Paramagnetic defects in free standing polycrystalline diamond films made by chemical vapour deposition (CVD) have been studied using electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR) and infrared absorption. EPR experiments at a range of frequencies (1-35 GHz) confirm the ¹H hyperfine parameters for the recently identified H1 defect (Zhou et al., Phys. Rev. B, 54:7881 (1996)). In the samples studied here, H1 is always accompanied by another defect at g=2.0028(1). Saturation recovery measurements are consistent with two defects centred on g=2.0028. The spin-lattice relaxation rate of H1 is a factor of 10-100 times more rapid than the single substitutional nitrogen centre (N⁰_S), which is known to be incorporated into the bulk diamond. ¹H matrix ENDOR measurements indicate that the H1 centre is in an environment with hydrogen atoms 2-10 A distant from the centre. The near neighbour hydrogen identified by the EPR was not detected in the ENDOR experiments. The concentration of H1 correlates with the total integrated C-H stretch absorption in the samples studied here. All the evidence is consistent with H1 being located at hydrogen decorated grain boundaries (or in intergranular material) rather than in the bulk diamond. The affect of annealing the films in vacuo up to 1900 K has been studied. On annealing at 1700 K it was found that some of the hydrogen on internal grain boundaries became mobile but was not lost from the sample, and the intensity of the EPR absorption at g=2.0028 decreased. Annealing at 1900 K severely degraded the optical properties of the samples, and a new defect with g=2.0035(2) was created. Infrared measurements show that hydrogen is lost from most CVD diamond samples when annealed to 1900 K for four hours. An EPR imaging (EPRI) probe was designed and built. This comprised a 3-loop, 2-gap loop-gap resonator and a pair of anti-Helmholtz coils providing a magnetic field gradient ∂B_z/∂z. Using this probe the distribution of N⁰_S was measured in the growth direction of four CVD diamonds to a resolution of 20 μm. The distribution of N⁰_S is shown to be different to the distribution of defects with g=2.0028. Two-dimensional images of the spin density of N⁰_S in single crystal type Ib diamonds made by the high temperature and pressure (HTP) method have been generated, demonstrating a resolution of 100 μm. A two-dimensional image of the spin density of g=2.0028 defects in a CVD sample is compared to a photograph of the same sample, showing the correlation between the distribution of the defects with the distribution of non-diamond material in the sample. The distribution of the [N-N]⁺ defect in a natural diamond has been examined using ∂B_z/∂B_ϰ field gradient coils.

PrBa₂Cu₃O₆₊_x is anomalous among the compounds which can be made by substituting different rare-earth ions for yttrium (Y) in YBa₂Cu₃O₆₊_x:it is the only compound which has the same structure as YBa₂Cu₃O₆₊_x, and yet does not superconduct. This unusual property makes it an important system to study, since the differences between the two compounds could produce theoretical insights into the mechanisms of cuprate, or high-temperature, superconductivity. This thesis describes neutron scattering investigations of the magnetic properties of PrBa₂Cu₃O₆₊_x. I have investigated the magnetic ordering in well-characterised, single-crystal samples, both with and without Al-contamination. In the data analysis, proper account is taken of the sample mosaicity and the different intrinsic peak shapes encountered. Some of the most striking new characteristics reported here are that (i), there is a clear interaction between the Pr ions and the Cu-O₂ planes, (ii), the ordered Pr moments tilt away from the c-axis (as suggested previously by Mossbauer spectroscopy), and (iii), there is two-dimensional ordering in the Al-containing crystals. I show how observed trends in the magnetic ordering, as O- and Al-contents vary, may be understood in terms of charge redistribution involving the hybridisation of the Pr ions. I have also examined the crystal field transitions of the praseodymium ion. Because of the difficulties previously encountered in measuring these in PrBa₂Cu₃O₆₊_x, I have used instead the compounds PrO₂ and Y_0.3Pr_0.7Ba₂Cu₄O₈, which contain praseodymium in a chemically similar environment. Y_0.3Pr_0.7Ba₂Cu₄O₈ shows the same broadening effects as PrBa₂Cu₃O₆₊_x; PrO₂ shows signs of mixed-valency, with temperature-dependent lifetime effects. The application of symmetry considerations and the point-charge model to the crystal field measurements is considered in some detail.

This thesis describes Nuclear Orientation and radiofrequency magnetic susceptibility experiments on the enhanced nuclear antiferromagnet holmium vanadate (HoVO₄) from 1 K down to about 1 mK and in applied fields of up to 2.4 tesla. Adiabatic demagnetisation of single crystals of HoVO₄ allowed a mean nuclear spin temperature of about 1 mK to be reached. R.f. magnetic susceptibility measurements at approximately 2.4 mK and in an applied field of about 1.5 x 10^-2 T identified the spin 'flop' phase of the nuclear anti-ferromagnet HoVO₄. The Néel temperature T_N was determined to be 4.0 (2) mK which is in good agreement with the value of 4.8 mK calculated for the dipole-dipole interaction of the enhanced magnetic moments at the Ho site. Gamma-ray anisotropy measurements of ^166mHoVO₄ showed that in low applied fields (B ≮ 0.5 x 10^-2 T) and temperatures of about 1 mK the antiferromagnetic domains were uniformly distributed in the a-a' plane of the tetragonal zircon crystal structure. This was interpreted in terms of pinning of the nuclei due to crystalline imperfections. The spin 'flop' phase was identified in an applied field of 1.2 x 10^-2 T which is in good agreement with the susceptibility measurements. Demagnetisation experiments down to the relatively high residual fields of 4.2 x 10</sup>-2</sup> T and 7.4 x 10</sup>-2</sup> T and temperatures of about 2 mK showed that the effective field at the nucleus was much smaller than the applied field thus indicating that there was some antiferromagnetic ordering even at these relatively high fields. The magnetic dipole moment of ^166mHo was estimated to be 3.60 (6) μ_N from gamma-ray anisotropy measurements of some gamma transitions of known NO parameters. This value is in good agreement with the value of 3.69μ_N calculated for the Nilsson configuration [6337./2]_n [5237/2]_p for the extra-core neutron and proton respectively. Mixing ratio (δ) measurements for gamma-ray transitions between the (K^π = 2⁺) γ-vibrational and (K^π = 0⁺) ground state rotational bands of the daughter nucleus ¹⁶⁶Er showed that Ml transition probabilities are predominantly via ΔK = 1 band mixing which is in good agreement with systematics for deformed nuclei in this mass region. Nuclear refrigeration experiments on a powdered compressed HoVO₄ - copper pill allowed a lattice temperature of 3.9 (3) mK to be reached. Thermal contact measurements on a similar pill showed that the effective area of contact between the salt particles and copper was only about 30 cm² which was a small fraction of the total surface area available. Preliminary work on a HoVO₄ - gold pill enabled a ⁶⁰Co ̲C̲o thermometer to be cooled to 6.0 (l) mK.

Powder neutron diffraction and magnetic susceptibility measurements of the antiferromagnetic phases of Mn_xNi_1-xO,Mn_xCo_1-xO, (Mn_xFe_1-x)_zO and (Co_xFe_1-x)_zO reveal that the magnetic moments of unlike ions are always effectively collinear, despite the presence of competing anisotropies. The magnetic moments of Mn_xNi_1-xO (x = 0.24,0.48 and 0.77) at 5K are confined to (111) planes by dipole-dipole forces, and the small trigonal exchangestrictions are the products of opposed antiferromagnetic Mn²⁺-Mn²⁺ and ferromagnetic Ni²⁺-Mn²⁺ nearest neighbour interactions. In Mn_xCo_1-xO (x = 0.05, 0.10, 0.25,0.36) at 5K, the orbital degener- acy of Co²⁺ is removed by both Jahn-Teller (J.T) and spin-orbit coupling (S.O) mechanisms, leading to orthorhombic or triclinic symmetries and partially quenched Co²⁺ moments. Monoclinic symmetry is observed for 0.51<x<0.67, and Jahn-Teller stabilisations and spin-only Co²⁺ moments are evident. The cobalt concentration is too small to support a cooperative J.T stabilisation in Mn_0.83Co_0.17O, where a magnetostriction (c/a < 1) is observed (in agreement with previous AFMR results). Previous results for Co_xNi_1-xO, and those for Mn_xCo_1-xO, (Mn_xFe_1-x)_zO (x = 0.05, 0.1, 0.12, 0.23, 0.36,0.56,0.66,0.89) and (Co_xFe_1-x)_zO (x = 0.04,0.12,0.50,0.63,0.81) at 5K indicate that the anisotropy order for the iron group monoxides is CoO>MnO»Fe_zO≈NiO. The weak trigonal anisotropy of Fe²⁺ correlates with the near-cubic symmetries of (Co_xFe_1-x)_z( (x = 0.04,0.12) and (Mn_xFe_1-x)_zO (0.1<x<0.66), and a tetragonal (c/a>1) magnetostriction of Fe²⁺ is observed in (Co_xFe_1-x)_zO with x > 0.5. Measurements of vacancy-ferric interstitial ratios for (Mn_xFe_1-x)_zO and (Co_xFe_1-x)_zO suggest that non-stoichiometry is accommodated by 6:2 or 8:3 defect clusters in the former, and by larger units in the latter. The observed magnetic moments of defective samples are normally larger than those calculated according to a previous model for Fe_zO, and require the postulation of partial antiferromagnetic order around the clusters. The room temperature Mossbauer effect parameters of (Mn_xFe_1-x)_zO (0<x<0.975,0.910<z<1.0) indicate that while Fe²⁺ and Mn²⁺ ions are randomly distributed over octahedral sites, ferric ions are localised around defect clusters.

The conduction band of InAs lies lower in energy than the GaSb valence band. In order to preserve continuity of the Fermi level across the interface, charge transfer takes place resulting in a confined quasi two dimensional electron gas (2DEG) in the InAs and a confined quasi two dimensional hole gas (2DHG) in the GaSb. This study is an investigation into the vertical transport in an n-InAs/p-GaSb single heterojunction (SHET). Application of a forward bias (InAs negative with respect to GaSb) increases the 2DEG and 2DHG concentrations and, therefore, their confinement energies. Eventually a critical bias is reached where the electron confinement energy moves above the hole confinement energy (the theoretical voltage induced semimetal/semiconductor transition V_c). Any subsequent increase in voltage is expected to result in a current decrease, and a region of negative differential resistance (NDR) should occur. The SHET can be grown with two distinct interface types, 'InSb-like' and 'GaAs-like'. This in turn affects the vertical transport characteristics of each type. Experimental IV traces at various pressures are compared with the corresponding results from sophisticated self-consistent band profile calculations taking into account band mixing effects for the first time through a k.p theory framework. Experimental IV traces of the SHETs under a magnetic field parallel to the interface are also compared with results from calculations that take into account the coupling of the growth and in-plane electron and heavy hole motions. Both sets of analysis support earlier conclusions that NDR occurs after V_c for both interfaces, and that each interface supports a different conduction mechanism. Evidence of multiple phonon processes occurring in both sample types is observed for the first time and is proposed to reconcile the above experimental observations with theory. This data is found to offer explanations for a number of other observations. Field perpendicular to the interface leads to the observation of features beyond the NDR region in both sample types. In samples with an 'InSb-like' interface, applying additional hydrostatic pressure leads to very strong features beyond the main NDR. Through a complex self-consistent decoupled model taking into account electrons and heavy holes, all these features are proposed to be due to a filling of an integer number of Landau levels. The band profile is predicted to alter dramatically at this point. The same model explains the observation of weaker features at 1 bar at high fields (~ 40T). A variation of NDR position is found with a rotation of an-plane field.

The magnetic spinel ferrite nanoparticle is exceptionally intriguing nanocrystal system due to the industrial importance of various technical applications and the scientific significance of studying the quantum origin of magnetism. Studies of quantum influences upon magnetic properties have revealed that the spin-orbit coupling and the net magnetization greatly affect the net magnetic properties of each spinel ferrite system differently. In case of cobalt ferrite where spin-orbit coupling is relatively large, increasing Cr3+ doping concentration, which has smaller magnetic moment and zero angular moment, decreases blocking temperature, saturation magnetization, remnant magnetization and coercivity. However, in case of manganese ferrite where spin-orbit coupling is relatively small, increasing Cr3+ doping concentration, reduces all the magnetic parameters except coercivity. The coercivity increases due to smaller magnetocrystalline anisotropy energy constant which forces the coercivity to increase as saturation magnetization decreases in accordance with Stoner-Wohlfarth theory. In order to improve product quality and quantity, synthesis routes in hot oleylamine and aminolytic reaction were developed. Both methods were proven to be extremely effective, environmental friendly, inexpensive, and simple routes in the synthesis of a variety of spinel ferrite systems including CoFe2O4, MnFe2O4, NiFe2O4, and ZnFe2O4 from a single source metal precursor.

Thesis (Ph.D.) OGI School of Science & Engineering at OHSU, June 2008. / Includes bibliographical references.

Thesis (Ph.D.)--Materials Science and Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Snyder, Robert; Committee Co-Chair: Wang, Zhong Lin; Committee Member: El-Sayed, Mostafa; Committee Member: Milam, Valeria; Committee Member: Summers, Christopher; Committee Member: Wong, C. P.

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Kenneth A Cunefare; Committee Member: Christopher Lynch; Committee Member: Massimo Ruzzene; Committee Member: Nader Sadegh; Committee Member: Reginald DesRoches. Part of the SMARTech Electronic Thesis and Dissertation Collection.

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