Correlation between microstructure, morphology and magnetic propertiesof metallic nanoparticles (fe,co,ni) inserted into an activated porous carbon matrix
- Autores: Maria Paz Fernandez Garcia
- Directores de la Tesis: Pedro Gorria Korres (dir. tes.), Jesús Ángel Blanco Rodríguez (dir. tes.)
- Lectura: En la Universidad de Oviedo ( España ) en 2010
- Idioma: español
- Tribunal Calificador de la Tesis: Maria Dolores Baró Mariné (presid.), Jesús Daniel Santos Rodríguez (secret.), Luis Fernández Barquín (voc.), A. Benito Fuertes Arias (voc.), Joao Pedro Esteves de Araujo (voc.)
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- Resumen
The synthesis, characterization and study of the physico-chemical properties of magnetic nanoparticles (NPs) have received great interest in the last two decades due to their potential use in technological and biomedical applications. From the fundamental point of view, it is essential to understand how the properties of these materials are modified with the reduction of their size because the surface to volume ratio plays a major role at the nanometer length scale.
A parallel study of Fe, Co and Ni NPs allows us to investigate the variation of the magnetism exhibited by the three systems when compared with their bulk counterparts. For instance, the Co bulk has a hexagonal close packed (HCP) crystal structure under ambient conditions (temperature and pressure), but a reduction of the crystalline size down to tens of nanometers favours the face centered cubic crystal structure (FCC). On the other hand, the high temperature (T~1200 K) stable phase of Fe with FCC crystal structure (g-Fe), can be attained at room temperature in several Fe NPs systems under adequate synthesis conditions.
The aim of this thesis work is to show a relationship between the morphology, microstructure and magnetism of Ni, Fe and Co NPs inserted on a porous amorphous matrix of activated carbon. This study has been done by combining several long and short-range order characterization techniques (X-ray diffraction, scanning and transmission electron microscopy and, X-ray absorption) with Mössbauer spectroscopy and magnetic measurements. In addition, the value of some characteristic magnitudes or parameters, such as the effective magnetic anisotropy constant and the saturation magnetization, together with the average NP size have been obtained from the fit of the low field magnetization vs. temperature curves using a method based on the Stoner-Wohlfarth model.
The Fe, Co and Ni-NPs fabricated by a chemical synthesis procedure based on a pyrolysis process, are randomly distributed in the carbon matrix with sizes between 2 and 50 nm. The magnetism of these systems is strongly affected by the NP morphology. Although superparamagnetic (SPM) behaviour around room temperature could be expected for almost all the studied samples due to the metallic core diameters, the presence of oxide shells changes completely this picture. The magnetic exchange coupling between the antiferro- or ferrimagnetic metallic oxide shell and the metallic core produces an exchange bias effect (EB) and/or an increase in the value of the blocking temperature.
It was found that the addition of sucrose during the synthesis of Ni-based samples gives rise to an efficient protection of the NP against oxidation. However, this protection does not take place for the Co and Fe systems where a thin (2-3 nm) oxide layer is always present covering the metallic NP cores. These oxides layers are, in most of the cases, almost undetectable by X-ray powder diffraction but X-ray absorption is a powerful tool to prove their existence and even to quantify their relative amount. Therefore, it was found that the NPs display different onionlike morphological configurations made up of an internal metallic core and one or two several concentric shells with variable thicknesses. The configuration of Co NPs is Co-FCC / CoO / Co3O4 whereas the nano-onion Fe NPs exhibit a g-Fe / a-Fe / g-Fe2O3 microstructure.
The sucrose protected Ni NPs are one exception and, SPM regime is reached below room temperature. From the point of view of both the morphology and the magnetic behaviour, the Fe samples display the most complex scenario, because competing magnetic interactions between the different phases in the g-Fe core and the a-Fe / Fe-oxide double shell configuration are present. In the case of Fe NPs, Mössbauer spectrometry was important to identify and quantify the different phases containing Fe and their evolution with temperature. Finally, the Co-NP systems exhibit the larger values for the EB effect (above 500 Oe at T = 4 K) and, a nonconventional dependence with the applied magnetic field.
