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No description available.

We present a study of the synthesis, structural and magnetic properties of bulk and nanosized (Zn, Cd, Cu)0:5Ni0:5Fe2O4 and NiFe2O4 compounds. The e®ects of electronic con¯guration and atomic sizes of Zn, Cd, Cu and Ni on the magnetic properties of the ferrites are the primary focus of the study. Di®erent synthesis routes, preparation conditions and how they a®ect single phase formation are explored. The synthesis was undertaken by solid{state reaction, combustion, hydrothermal and glycothermal techniques. The structure determination was by Xray di®raction. The magnetic measurements were performed using MÄossbauer spectroscopy (from 79 K to about 850 K) and a vibrating sample magnetometer (at about 300 K). The bulk densities of the sintered pellets were deduced by Archimedes principle. The bulk oxides were produced by solid{state reaction and combustion techniques. Fine powders with grain sizes of about 10 nm were produced from bulk compounds by a Retsch planetary ball mill and by the hydrothermal and glycothermal processes. The e®ects of the applied pressure used to make pellets (related to green density of the raw pellets) and the sintering temperature on the properties were investigated. An anomalous variation of bulk densities of (Zn, Cd)0:5Ni0:5Fe2O4 oxides with increase in pelletizing pressure was observed which appears to suggest evidence for trapped porosity. Di®erent states of pelletizing the samples appear to be related to a systematic change of the hyper¯ne ¯eld distributions derived from the MÄossbauer spectra. The temperature dependence of the magnetic hyper ¯ne ¯elds at tetrahedral (A) and octahedral (B) sites were observed to vary with temperature according to the equations Bhf (T) = Bhf (0)[1 ¡ (T=TC)n]¯n where n = 1 (based on the Landau{Ginzburg theory) and n = 2 (based on the Stoner theory). The equation Bhf (T) = Bhf (0)[1¡(T=TC)2]¯2 appears to ¯t the hyper¯ne ¯eld data over a wider temperature range. The Zn{ and Cd{based oxides were found to be ferrimagnetic with Curie temperature TC = 548 § 3 K (measured by zero velocity technique). The Cu{based compound exhibited antiferromagnetic behavior with a magnetic transition temperature of 825 § 3 K. The di®erence in behavior between Zn{, Cd{ and Cu{based compounds is due to di®erence in electronic con¯guration and atomic or ionic sizes. The stronger magnetic coupling between spins in the Cu{based sample can be explained by the presence of RKKY interactions in addition to superexchange interactions. The larger ionic size for Cd appears to favour smaller grain sizes in Cd{based oxides. An anomalous increase in TC is obtained in the Zn0:5Ni0:5Fe2O4 compound with reduction in grain size. This increase in TC is attributed to a distribution of Zn ions on both A and B sites. The MÄossbauer spectra of the milled nanosized samples show a combination of ferrimagnetic and paramagnetic behavior. The coercive ¯eld (HC) at room temperature was found to increase with reduction in grain size (G) according to the equation HC = am+bm=G, which is consistent with multidomain particles. With further reduction in grain sizes, the coercive ¯eld reduced according to the equation HC = as ¡bs=G2. This equation is associated with the onset of single domain particles. The samples produced by hydrothermal and glycothermal processes show evidence of transformation from single domain to multidomain structure with increasing sintering temperature. The ease of single{phase formation in the compounds studied is shown to depend on the technique used to prepare the samples. Single phase formation of the spinel structure was easier to achieve in samples prepared by wet chemical methods because lower sintering temperatures (T < 1000 oC) were required. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2007.

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This thesis discusses the investigation of the two-dimensional S = œ Heisenberg antiferromagnet copper formate tetrahydrate (CFTH) and the layered triangular lattice material NaTi0₂ which was thought to be a candidate for the possession of a non-classical antiferromagnetic ground state due to the frustration inherent in a triangular antiferromagnet. Single crystals of CFTH are investigated using elastic, quasielastic and inelastic neutron scattering, electron spin resonance (ESR) spectroscopy and DC magnetometry. Using the appropriate theory, these measurements are used to determine values for the terms in the Hamiltonian which describes the interactions between the Cu²⁺ ions in this material. The Hamiltonian is compared with that of La₂Cu0₄, and the two are determined to be quite similar. There does not appear to be any evidence for a non-classical ground state in either material. This was proposed to exist in La₂Cu0₄ to account for the superconductivity of doped samples. The difficulties in preparing pure NaTi0₂ have been surmounted, and a technique is described for preparing powder samples with very reproducible structural and magnetic properties. The structural properties have been investigated using X-ray and neutron diffraction, the latter at temperatures between 100 K and room temperature. The magnetic properties have been probed using DC magnetometry and ESR spectroscopy. The sample dependence of the structural and magnetic properties of samples prepared in slightly different ways is discussed. The temperature dependence of the magnetic susceptibility and of the structural parameters is explained by comparison with other oxides and chlorides of trivalent Ti and V, many of which show similar behaviour. The magnetic and structural changes which occur between 200 and 250 K are assigned to changes in the Ti-Ti bonding within the layers, which leads to a transition between two metallic states with subtle differences in their band structures.

This work presents three new classes of 6-oxoverdazyl radicals. Each of these classes of radicals bears a different substituent in the 3-position of the verdazyl ring. These classes include N-heteroaromatic monoverdazyls, oligopyridine diradicals, and ferrocenyl-based verdazyl radicals and diradicals. With the exception of the ferrocenyl-based radicals, these verdazyl radicals have been designed to serve as ligands and the direct metal-radical interactions have been explored. The ferrocenyl verdazyls have been designed to investigate the indirect interactions between iron(II) and the covalently linked verdazyl radical. All verdazyl radicals and precursors were fully characterized and the metal-radical magnetic interactions were investigated where structural characterization was available. A series of bidentate verdazyl radical ligands were prepared and the metal-radical magnetic interactions have been investigated. The magnetic susceptibility data for the octahedral complexes indicates that cobalt(II) couples ferromagnetically (Jco_vd = +95 cm-l) and iron(II) antiferromagnetically (JFe_vd = -66 cm-1) to the verdazyl radical. The nature of these interactions appears to be dictated by orbital symmetry and is consistent with previously reported nickel and manganese verdazyl complexes. This work also demonstrates that imidizole-based verdazyl radicals are effective ligands in tetrahedral copper(I) complexes. Oligopyridine-based diradicals have been designed as ligands, but decompose in solution preventing metal complexation. An attempt was made to construct grids as a higher order molecular structure. To this end, a diradical ligand with the topology necessary to form discrete grid architectures was synthesized, but has not yet demonstrated the ability to coordinate to metal ions. Other monoverdazyl radicals that are symmetrically substituted in the N1 and N5 positions were also investigated as potential grid forming ligands. A series of ferrocenyl verdazyl and methylated ferrocenyl verdazyls were prepared to investigate the electronic and magnetic interactions between the ferrocene and verdazyl electrophores. As evidenced by UV-Vis and electrochemical solution measurements, the two electrophores exhibit mutual electronic perturbations. In the case of the methylated ferrocene derivatives the degree and pattern of methylation appears to have a regiospecific influence on the verdazyl electrochemistry. In the solid state Mössbauer data is consistent with a Fe2+ ground state and there is no evidence of Fe3+ at or below room temperature. Weak antiferromagnetic behaviour (|J| < -13 cm-1) was observed within and between pairs of ferrocenyl monoverdazyls in the solid state. In order to investigate the interactions between radicals separated by an organometallic spacer, a ferrocenyl verdazyl diradical was prepared. The ferrocene diradical demonstrated significant differences between the solid state and solution phase. While the ferrocene diradical and ferrocene monoverdazyl exhibited similar solution electronic properties, the magnetic properties were vastly different. In solution the spins associated with the diradical appeared to be weakly coupled, but in the solid state the diradical has been characterized as a strongly coupled antiferromagnetic π-dimer. This is the first example of a verdazyl π-dimer. The π-dimer appears to be diamagnetic with the lower limit of exchange estimated at Jinter ≈ 2000 cm 1.

The primary focus of this thesis is to study the effect of doping on magnetic properties in amorphous Fe100−xZrx alloys. Samples with compositions of x = 7,11.6 and 12 at.% were implanted with different concentrations of H. Moreover, the samples with a composition of x = 7 at.% were also implanted with He, B, C and N. Magnetic measurements were performed, using SQUID magnetometry and MOKE, in order to compare the as-grown and the implanted films. The Curie temperature (Tc) increases and the coercivity (Hc) decreases, with increasing dopant volume. We also found that Hc increases with temperature for B and C doped samples. Magnetization curves at low temperature validate the presence of non-collinear spin configurations in the as-grown films, which is suppressed after doping, resulting in films with tunable soft magnetic properties. We have also studied the effect of interlayer mixing and finite size effects on FeZr in Fe92Zr8/AlZr multilayer films, and found an anomalous increase of Tc with decreasing thickness. Strain induced changes in the magnetization of an amorphous Co95Zr5 film at the orthorhombic phase transition of the BaTiO3 substrate, was also studied. The results show that structural modifications of the substrate increases the stress and hence changes the magnetic anisotropy in the amorphous Co95Zr5 layer. Finally, the magnetization reversal of Co and CoX heterostructures, with X being Cr, Fe, Ni, Pd, Pt and Ru, has been studied. For this purpose a synthetic antiferromagnet structure, FM/NM/FM, was used, where FM is a ferromagnetic Co or CoX layer and NM is a nonmagnetic Ru spacer layer. The FM layers are coupled antiferromagnetically across the NM layer. For a range of FM layer thicknesses, the exchange stiffness parameter Aex and the interlayer coupling (JRKKY ) of the Co or CoX layers were obtained. This is done by fitting M(H) curves, measured by SQUID magnetometry, to a micromagnetic model. The alloying in CoX resulted in a decreasing Aex and also a reduced MS. The experimental results are in a good agreement with DFT calculations.