Browsing by Subject "Phase separation"

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    Magneto-electronic phase separation in doped cobaltites.
    (2009-09) He, Chunyong
    This thesis work mainly focuses on magneto-electronic phase separation (MEPS), an effect where chemically homogeneous materials display inhomogeneous magnetic and electronic properties. A model system La1-xSrxCoO3 (LSCO) is chosen for the study of MEPS. The doping evolution of MEPS in LSCO single crystals is extensively studied through complementary experimental techniques including heat capacity, small angle neutron scattering, magnetometry, and transport. It is found that there exists a finite doping range over which MEPS occurs. The doping range determined from different experimental techniques is found to be in good agreement. Also, this same doping range is reproduced by statistical simulations incorporating local compositional fluctuations. The excellent agreement between experimental data and statistical simulations leads to the conclusion that the MEPS in LSCO is driven solely by inevitable local compositional fluctuations at nanoscopic length scales. Such a conclusion indicates that nanoscopic MEPS is doping fluctuation-driven rather than electronically-driven in LSCO. The effect of microscopic magneto-electronic phase separation on electrical transport in LSCO is also examined. It is demonstrated (i) that the T = 0 metal-insulator transition can be understood within double exchange-modified percolation framework, and, (ii) that the onset of a phase-pure low T ferromagnetic state at high x has a profound effect on the high T transport. In addition, a new origin for finite spin Co ions in LaCoO3 is revealed via a Schottky Anomaly in the heat capacity, which was not previously known. Such a discovery casts a new understanding of the spin state at low temperature. Via small-angle neutron scattering and d.c. susceptibility, it is revealed that short-range ordered FM clusters exist below a well-defined temperature (T*) in highly doped LSCO. It is demonstrated that the characteristics of this clustered state appear quite unlike those of a Griffiths phase. Finally, through magenetometry and SANS, the magneto-crystalline anisotropy of highly doped LSCO is studied and the easy and hard magnetization axes are determined.
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    The role of iron and manganese in elucidating the temperature of subseafloor hydrothermal reactions: insights from experimental and field data
    (2012-11) Pester, Nicholas Jon
    Field/experimental investigations demonstrate the chemistry of mid-ocean ridge hydrothermal fluids reflects fluid-mineral reaction at temperatures higher than typically measured at the seafloor. In order to better constrain sub-seafloor hydrothermal processes, we have developed an empirical geothermometer based on the dissolved Fe/Mn ratio in high-temperature fluids. Using data from basalt alteration experiments, the relationship; T(°C) = 331.24 + 112.41*log[Fe/Mn] has been calibrated between 350 °C and 450 °C. The apparent Fe-Mn equilibrium demonstrated by the experimental data is in good agreement with natural vent fluids, suggesting broad applicability. When used in conjunction with constraints imposed by quartz solubility, associated sub-seafloor pressures can be estimated for basalt-hosted systems. This methodology is used to interpret new data from 13°N on the East Pacific Rise (EPR) and the Lucky Strike Seamount on the Mid-Atlantic Ridge where high-temperature fluids both enriched and depleted in chloride, relative to seawater, are actively venting within a close proximity. Accounting for these variable salinities, phase separation is also a dominant process affecting the chemistry of hydrothermal fluids; and vapor-liquid partition coefficients for accessory metals (Na-normalized as NaCl dictates phase equilibria) have therefore been experimentally derived for temperature between 360 and 460 °C, where those > Na represent increasing affinity for the liquid phase. These data reveal the relationship Cu(I) < Na < Fe(II) < Zn < Ni(II) ≤ Mg ≤ Mn(II) < Co(II) < Ca < Sr < Ba and a dependence on the cationic radii. The depth below seafloor of the magmatic heat source at Lucky Strike is greater than that of faster spreading systems such as EPR 9-13°N, which host higher temperature vents with greater transition metal concentrations. The combined experimental/field data suggest the resulting increased residence time in the up-flow zone allows re-equilibration at temperatures lower than those resulting in phase separation. Fluid chemistry measured in the immediate aftermath of eruptions at EPR 9-10°N conversely shows large deviations from Fe/Mn equilibrium consistent with partitioning due to phase separation. The eruptive data also exhibit that P-T conditions were sufficiently extreme that Fe is behaving in a volatile manner observed in the experiments and in some instances subseafloor halite saturation may have occurred.