Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references. Available also in a digital version from Dissertation Abstracts.

Thesis (M. Phil.)--University of Hong Kong, 1993.

Thesis (Ph. D.)--University of Hong Kong, 1996. / Includes bibliographical references (leaf 187-199).

Thesis (Ph. D.)--University of Hong Kong, 1998. / Includes bibliographical references.

Thesis (Ph.D.)--University of Hong Kong, 1996. / Includes bibliographical references (leaf 187-199). Also available in print.

Thesis (Ph. D.)--Florida State University, 2006. / Advisor: Peng Xiong, Florida State University, College of Art and Sciences, Dept. of Physics. Title and description from dissertation home page (viewed June 9, 2006). Document formatted into pages; contains xiv, 78 pages. Includes bibliographical references.

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In this thesis, the potential of all-spinel oxide tunnel junctions in the field of spintronics has been investigated. In particular, the suitability of the metallic and superconducting LiTi2O4 as non-magnetic electrode in an almost defect-free CoFe2O4-based spin-filter tunnel junction has been explored. For this purpose, high-quality spinel LiTi2O4 and CoFe2O4 thin films have been deposited by pulsed laser deposition on MgAl2O4 substrates. Both films were extensively characterised in terms of structural, surface, magnetic and transport properties. The LiTi2O4 showed metallic and superconducting properties, and the CoFe2O4 had insulating and ferromagnetic properties. A careful tuning of the different growth conditions of these oxides followed in order to grow CoFe2O4/LiTi2O4 bilayers in which LiTi2O4 maintains its metallic and superconducting properties and CoFe2O4 its insulating ferromagnetic characteristics. Transport measurements at low temperature have been carried out to explore details of the tunnelling in symmetric tunnel junctions of the form LiTi2O4/CoFe2O4/LiTi2O4. The measured current–voltage characteristics of these junctions revealed clear Josephson junction behaviour due to superconductivity of the LiTi2O4 electrodes. This conclusive evidence of the tunnel nature of these junctions proves that LiTi2O4 can be used as bottom electrode in all-spinel oxide tunnel junctions.

The high resolution magneto-optic method using the Faraday effect in thin films of EuS:EuF2has been used to observe the dynamic intermediate state induced by the passage of an electric current or a heat current through thin slabs of the superconductors Pb, In and Sn. The ease with which the various intermediate state topologies could be made to move has been studied and several features of the interaction of moving flux with pinning sites have been noted. In the case of the current induced motion the measured characteristics of flux flow velocity versus current have been found to exhibit two distinct regions. Firstly, a linear region where the observed velocity was found to agree reasonably well with the predictions of the recent general theory of Andreev and Dzhikaev when allowance was made for the effects of pinning by the introduction of a velocity independent pinning force. Secondly, a curved region was found for currents close to the critical current J0 in agreement with earlier work using other methods of observation. Possible reasons for the existence of this curvature were examined in detail, and. it was found, that a phenomenological model based, on the presence of a Gaussian distribution of critical current values throughout the sample could account satisfactorily for the observations. The presence of such a Gaussian distribution was confirmed by observing the variations in distance travelled by a domain subjected, to a pulsed, driving current. The curvature was found in disagreement with the theory of thermal activation and. no evidence could be found, for the presence of a velocity dependent pinning force. In the case of the thermally induced, motion which was investigated only in Pb, it was found, in agreement with the very recent work of Laeng and Rinderer, that there exist two competing mechanisms driving the flux. The first of these, that treated, by Andreev and Dzhikaev in their general theory, which acts in a direction parallel to the heat flow, was found to be effective at low temperatures (T ≪ 4.2K) but to be negligible at high temperatures. The magnitude of the velocity produced by this mechanism agreed reasonably well with the theory at low temperatures but was in complete disagreement at high temperatures when the theory predicts that it should still be observable. The second mechanism which acts perpendicularly to the heat flow dominated the motion at high temperatures but its magnitude did not agree with the predictions of the recent theory of Rothen, which ascribes the effect to the thermoelectric power of the normal state. It should be noted that to overcome the pinning all of the low temperature observations (T ≪ 4.2k) were performed in the presence of an electric current. Clearly, much more work requires to be done to clarify these observations.

The possibility of making a refrigerator by mechanically induced flux flow to transfer heat in a type II superconductor has been examined. Heat generation due to flux flow was sufficient to destroy the cooling effect over most of our experimental temperature range, although some cooling was obtained near T[sub]c. The dissipation due to flux pinning was determined by the pinning strength of the specimen. The pinning forces in Nb and Pb-In were determined by mechanically sweeping a magnetic field over them and measuring the resulting force. Three different forces due to the interaction of the magnetic field, with (a) the trapped flux lines, (b) the Meissner screening current, and (c) the pinning sites of the specimen, were identified. The magnitude of the force due to the screening current was proportional to the field strength, and was reduced by the trapped, flux lines inside the specimen. The magnitude of the bulk pinning force was in agreement with that calculated from the magnetization via the Irie-Yamafujii model; F_p = αB^γ. The dependence of the pinning force and the magnetic properties on the temperature and on the surface treatment of the specimen was also studied. The temperature dependence of the pinning force was found to be given by F[sub]p(T) = F[sub]p(o)(1 -T/T[sub]c ) it is suggested that this can be understood in terms of Anderson's flux creep model. The critical current of various specimens was obtained from the pinning force and the magnetization. In applying the Irie-Yamafuji model, the demagnetization and the surface effects of a specimen had to be taken into account, and methods of doing these have been suggested and experimentally checked. The role of the magneto-caloric effect on the mixed state specific heat was discussed. A 'two-fluid' type equation was proposed and used to calculate the mixed state specific heat. The results were compared with the experimental measurements. With the above experiments, the dissipation associated with flux flow was studied in terms of the pinning and viscous forces. It was shown that dissipation due to pinning was dominant in low fields and that viscous forces gradually became important as the field increased. It was found that the motion of the flux lattice inside a type II superconductor could not be induced by moving the source of a uniform magnetic field. This showed that the concept of 'lines of flux' has no meaning for the case of a uniform field.