Carlos Moya

Inducing glassy magnetism in Co-ferrite nanoparticles through crystalline nanostructure

This work reports on the study of three samples of 8 nm Co-ferrite particles prepared by standard methods based on the thermal decomposition of metal–organic precursors. Although all samples are single phase according to conventional techniques of structural and chemical characterization, they show strongly sample-dependent magnetic properties ranging from bulk-like ferrimagnetism to glassy magnetic behaviour. We show that the presence of crystallite domains associated with crystallographic defects throughout the particles leads to highly-frustrated ferrimagnetic cores that are responsible for the glassy phenomenology, while only samples almost free of structural imperfections show bulk-like magnetic properties. These results suggest the key role of the crystal quality in the large variability of magnetic properties previously reported for Co-ferrite nanoparticles. Besides, achieving control of the amount of structural defects in monodisperse, stoichiometric Co-ferrite nanoparticles can be an additional nano-structuring procedure to tailor their final magnetic properties.

Probing the variability in oxidation states of magnetite nanoparticles by single-particle spectroscopy

We have studied the electronic and chemical properties of a variety of ensembles of size- and shape-selected Fe3O4 nanoparticles with single-particle sensitivity by means of synchrotron-based X-ray photoemission electron microscopy. The local X-ray absorption spectra reveal that the oxidation states and the amount and type of cations within the individual nanoparticles can show a striking local variability even when the average structural and magnetic parameters of the monodisperse ensembles appear to be compatible with those of conventional homogeneous magnetite nanoparticles. Our results show the key role played by oleic acid concentration in the reaction mixture on the formation and compositional homogeneity within individual nanoparticles. When the concentration of oleic acid is high enough, the nanoparticles are composed of a Fe3O4 core surrounded by a thin γ-Fe2O3 shell. However, at a low concentration of the fatty acid, the Fe3O4 nanoparticles are likely inhomogeneous with small inclusions of FeO and Fe phases, as a result of an uncontrolled reduction of Fe3+ cations. All the foregoing underlines the importance of combining both advanced synthesis techniques and complementary single-particle investigations performed on a statistically significant number of particles so as to improve the understanding and control over electronic and magnetic phenomena at the nanoscale.