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Identifier

000448924

Title

Ρύθμιση απόκρισης νανοκρυστάλλων οξειδίων του σιδήρου : έλεγχος διαταραχών δομής και μαγνητικά οδηγούμενη λειτουργικότητα

Alternative Title

Optimization of iron oxide nanocrystals' response: structural disorder control and magnetically-mediated functionalities

Author

Αντωναρόπουλος, Γεώργιος Λ.

Thesis advisor

Τρικαλίτης, Παντελής
Λάππας, Αλέξανδρος

Reviewer

Αρματάς, Γεράσιμος
Βασιλικογιαννάκης, Γεώργιος
Δημάδης, Κωνσταντίνος
Μήλιος, Κωνσταντίνος
Φρουδάκης, Γεώργιος

Abstract

The presentthesisexplores the possibilities forsynthesis, shaping and chemical modification of colloidal, core-shell, iron oxide nanocrystals, in the sub-20 nm size range. It is mainly an in-depth study of the structure of the fabricated nanocrystals and the correlation between their morphological and structural characteristics and their magnetic behavior. The studied particles consist of anantiferromagnetic (AFM) core, adopting a rock-salt crystal structureand a ferrimagnetic (FiM) shell of spinel structure. This coexistence in a magnetic heterostructureis intentional, sincethe macroscopic interaction betweenthetwomagneticphases (AFM-FiM) via exchangespininteractions at the interface of the two distinct crystallographic structures, leads to a technologically exploitable phenomenon, known as exchange bias.The potential use of such systems as heat mediators in an important biomedical application, magnetic hyperthermia, is being evaluated among others. Magnetic hyperthermia is a promising therapeuticapproach for treatment of cancer tumors. A systematic effort was made to understand the mechanisms by which the magnetically induced thermal response can be controlled, mainly by adjustingthe nanocrystal shape, structure and chemical composition. In order to optimize this response, careful and fine tuning of magnetization and magnetic anisototropyhasto be achieved. Anisotropy could be introduced in the system, in the form of shape andsurface anisotropy, exchange anisotropy, due to exchange interactions at interfaces and magnetocrystalline anisotropy. The way in which atomic,crystal structural defects act as spin pinning centers due to uncompensated spins,is being studied in detail.Structural defects in these systems are mainly related to vacant crystallographicmetal sitesin the core and the shell (octahedrally or tetrahedrally coordinated metal ion sites in the spinel shell) and their populations varydepending on the particle size and shape. Thesepinning centers introduce additional exchange anisotropy in the system and surprisingly, even in nanocrystal particles of a single magnetic phase (FiM spinel), a considerable exchange bias field has been measured experimentally. The substitution of cobaltforiron (at different substitution rates), apart from the expected increase ofthe magnetocrystallineanisotropy, highlighted a possible pathway for controlling the nanocrystal structure, more specifically the core to shell volumetric ratio, the population of vacant crystallographic sites and their distribution in the core and shell crystallographic phases. A powerful tool has been used for this structural study, namely the novel Pair Distribution Function (PDF) analysis ofexperimental data acquired from synchrotron-based X-ray total scattering.Monte Carlo simulationsbased on theoretical models,combined with the experimentally observed trends, were also considered,to fully understand how the nanocrystalline structure affects the particles’magnetic behavior. Deep understanding of the mechanisms involved in the effort to tune the magnetic response of such nanocrystals, paves the way for designing similar nanocrystalline systems with controlled functionalities.Motivated especially bythe potential use of these particles as magnetic hyperthermia agents, the desirable morphological and structural characteristics of such nanocrystalline core-shell systems,with optimized magnetically-mediated heat performance, are being reportedas well.

Language

Greek

Subject

Cobalt ferrites

Colloidal magnetic nanoparticles

Core-shell nanocrystals

Crystal structure distortions

Defects

Exchange bias

Iron oxides

Magnetic anisotropy

Magnetic hyperthermia