The origin of the magnetism of maghemite (γ-Fe2O3)-based core/shell nanoparticles
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Date
2017
Authors
Skoropata, Elizabeth Marie
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Abstract
The study of core/shell nanoparticles has gained considerable interest due to the potential to engineer interactions at interfaces to provide a rich variety of properties which are not achievable with single phase counterparts. However, an open question remains: What is the precise nature of the core/shell interface and how does it produce the nanoparticle properties? For magnetic systems this is exemplified by interfacial spin disorder and exchange interactions that determine the magnetic coupling between the core and shell. To answer this question necessitated a detailed understanding of interfacial intermixing effects.
In this work, these questions are addressed by describing in detail the relationship between interfacial intermixing, interfacial magnetic interactions, and the overall magnetism of γ-Fe2O3-based core/shell nanoparticles with NiO, MnO and CoO shells. Using element-specific techniques (Mossbauer spectroscopy, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism spectroscopy) interfacial intermixed layers were identified as being formed by transition metal ions from the shell that migrated into the octahedral and tetrahedral sites within the surface layers of the γ-Fe2O3. A detailed analysis of the element and site-specific magnetism revealed a clear relationship between the nature of core/shell intermixing, interfacial spin disorder and exchange interactions that modified directly all aspects of the nanoparticle magnetism (e.g. relaxation, anisotropy, and exchange bias). Furthermore, chemical changes induced by intermixing resulted in a temperature dependent transition that modified the interfacial metal-oxygen bonding and that was correlated with the onset of the exchange bias. By disentangling the element- and site-specific magnetism of each species at the core/shell interface, this work provides a clear description of the origin and the variability of the core/shell nanoparticle magnetism based on a precise understanding of interfacial intermixing, and reveals the critical role of the interfacial superexchange pathway responsible for the magnetism.
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Keywords
Magnetism, Nanoparticles