This thesis contains three major areas of work. Chapter 1 describes the various film deposition techniques available for the growth of thin films. These different growth methods, substrate preparation and vacuum techniques, are discussed in the context of the effect of the growth process and deposition environment on film structure and quality. Chapter 2 describes the two film growth chambers. Chapter 3 describes the origin of giant magnetoresistance (GMR) in magnetic multilayered structures, and investigates the magnetoresistive properties of ion-beam sputtered CoCu multilayered thin films. Different film structures and deposition conditions have been investigated in order to maximise the size of negative magnetoresistance seen. Chapter 4 investigates the magnetic and giant magnetoresistive properties of sputtered CoCu and CoAg heterogenous alloys. The magnitude of the GMR effect is found to depend on the Co concentration and Co particle size. Chapter 5 describes an investigation of molecular beam epitaxy (m.b.e.) grown Dy/Y multilayer structures. The temperature dependences of helical magnetic structure in the Dy layers is investigated in two samples of different multilayer structures. Chapter 6 describes an acoustic interferometer which has been used to measure velocity and attenuation changes in an ultrasonic signal at frequencies of order IGHz. Also described is the production of the thin film piezoelectric transducers used in this technique, which are grown directly onto a single crystal of the material under investigation. The acoustic interferometer is used to investigate the dynamics of of a relaxation process between two time-reversed antiferromagnetic states in DyAlG, and a model for domain growth in this system is postulated. Chapter 7 investigates the phase diagram of DyPO₄ using the acoustic interferometry technique. The observations of hysteresis effects are used to define a mixed phase region below the tricritical point.

In this study, new compositions of Tsai-type 1/1 quasicrystal approximants Gd(14)Au(70)Si(16), Gd(14)Au(67)Ge(19), Tb(14)Au(70)Si(16) and Yb(16)Au(65)Ge(19)are synthesized using both self-flux and arc-melting-annealing techniques. Both syntheses routes resulted single phase samples. The crystal structures of the compounds are determined by collecting single crystal X-ray and/or powder X-ray and powder neutron diffraction intensities.  The atomic structure refinements indicated that the compounds are essentially iso-structural with the prototype Tsai-type 1/1 approximant crystal, YbCd6. However, there are subtle structural variations at their cluster centers and in the so-called cubic interstices which affects some of their physical properties. Thermoelectric and magnetic properties of the compounds are investigated. Significant differences are observed in the thermoelectric properties of Gd(14)Au(70)Si(16), Gd(14)Au(67)Ge(19) and Yb(16)Au(65)Ge(19) compounds which are explained on the bases of their crystal structures and chemical compositions. Magnetic susceptibility and specific heat measurements revealed ferromagnetic transitions at low temperatures, Tc ≈ 22.5 K for Gd(14)Au(70)Si(16) and Tc ≈ 13.1 K for Gd(14)Au(67)Ge(19), whereas, for the Tb(14)Au(70)Si(16) compound a ferrimagnetic-like transition is observed at Tc ≈ 9 K. Moreover, a re-entrant spin-glass transition is observed at TRSG ≈ 3.3 K for Gd(14)Au(67)Ge(19) compound. Finally, the magnetic structure of the Tb(14)Au(70)Si(16) compound was determined from powder neutron diffraction data which is to our knowledge the first magnetic structure refinement report in the family of quasicrystals and approximants.

Superconductivity in high magnetic fields has attracted considerable atten- tion in recent years. The topic is important both for our fundamental un- derstanding of superfluids and for numerous practical applications. In this thesis, we consider several effects originating from the interplay between the Landau level structure of the normal state quasiparticle spectrum, and the tendency of the quasiparticles to form Cooper pairs below the critical tem- perature. A formalism designed to describe extreme type-II superconductors close to the upper critical field H_c2 is developed. The theory which utilizes the selection rules coming from the symmetry properties of the vortex lattice, simplifies the algebra describing a superconductor in the mixed state signifi- cantly. We are, on the mean field level, able to include the quantizing effects of the magnetic field on the electron motion exactly. A main achievement is the exact calculation of the expansion coefficients giving the grand canonical potential of a superconductor in terms of a power series in the size of the or- der parameter. The result is an expression for the grand canonical potential in terms of a polynomial in a finite set of variables close to H_c2. Using this formalism, a theory for the experimentally observed damped de Haas-van Alphen (dHvA) oscillations in the mixed state of a 2-dimensional (2D) superconductor is presented. The theory is compared with numerical results and the agreement is found to be good. A simple physical interpreta- tion of the damping is provided. The dependence of the damping on a finite Zeeman term, temperature, and the magnetic field is considered. A compar- ison of the theory with experimental data for the quasi-2D superconductor K-(ET)₂Cu(NCS)₂ yields good agreement. The attenuation of a longitudinal sound wave in the mixed state is then calculated. In analogy with the dHvA effect, we predict that there should be damped oscillations in the sound attenuation in the mixed state as the exter- nal magnetic field is varied. Furthermore, the dependence of the oscillations on the sound frequency and temperature is shown to yield information on the low lying quasiparticle spectrum. Especially, the presense of gapless excita- tions due to the magnetic field makes the attenuation qualitatively different as compared to the attenuation in the Meissner state. Some formal convergence properties of the Gor'kov theory for type-II su- perconductors close to H_c2 are derived. We show that the theory is essentially a high temperature expansion; the convergence radius of the Gor'kov series is proportional to k_BT when there is a Landau level at the chemical potential.

The crystallographic and magnetic properties of manganese oxides belonging to the Ruddlesden Popper (RP) series, described by the formula A_n+1B_nO_3n+1, are presented. Compounds having n = 2, 3 and ∞ are discussed in Chapters 3, 4 and 5, respectively, their properties being studied by powder X-ray and neutron diffraction, SQUID magnetometry and magnetotransport measurements. In Chapter 3 (A₃B₂O₇) it is shown that Sr_2-xNd_1+xMn₂O₇ (0 ≤ x ≤ 0.5) and Sr₂PrMn₂O₇ exhibit colossal, negative magnetoresistance (CMR) below 150K. The zero field resistivity signal is reduced by 2-3 orders of magnitude in a field of 14T, at ~4.2K. The compounds Sr₂HoMn₂O₇ and Sr₂YMn₂O₇ show no significant magnetoresistance. In contrast to the tetragonal Sr,Nd and Sr,Pr compositions, these compounds show a symmetry lowering to space group P4₂/mnm, and spin glass freezing on the Mn sublattice at ~20K. Chapter 4 (A₄B₃O₁₀) focuses on A- and B-cation substitutions in the parent compound Ca₄Mn₃O_10-δ. Substitution of Ca by Sr yields Sr₄Mn₃O_10-δ, an orthorhombic (Cmca) compound, composed of trimers of face sharing octahedra. The magnetic susceptibility of this sample is interpreted in terms of direct and indirect antiferromagnetic (AFM) Mn-Mn exchange interactions. The orthorhombic (Pbca) RP phase Ca_3.95La_0.05Mn₃O_10-δ has a magnetic transition at ~114K, suggesting that the antiferromagnetic groundstate, with associated weak ferromagnetism arising through the Dzyaloshinsky-Moriya interaction observed in Ca₄Mn₃O_10-δ, is preserved on introduction of a fraction of La³⁺ dopant cations. Ca_3.95La_0.05Mn₃O_10-δ displays CMR at 4K, with the resistivity signal reduced to 18% of the zero field value, in 14T. B-cation substitution yields Ca₄Mn₂TiO_9.93, an orthorhombic (Pbca) RP phase in which the Mn:Ti cation distribution, deduced from the combined results of anomalous dispersion X-ray experiments and neutron diffraction studies, is 59.8%(2.6):40.2%(2.6) and 70.1%(1.3):29.9%(1.3) across the 4b and 8c octahedral sites, respectively. Neutron diffraction studies at 5K show the presence of only short range magnetic interactions in this insulating material, the resistivity of which is reduced by just 10% at 75K in 14T. Chapter 5 (ABO₃ or A₂BB'O₆) describes the mixed B-cation phases, La₂GaMnO₆ and Nd₂GaMnO₆, containing Mn³⁺, 3d⁴ cations. Both are cation disordered, orthorhombic (Pnma) materials, and Nd₂GaMnO₆ exhibits a static, cooperative Jahn-Teller (JT) distortion. La₂GaMnO₆ contains a relatively higher proportion of dynamic, cooperative JT distortions, and as such exhibits isotropic ferromagnetism at 5OK and 5K (2.80(5)μ_B per Mn aligned along y at 5K), predicted by the 'quasistatic hypothesis', which describes the correlation between electron spin configurations of neighbouring JT cations. The magnetic structure of Nd₂GaMnO₆ at 5K and 1.7K is modelled as 'A_X F_Y G_Z' for the Mn sublattice, and 'F_γ' for the Nd sublattice, using Bertaut's notation, resulting in competing ferromagnetic and antiferromagnetic superexchange interactions on the Mn sublattice along the [010] direction of the unit cell. Both materials are highly insulating and neither displays CMR, with resistivity values in each being reduced to just 96% of the zero field value at 200K, in a field of 14T.

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[Truncated abstract] In the past decade, the synthesis of superparamagnetic iron oxide nanoparticles (SPIONs) has received considerable attention due to their potential applications in biomedical fields. However, success in size and shape control of the SPIONs has been mostly achieved through organic routes using large quantities of toxic or/and expensive precursors in organic reaction medium at high reaction temperature. This has limited the biomedical applications of SPIONs and therefore, development of a synthetic method under aqueous condition that is reproducible, scalable, environmentally benign, and economically feasible for industrial production is of paramount importance in order to fully realise their practical applications. Spinning Disc Processing (SDP) has been used to synthesise superparamagnetic magnetite (Fe3O4) nanoparticles at room temperature via a modified chemical precipitation method under continuous flow condition and offer a potential alternative to be applied to SPIONs production. SDP has extremely rapid mixing under plug flow conditions, effective heat and mass transfer, allowing high throughput with low wastage solvent efficiency. The synthesis process involves passing ammonia gas over a thin aqueous film of Fe2+/3+ which is introduced through a jet feed close to the centre of a rapidly rotating disc (500-2500 rpm). Synthetic parameters such as precursor concentrations, temperature, flow rate, disc speed, and surface texture influence the particle sizes. ... Magnetic silica microspheres are receiving great attention for possible applications in magnetic targeting drug delivery, bioseparation and enzyme isolation. However, the current available methods for preparation suffer from the setback of low loading of Fe3O4 nanoparticles in the silica microsphere, which result in low magnetic moment, thereby limiting their practical applications. Therefore it is of considerable importance to develop new alternative synthetic methods for fabricating magnetic silica microspheres with high magnetic nanoparticles loading. Superparamagentic Fe3O4 nanoparticles (8-10 nm diameter) and curcumin have been encapsulated in mesoporous silica in a simple multiplestep self assembly approach process with high Fe3O4 nanoparticles loading (37%). The synthesis involves loading of curcumin in the Cetyltrimethylammonium bromide (CTAB) micellar rod in the presence of superparamagnetic Fe3O4 nanoparticles via a parallel synergistic approach. The synthesised magnetic mesoporous silica composite material is stable, superparamagnetic with high saturation magnetisation before and after curcumin leaching experiment. Under physiological pH in phosphate buffer, the curcumin is slowly released over several days. These magnetic mesoporous silica are expected to have great potential as targeted drug delivery systems.

Thesis (M. Phil.)--University of Hong Kong, 1999. / Includes bibliographical references (leaves 124-138).

Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2009. / Committee Chair: Z. John Zhang; Committee Member: Angus Wilkinson; Committee Member: C P Wong; Committee Member: E. Kent Barefield; Committee Member: Mostafa El-Sayed. Part of the SMARTech Electronic Thesis and Dissertation Collection.