Phillips, Peter John, 1961- (1991) Conduction processes in spinel ferrites. PhD thesis, University of Warwick.

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Abstract

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.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics Q Science > QE Geology
Library of Congress Subject Headings (LCSH):	Spinel group, Ferrites (Magnetic materials), Ferrites (Magnetic materials) -- Thermal properties
Official Date:	July 1991
Dates:	Date Event July 1991 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Whall, Terry E.
Sponsors:	Science and Engineering Research Council (Great Britain)
Format of File:	pdf
Extent:	xviii, 95 leaves : illustrations
Language:	eng

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