Branford, William Richard; (2002) Relationship of structural and electronic properties in transition metal oxides. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

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.

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