D.Phil. (Physics) / Most of previous magnetic investigations performed on uranium chalcogenides have been limited to polycrystalline specimens. To extend the knowledge on magnetization processes in uranium monosulphide, single crystals were grown. The magnetization of these single crystals was determined in the ferromagnetic and paramagnetic regions as a function of temperature (4-300 °K), field strength (0-20 k0e) and orientation ...

No description available.

This dissertation reports on a detailed systematic study of the investigation into using Indium Oxide based materials in next generation spin-transport electronic applications. Initial studies focused on the optimisation of the electrical properties of Indium Oxide (In2O3) and Tin(Sn)-doped Indium Oxide (ITO) thin films grown using DC magnetron sputtering. The manipulation of various deposition parameters allowed the electrical properties to be tuned effectively. With the desire to create multi-functional spintronic devices, a dilute magnetic oxide system is developed where the In2O3 and ITO matrices are doped with low levels of transition metals, in particular, Co. Using a number of characterisation techniques, the origins of the magnetic response in these thin films is explored in great detail. In particular, powerful probes such as x-ray and optical magnetic circular dichroism are utilised. The major finding from these investigations is that the magnetism does not necessarily emanate from the Co dopants alone. In fact, Co dopants give a strictly paramagnetic response, suggesting that the magnetism observed may be a result of polarised electrons in localised donor states in the In2O3 and ITO hosts. Therefore, we believe that the origins of magnetism in these films is related to a hybridisation and charge transfer of electrons from a broad donor/defect-derived impurity band to a band of unoccupied 3d states at the Fermi level. The emergence of a very weak magnetic signal in pure ITO raises further questions as to the true origins of the ferromagnetic behaviour and supports a defect-related mechanism. To explore the suitability of ITO for a future in spintronics further, the performance of some metal ferromagnet/oxide multilayered structures was investigated. The investigations revealed a significant contribution to both the magnetic and magnetotransport properties from a superparamagnetic component giving some insight into the importance of the quality of interfaces between the metal ferromagnet/oxide layers and heterostructures. Using a three-dimensional focused-ion beam etching technique to fabricate submicronspin-valve devices with ITO spacer layers, current-perpendicular-to-plane magnetoresistance measurements were carried out to estimate the spin diffusion length of ITO at room temperature. In conjunction with a simplified Valet-Fert model, a spin asymmetry ratio for Co of 0.55 and spin diffusion length of 6±1 nm in semiconducting ITO at room temperature was estimated. These findings imply that spin information can be conserved and transported through In2O3 and ITO even up to and beyond room temperature.

The aim of the research project presented in this thesis is to investigate the effects of electrostatic gating on the magnetic properties of carrier-mediated ferromagnetic Ga1-xMnxAs diluted magnetic semiconductors. (Ga,Mn)As can be regarded as a prototype material because of its strong spin-orbit coupling and its crystalline properties which can be described within a simple band structure model. Compressively strained (Ga,Mn)As epilayer with more complex in-plane competing cubic and uniaxial magnetic anisotropies is of particular interest since a small variation of these competing anisotropy fields provide a means for the manipulation of its magnetization via external electric field. An all-semiconductor epitaxial p-n junction field-effect transistor (FET) based on low-doped Ga0.975Mn0.025As was fabricated. It has an in-built n-GaAs back-gate, which, in addition to being a normal gate, enhances the gating effects, especially in the depletion of the epilayer, by decreasing the effective channel thickness by means of a depletion region. A shift in the Curie temperature of ~2 K and enhanced anisotropic magnetoresistance (AMR) (which at saturation reaches ~30%) is achieved with a depletion of a few volts. Persistent magnetization switchings with short electric field pulses are also observed. The magnitude of the switching field is found to decrease with increasing depletion of the (Ga,Mn)As layer. By employing the k . p semiconductor theory approach (performed by our collaborators in Institute of Physics, ASCR, Prague), including strong spin-orbit coupling effects in the host semiconductor valence band, a change in sign of Kc at hole density of approximately 1.5x1020 cm-3 is observed. Below this density, the [110]/[1⁻10] magnetization directions are favoured, consistent with experimental data. A double-gated FET, with an ionic-gel top-gate coupled with a p-n junction back-gate based on the same material, was also employed in an attempt to achieve larger effects through gating. It reaffirms the results obtained and demonstrates enhanced gating effects on the magnetic properties of (Ga,Mn)As.

This thesis discusses experiments that have been performed to investigate ultrafast acoustoelectric effects in semiconductor devices. Current commonly employed techniques to generate ultrafast acoustic pulses and detect them with spectral resolution require a powerful pulsed laser system that is bulky, expensive and complicated. If the acoustic pulses could instead be generated and detected by electrical methods, picosecond acoustic techniques could become more readily available as a tool for other users. This thesis focusses on the electrical detection of acoustic pulses with spectral resolution. In many of the key experiments described in this thesis a picosecond strain pulse was generated optically on the opposite face of the sample to the semiconductor device of interest. The strain was generated either in a thin Al film thermally deposited on the sample surface, or directly in the GaAs substrate. Acoustic phonons generated by this method propagated across the substrate to the device. Transient voltages across the semiconductor device caused by the incident phonons were detected using a high frequency real-time oscilloscope. The first evidence of heterodyne mixing of coherent acoustic phonons with microwaves was obtained, for frequencies up to about 100 GHz. First, it was confirmed that Schottky diodes can produce a fast transient voltage in response to an incident acoustic wavepacket. The detection process occurs at the semiconductor-metal interface, and is due to the deformation potential. Bow-tie antenna fabricated directly onto the GaAs substrate proved to be ineffective at coupling microwaves from free space to the Schottky diode. A waveguide-coupled beam-lead Schottky diode provided by e2v had a sufficient response to the incident microwaves to proceed with the mixing experiments. The microwave local oscillator signal was mixed with a tunable narrow frequency band acoustic signal that was produced using a Fabry-Perot etalon external to the laser cavity. The intermediate frequency components were in the range of 1-12 GHz, which could be detected on the oscilloscope. Mixing was performed using both the fundamental frequency acoustic wave and the second harmonic generated in the sample. Semiconductor superlattices were also investigated as electrical detectors for ultrafast acoustic pulses. In this case, the transient voltage measured across the device contained an unexpected contribution in the form of a peak with a width of approximately 2 ns. This signal is too slow to be caused by a strain pulse and too fast for a heat pulse. It is proposed that this peak is caused by long-lived phonon modes from the centre of the mini-Brillouin zone being confined in the superlattice due to Bragg reflections. The peak caused by confined phonons and the two peaks caused by heat pulses also present in the detected signal were investigated for a range of experimental conditions. This allowed comparisons to be made to previous works. A similar superlattice structure had a very different response to the incident acoustic wavepacket. The polarity of the transient voltage detected was inverted and there was no evidence of an electronic response to the confined phonon modes, which would have been present in both samples. It is proposed that the barriers of the NU1727 superlattice sample are thicker than expected, and this strongly affects the electron transport through the structure. This thesis shows that semiconductor devices can be suitable for the electrical detection of ultrafast strain pulses. For this technique to reach its full potential, it is also necessary to be able to generate these strain pulses electrically. A step recovery diode has been considered for this purpose as part of the suggested future work.

The aim of this project is the conception, implementation, and application of a simulation tool for the accurate modeling of electromagnetic fields within inhomogeneous materials with complex shapes and the propagation of the resulting fields in the surrounding environment. There are many methods that can be used to model the scattering of an electromagnetic field, however one of the most promising for hybridisation is the Boundary Element Method (BEM), which is a surface technique, and the Unstructured Transmission Line Modeling (UTLM) method, which is a volume technique. The former allows accurate description of the scatterer's boundary and the field's radiation characteristics, but cannot model scattering by materials characterized by a non-uniform refraction index. The latter, on the contrary, can model a very broad range of materials, but is less accurate, since it has to rely on approximate absorbing boundary conditions. A method resulting in the hybridisation of BEM and UTLM can be used to construct a tool that takes into account both the interaction with non-uniform tissue and propagation in its environment. The project aims to describe in detail the implementation of the novel method, and deploy it in a heterogeneous distributed computing environment.

No description available.

No description available.

The magnetic behaviour (magnetization curves, Meissner effect, hysteresis losses, remanent flux and flux line cutting) for high $T\sb{c}$ samples of Nd and $YBa\sb{2}Cu\sb{3}O\sb{7-x}$ at 77 K, semi-reversible PbIn and hysteretic VTi at 4.2 K have been investigated and compared. The magnetic behaviour of the high $T\sb{c}$ samples and of the PbIn are remarkably similar but contrasts dramatically with the phenomena observed in the VTi. A hump structure appears in the low field region of the initial magnetization of the high $T\sb{c}$ samples. This phenomenon occurs because the sintered samples consist of a compact agglomeration of small irregularly shaped grains which are electrically coupled before the hump structure but are isolated after this feature. We show that the appropriate calibration should be based on the diamagnetic response of the uncoupled grains. A large ratio of $j\sb{c\Vert}$ to $j\sb{c\perp}$ (the critical current densities $\Vert$ and $\perp$ to the flux line density) accounts for the observations on the VTi in the flux cutting regime whereas $j\sb{c\Vert} \approx j\sb{c\perp}$ is indicated by the behaviour of the high $T\sb{c}$ and PbIn samples.

We have developed a novel experimental approach for the study of the interaction of sheets of non parallel flux lines in hysteric type II superconductors. We continuously monitor the evolution of the components of the magnetic flux density $\Vert$ and $\perp$ to $H\sb{a}$ (i.e. $\langle B\sb{z}\rangle$ and $\langle B\sb{y}\rangle$) as $H\sb{a}$ is raised to various intensities, and then reduced to zero. In our investigation of a high $T\sb{c}(YBa\sb{2}Cu\sb{3}O\sb{7-x}$) ceramic, $H\sb{a}$ exceeded $H\sb{*}$, the first full penetration field. We applied the phenomenological Clem general critical state model to the analysis of our extensive observations. A computer program was developed to solve the four coupled differential equations of this theory with appropriate physical constraints for the situations prevailing in our experiment. This analysis provides detailed insight into the evolution of the intricate configurations of the magnetic flux density $\vec B$(x), the critical current density, $\vec J$(x), and electric field $\vec E$(x) patterns as the injected and trapped flux lines are made to interact, unpin, migrate and undergo flux cutting processes. The model is seen to generate the variety of complicated measured curves of $\langle B\sb{y}\rangle$ and $\langle B\sb{z}\rangle$ vs $H\sb{a}$ very satisfactorily. (Abstract shortened by UMI.)