This study focuses on investigating the effect of the electronic spin transition of iron on the elasticity of the candidate lower mantle ferropericlase (Mg,Fe)O. This may be relevant to our understanding of the seismic velocity structures of the Earth’s lower mantle. The elastic constants of (Mg₀.₉₂Fe₀.₀₈)O at high-spin (HS) state, low-spin (LS) state, and through the pressure-induced HS-to-LS transition has been measured using both Brillouin Light Scattering (BLS) and Impulsive Stimulated Scattering (ISS). There is a large pressure range in which c₁₁ and c₁₂ exhibit a softening, while c₄₄ does not register such an anomaly. Compared with previously published data of ferropericlase with similar compositions ([Marquardt et al., 2009b], BLS measurement of (Mg₀.₉Fe₀.₁)O and [Crowhurst et al., 2008], ISS measurement of (Mg₀.₉₄Fe₀.₀₆)O), this study provides more reliable elastic constants measurements by taking the advantage of simultaneous measurements on Vp and Vs using both BLS and ISS. Our results show that bulk sound velocity of ferropericlase has a large but smooth softening in the spin transition pressure region. The elastic constants of ferropericlase at the spin transition region and the LS state have been well studied in this thesis, and a relaxation behavior has also been observed in this study. Those two subjects are not well documented in literature. The temperature effect of the spin state transition and its consequential effect on mineral’s elastic properties have not been studied in this project, but further research on this subject will follow. However, even in the room temperature, our results don’t show sudden changes in seismic velocities. Moreover, current theoretical and experimental studies [Sturhahn et al., 2005, Tsuchiya et al., 2006, Lin et al., 2007] indicate that the spin transition takes place over an extended range of depth along an expected lower-mantle geotherm, where sudden changes in compressional and bulk sound velocity are not expected. / text

In this work we studied the magnetic properties referring to the MgFe2O4 nanoparticles precipitated in monocrystalline solid solution of [Mg, Fe]O, with 2.2% iron, obtained at different temperatures and times. These particles grow coherently with the matrix lattice and present shapes of small octahedrons whose diagonals are parallel to the [100] directions. The results presented here refer to samples treated in the temperature range between 400 and 900 º C for 2, 6 and 10 hours. They were analyzed by X-ray diffraction (XRD), magnetization as a function of field (hysteresis curves) and magnetization as a function of temperature (Zero Field Cooled e Field Cooled Warming). It was observed that in samples treated for 6 hours, the system shows increasing values of magnetization remanent and coercive field for temperatures of precipitation from 600 º C, which can be attributed to the precipitation and growth of nanoparticles magnesioferrite matrix. The variation of coercive field with temperature measurement was studied using two different theoretical models. By comparing the remanent magnetization and coercive field of the samples, it was found that the variation of the inversion parameter of magnesioferrite in the studied range of precipitation temperature decreases as reported by other authors. / Neste trabalho foram estudadas as propriedades magnéticas do sistema de nanopartículas de MgFe2O4 obtidas por precipitação em solução sólida monocristalina de [Mg;Fe]O, com 2,2% de ferro, em diferentes tempos e temperaturas de precipitação. Estas partículas crescem de forma coerente com a matriz, exibindo a forma de pequenos octaedros cujas diagonais são paralelas às direções [100] da rede cristalina da matriz, que possui simetria cúbica. Os resultados aqui apresentados são referentes a amostras tratadas no intervalo de temperaturas entre 400 e 900ºC durante 2, 6 e 10 horas. As mesmas foram analisadas por difração de raios X (DRX), magnetização em função do campo (curvas de histerese) e magnetização em função da temperatura (Zero Field Cooled e Field Cooled Warming). Observou-se que, em amostras tratadas durante 6 horas, o sistema apresenta valores crescentes de magnetização remanente e campo coercivo para temperaturas de precipitação a partir de 600ºC, fato que pode ser atribuído à precipitação e ao crescimento de nanopartículas de magnesioferrita na matriz. A variação do campo coercivo com a temperatura de medição foi estudada utilizando-se dois diferentes modelos teóricos e, a partir da comparação da magnetização remanente e do campo coercivo das amostras tratadas em diferentes temperaturas, verificou-se que a variação do parâmetro de inversão da magnesioferrita na faixa de temperaturas de precipitação estudada decresce como reportado por outros autores.