To measure the bottom soft layer transparent glass substrate was used for thin film deposition. We first observed an asymmetric hysteresis loop of the double exchange spring system by MOKE measurement. This is due to the existence of quadratic MOKE (QMOKE) signal caused by the in-plane magnetic anisotropy. Linear MOKE (LMOKE) and QMOKE signal were separated from the general MOKE signal using a symmetry operation. In general, the samples that have the induced easy axis have less significant QMOKE signal than those that do not have the easy axis. Second, from the LMOKE measurement, we found that the magnetic hysteresis loops for the bottom and the top S layers are not the same. The magnetic hysteresis loop from LMOKE measurements data was compared with the one measured by Alternating Gradient Magnetometer (AGM). AGM measures the total magnetization of the whole structure. Our data shows that the coercivity of the bottom S measured from LMOKE is closer to the coercivity of the first switching measured from AGM and is much smaller than that of the top S from MOKE. The coercivity of the top S from LMOKE is closer to that of the second switching from AGM. This indicates that the top S is, to some extent, strongly coupled with the hard layer causing them to switch together; while the bottom S is much less coupled so that its switching is spiral. In other words, the bottom S provides the non-collinearity as we expected from exchange spring magnets, while the top S does not. We found experimentally that within an acceptable uncertainty, the non-collinearity provided by the two soft layers is not symmetric despite the symmetry of the structure. A possible explanation of this general trend is that it is the difference in the surface roughness between the bottom Py/SmFe surface and the top Py/SmFe surface that causes the asymmetric non-collinearity, since we found that the interface between the top Py and the SmFe is rougher than the one between the bottom Py and the SmFe. Further investigation needs to be done to experimentally explain how the difference in the surface roughness affects the magnetic coupling between the soft and hard magnetic layers. </p>

The x-ray total structure factor of the glassy-metal alloys $Mg sb{70}Zn sb{30}, Ca sb{70}Mg sb{30}$ and $Mg sb{85.5}Cu sb{14.5}$ has been measured at three temperatures: 9K, 150K, and 300K. The data have a statistical precision of about.8% and an absolute accuracy of roughly 3%. / Percus-Yevick hard sphere structure factors may be fitted quite accurately to the data in the region of the first peak. In addition, the variation of the experimental structure factor with composition is found to be consistent with the Percus-Yevick theory. At low k values, Percus-Yevick and other theoretical model structure factors are in poor agreement with the data. / Within experimental error, the temperature dependence of the structure factors is in agreement with the Debye plane wave phonon model of atomic vibrations. / The measured structure factors are used to calculate the electrical resistivity from the Faber-Ziman equation. In most cases, the calculations yield both the correct magnitude of resistivity and sign of the temperature coefficient of resistivity.

The kinetics of spinodal decomposition has been studied by Monte Carlo renormalization group method. Using a standard block-spin transformation, we numerically renormalize the evolving configurations during the phase separation of a kinetic Ising model with spin-exchange dynamics. We find that in the scaling regime, the average domain size $R$($t$) grows in time as $R$ $ sim$ $t sp{n}$, with $n$ = 0.338 $ pm$ 0.008 consistent with the classical $n$ = 1/3 result of Lifshitz and Slyozov. A scaling form for the structure factor is obtained, which is invariant under the renormalization group transformation. / The fluctuations around the average domain growth in Model A and Model B are studied using Monte Carlo simulation. The fluctuations, which are non-self-averaging, were found to have multiscaling properties and exhibit 1/$f$-like properties in the scaling regime. These properties point to the existence of analogies between dynamical and random systems, which need to be further explored.

A phase-field approach describing the formation and evolution of an hexagonally patterned surface is presented. We studied a free-energy and a time-dependent Ginzburg-Landau equation for which the order parameter is non-conserved. We give a review of the scaling phenomena in general and of hexagonal systems in particular, both from theoretical and experimental points of views. A squared shaped grid of varied sizes, with periodical boundary conditions, was used for the simulations. First, we studied the evolution of surfaces themselves. Then, we considered the evolution of the structure factor's maximum amplitude, and full width at half maximum. Scaling laws as a function of system size were found for these variables. We also made a study of surface energies. We monitored the evolution of the surface energies with time, and propose a scaling law for the energies. Finally, we studied the evolution of temporal correlation functions. We propose a further scaling law for the temporal correlation functions.

This work focuses on the atomic force microscope: its hardware, modes of operation, and applications. / The construction of a x-y sample positioner, equipped with position dependent capacitive sensor, is presented. The implementation of a temperature-controlled laser for cantilever detection, via interferometry, is also discussed. / Two modes of atomic force microscopy are used. Amplitude modulation mode images are done in vacuum using Q-control to reduce the apparent Q-factor of the cantilever. Frequency modulation mode is used to obtain non-contact images and force curves above a quantum dot or gold sample. The former leads to detection of single electron charging events from a buried 2D electron gas to the surface layer of the sample. The latter was done to determine the geometric behaviour and capacitance of high-aspect ratio cantilever tips; a method for which is presented where the height, cone angle, radius of curvature and angle to the sample can be controlled.

The zero-temperature equilibrium conductance of mesoscopic devices due to single-particle resonant tunneling was first described by Landauer [1]. The Landauer formula was later extended to the multi-channel case by Fisher and Lee [2], who reduced the problem of calculating electronic transport properties to the problem of solving for the Green's function for a given system geometry. / In this work, the single-particle formalism is extended to the study of higher-order two-particle cotunneling processes by considering many-body Green's functions. The effect of attaching leads to the system is described in terms of a two-particle self-energy, whose analytical form is written in terms of a Feynman path integral over all possible tunneling processes between the leads and the device. In addition, an efficient numerical technique for the calculation of the fully dressed Green's function of a device region attached to two-particle leads is presented. / The problem of two-particle transport is then approached, and an analogy to single-particle transport on the infinite plane is drawn. It is shown that, for nonspin flip cotunneling processes, the two-particle transport result can be related to the single-particle conductance by way of a simple convolution. Finally, results for the cotunneling contribution to the conductance of double quantum dots, or charge qubits, are presented.

X-ray microdiffraction is a powerful technique to study the microstructure of materials. In this thesis we built two x-ray micro diffraction setups and demonstrated their capabilities by two case studies. In our microfocusing setup, synchrotron x-rays were focused to a micron-size spot using a Fresnel zone plate. To scan the sample, we built a 3-axis translation stage with 30 nm step size over 25 mm travel range. Our x-ray diffraction imaging (topography) setup consisted of a monochromator, a channel-cut analyzer to define the diffraction angle, and a CCD camera with 0.645 mum pixels size to record the images. In our first project, we studied the microstructure of 90° ferroelectric domains and domain walls in barium titanate (BaTiO3). We discovered a ~ 1 mum surface-like layer below the surface where domain walls angle, strain, and domains orientation deviate from the bulk values. These can be explained in terms of total energy minimization and domain-domain interaction. In our second project, we used x-ray topography to measure lattice deformations in the free and bent states of a silicon micro-cantilever used in scanning probe microscopy. We found that the cantilever was twisted by 8 mdeg with respect to the base and there were small strains in the cantilever and joint area. In the bent state, we measured 0.3 m average radius of anticlastic curvature and a maximum of 2 x 10-5 strain at the edges of the cantilever. We discuss possible causes of the twist and the non-zero strains. Our setups and the tools we developed can be used to study the microstructure of other similar systems, as well.

Since the discovery of a giant magnetocaloric effect in Gd5Si 2Ge2 that is associated with a first-order magnetostructural transition, the R5SixGe4- x materials (where R=Y,La-Lu) have come under intensive study. Systematic structural data has now been obtained for most of the R5Si xGe4-x compounds (Y, Pr, Nd, Gd, Tb, Er, and recently Yb), and from this structural data, trends have been begun to emerge. The Yb5SixGe 4-x system however defies all conventional wisdom and adopts the orthorhomic Gd5Si4 structure, independently of x, a unique feature in the R5SixGe 4-x family. The magnetic transition temperatures of the Yb5SixGe4- x compounds have been found to be monotonically increasing from 2.4 K to 3.2 K with increasing Ge content contrary to trend measured in other R5SixGe4- x compounds. Moreover, magnetic susceptibility suggests the presence of both Yb2+ and Yb3+ in all compounds at a 60:40 ratio. / In this study, we have performed cold-source 170Yb Mossbauer spectroscopy on samples that have been previously fully characterized, and analyzed our data with an exact solution to the full Hamiltonian. / We conclusively confirm the presence of both Yb2+ and Yb3+ ions in all samples, however with a ratio closer to 50:50. Previous attempts to assign specific crystallographic sites for them fail. We also find the magnetic transition temperature to be far lower than was indicated by the susceptibility data. More strikingly we find that the magnetic behavior of the x = 0 and 4 members is similar, with a single transition temperature at similar temperatures, whereas the middle members, x = 2, 3, have not only considerably different magnetic behaviour (we find two separate magnetic components with different ordering temperatures), but also lower transition temperatures. This system is considerably more complex than was originally thought.

<p>Multifunctional polymer nanocomposites (PNCs) are attractive for the design of tunable RF and microwave components such as flexible electronics, attenuators, and antennas due to cost-effectiveness and durability of polymeric matrices. In this work, three separate PNCs were synthesized. Magnetite (Fe₃O₄) and cobalt ferrite (CFO) nanoparticles, synthesized by thermal decomposition, were used as PNC fillers. Polymers used in this work were a commercial polymer provided by the Rogers Corporation (RP) and polyvinylidene fluoride (PVDF). PNCs in this thesis consist of Fe₃O₄ in RP, CFO in RP, and Fe₃O₄ in PVDF. Characterization techniques for determining morphology of the nanoparticles, and their resulting PNCs, include x-ray diffraction, transmission electron microscopy and magnetometry. </p><p> All magnetometry measurements were taken using a Quantum Design Physical Property Measurement System with a superconducting magnet. Temperature and external magnetic field magnetization measurements revealed that all samples exhibit superparamagnetic behavior at room temperature. Blocking temperature, coercivity and reduced remnant magnetization do not vary with concentration. Tunable saturation magnetization, based on nanoparticle loading, was observed across all PNCs, regardless of polymer or nanoparticle choice, indicating that this is an inherent property in all similar PNC materials. </p><p> Tunability studies of the magneto-dielectric PNCs were carried out by adding the PNC to cavity and microstrip linear resonator devices, and passing frequencies of 1&ndash;6 GHz through them in the presence of transverse external magnetic fields of up to 4.5 kOe, provided by an electromagnet. Microwave characteristics were extracted from scattering parameters of the PNCs. In all cases, losses were reduced, quality factor was increased, and tunability of the resonance frequency was demonstrated. Strong magnetic field dependence was observed across all samples measured in this study. </p>

Measurements of the complex dielectric constant of ceramic barium titanate have been made at temperatures ranging from 300&deg;K down to 2.4&deg;K. The frequencies employed ranged from 50 kilocycles per second up to 2 megacycles per second with the majority of the measurements at 500 kilocycles per second. The real part of the dielectric constant, K ', had a value of about 1100 at room temperature, decreasing to approximately 100 in the liquid helium temperature region (2--4&deg;K). The slope of K' against the temperature had a high positive value of 0.01 per &deg;K in agreement with Wul. The imaginary part of the dielectric constant, K&Prime;, had a value of about 4 at room temperature, rising to a very broad maximum of approximately 15 at 200&deg;K and steadily decreasing below 80&deg;K to room temperature values below 4 in the liquid helium region. The experimental work was done in the Collins cryostat of the low temperature laboratory of The Rice Institute, and the impedance measurements were made with a Boonton Q-meter and a General Radio Type 916-A impedance bridge. In addition to the pure barium titanate measurements, measurements were also made on specimens diluted with 15% and 30% strontium titanate respectively. The curves obtained for diluted specimens had the same general appearance as those for the pure barium titanate except that the sharp maximum in the K' curve, normally occurring at 390&deg;K, was brought near room temperature in case of the 15% dilution and to 274&deg;K for the 30% dilution. This lowering of the temperature of the peak is in accord with results reported by von Hipple, Coursey and Brand, and Jackson and Reddish.