Oriented attachment has created a great debate about the description of crystal growth throughout the last decade. This aggregation-based model has successfully described biomineralization processes as well as forms of inorganic crystal growth, which could not be explained by classical crystal growth theory. Understanding the nanoparticle growth is essential since physical properties, such as the magnetic behavior, are highly dependent on the microstructure, morphology and composition of the inorganic crystals. In this work, the underlying nanoparticle growth of cobalt ferrite nanoparticles in a bioinspired synthesis was studied. Bioinspired syntheses have sparked great interest in recent years due to their ability to influence and alter inorganic crystal growth and therefore tailor properties of nanoparticles. In this synthesis, a short synthetic version of the protein MMS6, involved in nanoparticle formation within magnetotactic bacteria, was used to alter the growth of cobalt ferrite. We demonstrate that the bioinspired nanoparticle growth can be described by the oriented attachment model. The intermediate stages proposed in the theoretical model, including primary-building-block-like substructures as well as mesocrystal-like structures, were observed in HRTEM measurements. These structures display regions of substantial orientation and possess the same shape and size as the resulting discs. An increase in orientation with time was observed in electron diffraction measurements. The change of particle diameter with time agrees with the recently proposed kinetic model for oriented attachment.

A suspension of monodisperse Au-particles of either 3 or 6nm were mixed with a dilution of 6nm Co-particles. The resulting mixture was employed for the formation of granular films and the transport properties of these assemblies were analyzed. An increased granular giant magnetoresistive response was observed for samples with a high content of Au-particles. The experimental data were compared to numeric solutions of the Landau–Lifshitz–Gilbert equation for discrete magnetic moments. The alteration of the magnetic properties can be related to the formation of a nanoparticular structure resulting from the minimization of the particle stray fields.

It was recently shown that the exposure of magnetic microbeads to a homogeneous magnetic field, which rotates around the axis perpendicular to the field direction, generates highly ordered two-dimensional particle arrays. In this work, the impact of downscaling such systems is analyzed. Dilutions of cobalt nanoparticles with an average diameter of 6 nm were brought into a rotating homogeneous magnetic field. A strong localization of the number of particles within a certain cluster size can be observed if the rotation frequency is adjusted to a specific particle concentration. In particular, we obtain an increase of 85 % of the maximum of the cluster size distribution, when changing the rotation frequency of the magnetic field from 300 to 750 rpm for a cobalt concentration of 35.95 mmol/l. We propose a heuristic model to explain the observed frequency dependence.

Cobalt nanoparticles are of large interest for applications in magnetic devices and in healthcare. We studied their properties by conducting atomic force microscopy on clusters of particles and single particles deposited on highly oriented pyrolytic graphite. Topography and conductance maps have been taken simultaneously and I-V curves were measured at predefined locations on nanoparticle clusters and single nanoparticles. The I-V curves on clusters corresponded to an energy band gap in the density of states of 3.7 eV which matches the band gap of CoO nanostructures while a single particle showed only a gap of 1.3 eV in the I-V curves which is similar to the indirect band gap of Co3O4. Moreover, we found a resistive switching, i.e., a change of the clusters’ resistance during sweeping the voltage. As a reason, we suggest a transition from CoO to Co3O4 due to heating effects.

This paper highlights recent advances in synthesis and magnetotransport properties of magnetic Co nanopartides. It is shown that magnetic Co nanoparticles self-assembled in nanoparticular monolayers revealing giant magnetoresistance similar to granular systems but with additional features resulting from dipolar interactions between small domains of nanoparticles. A spin-valve with one magnetic Co nanoparticular electrode is employed as a model to demonstrate that individual magnetic moments of Co nanoparticles can be coupled to a magnetic Co layer which in turn offers tailoring of the resulting giant magnetoresistance characteristics. In addition, it is demonstrated that combining a magnetic on-off ratchet with magnetic tunneling junctions integrated in the ratchet introduces a new biosensor concept enabling: (1) simultaneous transporting and separating biomolecules, (2) dynamical biomolecule detection when passing magnetic tunneling junctions in a 1D arrangement. It is projected that this biosensor concept could be applied for viruses as well as for bacteria.

This paper highlights recent advances in synthesis, self-assembly and sensing applications of monodisperse magnetic Co and Co-alloyed nanoparticles. A brief introduction to solution phase synthesis techniques as well as the magnetic properties and aspects of the self-assembly process of nanoparticles will be given with the emphasis placed on selected applications, before recent developments of particles in sensor devices are outlined. Here, the paper focuses on the fabrication of granular magnetoresistive sensors by the employment of particles themselves as sensing layers. The role of interparticle interactions is discussed.

Cobalt nanoparticles have been prepared with TOPO, subsequently a ligand exchange was carried out. Samples have been prepared by dropping particle solution on Si-wafer, which lead to samples which consist of multilayers partially. The samples were studied with respect to the 3D order to gain information about the influence of the ligand on the 3D structure of the particle array and the oxidation process in multilayered particles.