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Title: Conduction processes in spinel ferrites
Author: Phillips, Peter John
ISNI:       0000 0001 3489 3431
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1991
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An apparatus has been designed, constructed and tested for measurements of resistivity and thermopower, having the following novel features:- I) buffer amplifiers with driven shields to make measurements on highly resistive specimens, II) “in house” software and hardware developed to automate the measurements and, III) a new method of temperature measurement for thermopower studies. Measurements have been carried out on single crystal ZnxFe3_x04 and MnxFe3_x04 with 0 < X < 1 in the temperature range 4.2K-300K. The results strongly suggest that substitutional disorder plays an important role in the electrical transport properties of ferrites. The electrical conductivity for high X near 300K is interpreted in terms of nearest-neighbour hopping. The temperature dependence of G(a7’)=dLn(< 77’)/d( 1/T) between 300K and 100K and the concentration dependence of G(crT) at 100K provides evidence for the formation of a Coulomb Gap at low X. At low temperatures, variable-range and many-electron hopping for high and low X respectively is observed. An estimate of the effective radius of the carrier wavefunction is obtained from the conductivity measurements, which at ~0.4Á is comparable to that previously deduced for nickel ferrite. For all X a peak in the thermopower at about 50K was observed. The peak for high X has been attributed to the high temperature limit of variable- range hopping, while for low X interpreted as the transition from activation across a Coulomb Gap to many-electron hopping. In the high temperature range, the temperature variation is shown to be consistent with degenerate statistics for low X and non-degenerate statistics for high X. In the latter case it is possible to extract the dopant concentrations, which compare favourably with the values found by Electron Probe Micro-Analysis (EPMA). The high temperature thermopower and resistivity behaviour have been used to deduce the variation of the energy width of the conduction states, or bandwidth, with X and this is shown to be consistent with the expected behaviour. A polaron energy of ~0.04eV is calculated at 300K for high values of X, which compares favourably with previous work.
Supervisor: Not available Sponsor: Science and Engineering Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics ; QE Geology