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Title: Magnetic properties of the magnetite-spinel solid solution
Author: Harrison, Richard John
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1997
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The intrinsic magnetic properties of Fe-bearing solid solutions with the "spinel" crystal structure are determined to a large extent by the processes of non-convergent cation ordering and subsolvus exsolution. The aim of this dissertation is to investigate the interaction between these processes and the magnetic properties of the magnetite-spinel solid solution, with a view to assessing how these factors might influence the acquisition of natural remanent magnetization in other Fe-bearing solid solutions. Temperature and compositional variations in the state of non-convergent cation order are determined using a macroscopic thermodynamic theory, which is calibrated using cation ordering and phase equilibrium constraints from the literature. The cation distribution in the solid solution is calculated for various temperatures and used to derive the ideal variation in saturation magnetization as a function of composition. A compensation point is predicted at approximately 70 mol% MgA12O4, which is confirmed by experimental measurement of the saturation magnetization in synthetic samples. The magnetic properties of synthetic samples are sensitive to rapid changes in the distribution of Fe2+ and Fe3+ cations which occur during quenching. The kinetics of this ordering process are investigated using the Ginzburg-Landau rate law, which is used to calculate the ordering behaviour during quenching, isothermal annealing and temperature ramp experiments. The calculations show that rapid relaxation of the Fe2+-Fe3+ distribution occurs when intermediate members of the solid solution are heated above 400°C, and there is hysteresis in the degree of order during repeated heating and cooling cycles. Both these effects are confirmed by measurements of magnetic susceptibility versus temperature.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available