Self-assembled superstructures of magnetic nanoparticles: advanced nanofabrication and enhanced mechanical properties

Abstract

Artificial materials composed of self-assembled magnetic nanoparticles (i.e. magnetic superstructures) have attracted immense scientific interest in the past two decades due to their extraordinary collective behavior. An assembly of such ordered nanoscale building blocks results in synergistic effects endowing the superstructured material with unprecedented properties which can be tuned by the material type, size, shape and packing of the building blocks. Moreover, the magnetic properties of nanoparticles lead to novel self-assembled superstructure morphologies, such as rods, stripes and helices. Such attractive and tunable properties have led to the realization of applications ranging from nanoelectronics to nanomedicine. However, certain aspects of the collective behavior of post-assembled magnetic superstructure systems remain widely unexplored to date, a point which is vital to address in order to realize the full potential of this exciting and promising class of materials. This thesis aims to investigate new collective properties of magnetic superstructures composed of ferrite nanoparticles, namely the link between magnetic and mechanical properties, and behavior during exposure to a focused ion beam.

Fabricating advanced materials from self-assembled superstructures as starting points is an aspect that has been highly unexplored. During ion beam milling of these assemblies, it is demonstrated how collective merging of nanoparticles, as a result of partial milling and melting, results in a new way of fabricating nanoporous materials, which is a very attractive class of materials with potential use in numerous application areas. This forms the basis of the development of a new generic methodology for the fabrication of templateless and robust nanoporous structures with tunable porosity and independent of materials choice. It is further demonstrated, for the first time, how nanoscale magnetism in self-assembled magnetic superstructures leads to enhanced mechanical properties (i.e. increase in mechanical stability). The magnetic anisotropy of the nanoparticles, mediated by dipolar interactions, can be used as a means of modulating the mechanical properties, and yields, under the right conditions, reconfigurable mechanical anisotropy controllable by an applied magnetic field. Such smart magneto-mechanical systems are also shown to exhibit “super-magnetostriction”. Furthermore, a novel “super-size effect” is discovered in nanoparticle-based superstructures, indicating a destabilization as the overall superstructure size (i.e. number of particles) decreases, analogous to the familiar size effect in single nanoparticles. This opens up for size-controlled tuning of collective properties, expected to impact many areas of research.