Magnetic Proximity Effects in Nanoparticle Composite Systems and Macrocrystals

Assemblies of magnetic nanoparticles are of major interest for future applications e.g. in spintronic devices, high density data storage systems or biomedical applications. The reason is not only the obvious miniaturization, but also their novel properties emerging only at the nanoscale. Hence, arranging nanoparticles like atoms in a crystal enables the fabrication of a new class ofmaterials. To gain in-depth understanding of these systems, it is necessary to investigate them on all length scales. The present work provides a novel and extensive contribution to the understanding of the self assembly of iron oxide nanoparticle superstructures and their influence on polarizable matrix materials. Through the investigation of the samples at all stages of preparation, a comprehensive picture of the unique phenomena observed at the end is derived. For this purpose, oleic acid coated iron oxide nanoparticles were deposited on silicon substrates by spincoating to manufacture two-dimensional arrangements. Hereby, the influence of several parameters has been investigated and optimized. Afterwards, the organic surfactant shell was removed by oxygen plasma treatment. This process has been studied in detail, because it initiates a phase transformation that significantly influences the magnetic properties of the system (e.g. by reducing the blocking temperature). Thin palladium or platinum films were then respectively deposited to create a matrix material. Aside from magnetometry measurements, first order reversal curves were obtained in cooperation with the Max-Planck-Institute for Intelligent Systems, both revealing that the matrix materials significantly influence the inter-particle interaction and vice versa. However, only by performing x-ray magnetic circular dichroism experiments at the Advanced Photon Source of the Argonne National Laboratory, it could be evidenced unambiguously thatplatinum can be polarized by an oxide. Additionally, these systems were investigated concerning their electrical transport properties, whereby several nanoparticle phenomena could be observed. Another highlight of this work is the successful preparation of three-dimensional iron oxide nanoparticle assemblies on length scales of > 1000 m by carefully adjusting the parameters of a newly developed centrifuge assisted sedimentation method. Extensive SEM studies combined with magnetometry and sophisticated SAXS experiments resulted in a comprehensive overview on the morphology and magnetism of these so-called ’macrocrystals’, as well as on the nanoparticle arrangement inside them. Like this, highly correlated systems with a macroscopic expansion could be manufactured. Finally, a nanoparticle/palladium multilayer was prepared. It demonstrates that the polarization and magnetoresistance effects found in the two-dimensional systems can only partially be transfered to three dimensional assemblies. In conclusion, this work shows how two- and three-dimensional nanoparticle assemblies can be prepared, how their properties are modified at different stages of preparation and how a polarizable matrixmaterial influences the particles and vice versa

Macroscopic nanoparticle assemblies: exploring the structural and magnetic properties of large supercrystals

In the present work we demonstrate, how the self-assembly of nanoparticles provides a simple and straightforward way to fabricate ordered nanocomposites on length scales of up to 300…1000 µm. We realize this by employing a centrifugation assisted sedimentation technique. We start with magnetic iron oxide nanoparticles (NPs) with an average diameter of 15 nm, which are dispersed in toluene. The particles are coated with an organic shell to prevent unordered agglomeration of particles. After centrifugation and subsequent drying large macroscopic assemblies of NPs are obtained. The characterization of the samples using scanning electron microscopy (SEM), x-ray diffraction (XRD) and small angle x-ray scattering (SAXS) reveals that the samples are NP macro-polycrystals. By variation of the fabrication parameters as e.g. centrifugation speed and temperature we obtain a systematic study on how NP ordering, crystallinity and morphology depend on the fabrication parameters. Moreover, the magnetic properties of the NP macro-polycrystals are studied using superconducting quantum interference device (SQUID) magnetometry