Characterization of Magnetic Nanoscale Systems: From Molecules to Particles and Self-Assembled Chains

Abstract: This thesis characterizes magnetic nanoscale systems with increasing size and aspect ratio using synchrotron radiation-based spectroscopy and microscopy techniques, combined with conventional magnetometry. The systems studied are molecular magnets, bimagnetic nanoparticles and self-assembled magnetic nanochains.Single-molecule magnets are compounds that exhibit magnetic bistability and hysteresis at low temperatures. To investigate how their properties are affected when in contact with different surfaces, sub-monolayers of endofullereneDynSc3−nN@C80 (n = 1, 2) were prepared on conducting and insulating substrates by thermal evaporation and chemical deposition. In the latter case, the SMMs were functionalized with surface-anchoring thioether groups, which substantially affected the magnetic properties. However, magnetic hysteresis was observed on a surface of Au(111). The magnetic bistability was better preserved when sublimating the di-dysprosium compound (n = 2) onto Au(111), Ag(100), and MgO|Ag(100), exhibiting a wide substrate-independent hysteresis. While the magnetic bistability was unaffected, the orientation of the magnetic clusters was highly influenced by the choice of substrate.Bimagnetic nanoparticles composed of two different magnetically ordered phases were synthesized by means of a spark ablation aerosol method. Cr substituted Fe-spinel oxide particles with embedded FeO subdomains with sizes of 10, 20 and 40 nm were investigated in detail. The unique structure was attributed to the presence of Cr and its influence on the oxidation of Fe. The 40 nm NPs exhibited a large exchange bias and an increase in coercivity when cooled in an applied field across the Néel temperature of FeO. The influence of particle size on the formation of the FeO subdomains, as well as the emergence of exchange bias were investigated and the results point to a decrease in the exchange field as the particle size decreases, with virtually no exchange bias present for the 10 nm system.Nanochains were generated in a bottom-up approach by field-assisted self-assembly of aerosolized Co nanoparticles. Local magnetic properties were investigated by means of scanning transmission x-ray microscopy, complimented by micromagnetic simulations. While uniform chains have a single domain due to significant shape anisotropy, the inclusion of large multi-domain particles can facilitate domain formation.