Magnetite and maghemite : crystallography, Mössbauer spectral analysis and application to hyperthermic cancer therapy

This thesis describes studies into, firstly, a crystallographic examination of the magnetic iron oxide maghemite, y-Fe203, and secondly, applications of both magnetite, Fe^O^, and maghemite to hyperthermic cancer therapy. Maghemite has a complicated crystal structure because of the incorporation of vacancies to maintain charge neutrality. These vacancies can be either ordered or disordered, and their effect on the detailed interpretation of many physical measurements is not well understood. One of the results of this varying order is that the results of physical measurements becomes sample dependent. Some simple results of this are the assignment of four different space groups to structures with different order and the observation of different Mossbauer spectra, some of which are symmetrical about the mid point and most of which are not. To investigate this, experimental and simulation techniques were used to produce simulated Mossbauer spectra of vacancy-disordered and vacancy-ordered maghemite. Three samples of maghemite were investigated using infrared spectroscopy and Xray diffraction, and it was found that each of these experimental techniques allowed the determination of the vacancy-ordering regime of each sample. The same three samples were analysed using NMR (nuclear magnetic resonance) to determine the magnitude and distribution of their magnetic hyperfine fields. All samples revealed a broad distribution of hyperfine fields, with the vacancy-disordered sample displaying a significantly broader distribution of hyperfine fields than the vacancy-ordered samples. A significant determination from the NMR spectra was the allocation of hyperfine field values to all iron sites within both vacancy-disordered and vacancy-ordered maghemite using the crystallographic structure and site occupancies of the two vacancy-ordering regimes. A point charge model was implemented for both vacancy-disordered and vacancy ordered maghemite, and this predicted a large range of quadrupole splitting values for both vacancy-ordering regimes. It was shown that relatively small perturbations of the crystal lattice will have an appreciable effect on the magnitude, distribution and sign of the quadrupole splitting, as well as on the spherical polar angles of the magnetic hyperfine field direction in the electric field gradient principal axis system. The key finding of this investigation was that the quadrupole splitting should be considered when fitting Mossbauer spectra of maghemite, and not assumed to be zero, as has been done with most previous results in the literature. The experimentally determined magnetic hyperfine field values were combined with the modelled quadrupole splitting values to produce simulated Mossbauer spectra for both xiii Abstract vacancy-disordered and vacancy-ordered maghemite. A comparison of simulated Mossbauer spectra to experimental spectra for both vacancy-disordered and vacancy-ordered maghemite found the magnitude of the quadrupole splitting was overestimated, while the distribution of quadrupole splitting values was underestimated. A further significant finding was that the distribution of hyperfine fields observed in the NMR and the experimental Mossbauer spectra was partly due to inhomogeneities caused by vacancy distributions and partly due to magnetic dipole fields. Finally, the technique of ultrasonic atomisation was investigated for use in producing microspheres containing either magnetite or maghemite for possible applications to hyperthermic cancer therapy. Parameters influencing both the atomising solution and atomisation hardware were examined across a selected range of the parameter space, and a suitable parameter set selected to produce PLLA (poly(L-lactic acid)) microspheres in the required size range of 20 to 45 pm. Microspheres that encapsulated nanoparticles of both magnetite and maghemite within the PLLA bulk material were then manufactured, and their magnetic hysteresis properties assessed using a vibrating sample magnetometer. Additionally, a model for ideal magnetic microspheres for hyperthermic cancer therapy was proposed, taking into consideration the optimum size of single-domain magnetite and maghemite particles for maximum hysteresis loss, the saturation magnetisation of each of these two magnetic iron oxides, and the spacing required of the magnetic particles for the magnetic dipole interaction energy to equal the anisotropy barrier energy.