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Title: Magnetism and magnetotransport in Fe and Co nanocluster assemblies
Author: Maher, Matthew John
Awarding Body: University of Leicester
Current Institution: University of Leicester
Date of Award: 2002
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Vibrating sample magnetometry showed that dilute assemblies of unfiltered Fe and Co nanoclusters well above their blocking temperature are superparamagnetic and the magnetisation curves follow the Langevin function with a tight size distribution around a mean of 2.2 nm. Magnetisation curves of blocked non-interacting particles showed a small coercivity and a remanence value consistent with a uniaxial anisotropy in the particles. At low temperature, the films assume an asperromagnetic state and fits to this model indicated an enhanced anisotropy constant by a factor of ten over the bulk. Cluster assembled films with packing densities up to 30 at % concentration are described by the Langevin function with an increased mean particle size. As the packing density is increased further, the behaviour deviates from the super-paramagnetic model towards a correlated spin glass. At low temperatures the anisotropy energy of the particles is greater than the exchange energy and the sample is asperromagnetic. In pure Fe cluster assembled films (~500 A) thick capped by Ag a transition, as the temperature rises from 2 to 300K, from an asperromagnetic state to a correlated spin glass occurs. Uncapped Fe cluster films displayed only correlated spin glass behaviour even at 2K. Giant magnetoresistance has been observed in the cluster assembled systems Fex-Ag1-x, FexCu1-x and CoxAg1-x and investigated for varying cluster packing densities. The results were compared to models formulated for granular alloys and were found to be in good agreement for < 16% cluster films. As the cluster interactions are increased, the peak of the GMR, at H~0, was found to be quenched due to the magnetic correlations among the particles. Co cluster films created with Ag or SiO capping layers displayed a ferromagnetic - antiferromagnetic interaction caused by partial oxidation of the clusters. Magnetometry of an uncapped Co cluster film held in UHV has shown spin glass behaviour at 2K as found for Fe cluster films.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available