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Title: Relationship of structural and electronic properties in transition metal oxides
Author: Branford, William Richard
ISNI:       0000 0001 3477 4512
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 2002
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This thesis is an investigation into the relationship between the structural and electronic properties of quaternary manganese oxides, of the spinel structure, with general formula AI[BxMnIv(2-x)]O4 (x=0.5, 1; A=Li, Cu; B=MgII, NiII, CrIII, FeIII, CoIII, RhIII) and the series Cu1-y Ni0.5+y Mn1.5 O4 (0≤y≤1). Particular emphasis is placed on the interplay between cationic distribution, and sublattice ordering, and the observed magnetic and magnetotransport properties. Colossal magnetoresistance CMR, a large change in the electrical resistance upon application of a magnetic field, is associated with a paramagnetic-ferromagnetic transition. It is usually found in mixed valence MnIII/MnIv perovskites an considered with a double-exchange mechanistic framework. The mechanism was shown to be more complex by the CMR pyrochlore, Tl2Mn207 which contains manganese in the integral oxidation state, four, precluding double-exchange. The array of B cations in the spinel structure AB2O4 can be described as a pyrochlore network, thus spinels with magnetic ions solely on the B sites can be considered to have a pyrochlore magnetic network. This offers an interesting analogy with the CMR pyrochlore thallium manganate. This thesis reports the pursuance of this analogy by the synthesis and characterisation of a series of spinels containing MnIv on the octahedral sites. These materials show unusual magneticbehaviour; they undergo spontaneous magnetization below a well defined transition temperature, but the saturation magnetization is significantly smaller than predicted, and the low-field magnetization is time dependent. These observations are interpreted as being indicative of a semi-spin-glass magnetic structure, composed of an ordered ferrimagnetic component along one of the (equivalent) crystallographic axes and a disordered, glassy component perpendicular to this axis.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available