Nanostructured magnetic materials for sensors and biomedicine

Abstract

In the last twenty years the discovery of new nanostructured materials and nanoscale physical effects have paved the way for several technologies; thanks to these discoveries nanotechnology is now a word used in our everyday language. In the nanoworld the properties of materials are completely different from the bulk and can give rise to new and surprising solutions to problems arising in very different fields, from medicine, to energy production or telecommunications. Nanotechnology allows, for instance, to have smaller, easier to use, more sensitive and lighter sensors. In medicine, the day when nano-vectors and nano-manipulators will become reality is not so far. The research activity presented in this thesis focuses on different magnetic nanostructures, i.e., thin films, nanoparticles and nanocomposites, characterised by physical properties which can fruitfully be exploited in sensors and biomedicine. Novel properties in magnetic nanostructures can arise for a number of reasons, such as confinement of the magnetic material into two- or one-dimensional structures and close proximity with other materials in layered or nanocomposite structures. When specific functional magnetic properties are due to size reduction, an in depth study of the correlation between magnetism and size reduction is mandatory for the nanostructured material optimisation for a given application. For this reason, we have followed the same methodological approach for each studied material: starting from the material design, passing through a complete structural and magnetic characterisation has allowed us, in the most prominent cases, to model the magnetic properties and give an answer to application requirements. This thesis is made up of six chapthers. In the first chapter basic elements of magnetism and magnetic materials are recalled, which are useful to understand the experimental results. In particular the attention is on the effect of size reduction on magnetic phenomenology and magnetisation processes, on surface effects in thin films and nanoparticles and on the effect of interactions between magnetic nanoparticles. In the second chapter the experimental techniques used to characterise thin films and nanoparticles are shown. In the third chapter the main results obtained on amorphous thin films, showing soft magnetic properties, are reported. After a detailed investigation of the correlation between growth parameters and film properties, these have been optimised for exploiting the thin film as magnetic core in flux-gate and hall sensors. Chapter four presents the results on Iron oxide nanoparticles, which, after suitable surface functionalisation, have been studied as magnetic hyperthermia mediators and for magnetic separation. The mere effect of size reduction is here accompanied by other effects, such as aggregation, nanoparticles interactions, exchange-bias effect in core-shell nanoparticles, which enrich the systems phenomenology. Finally, chapter five summarises the results obtained on composite and multifunctional nanostructures, characterised by different functional properties. In particular, the superparamagnetic properties of magnetite nanoparticles are coupled to the semiconducting properties of Zinc oxide tetrapods and Silicon carbide nanowires. At the end of this chapter the results obtained on nanodisks of Ni-Mn-Ga Heusler alloy, which is an intrinsically multifunctional material, are reported.