Study of the dealloying process in Fe-Pd thin films through nano-mechanical properties measured by Atomic Force Microscope

The aim of this thesis is exploiting a nanoscale tool such as the Atomic Force Microscope (AFM), in order to combine mechanical and magnetic characterizations of advanced magnetic materials. Over the last few years, Fe-Pd alloys have gained a growing interest among the scientific community because it offers the possibility to combine magnetic metal properties (Fe) and noble metal (Pd) into a multifunctional alloy. In particular, writer’s attention is centered around Fe70 – Pd30 (at %) thin films having thicknesses in the 50-200 nm range, which are deposited on Si/Si oxide by sputtering and by electrodeposition. Those samples have been characterized by means of force curves obtained through stiff AFM cantilevers with a typical elastic constant in the 20-80 N/m range. To obtain nano-porous thin films with increasingly reduced Fe content and augmented porosity, the investigated materials did undergone a dealloying process under free corrosion condition, within hydrochloric acid. The changes in the morphology, density and microstructure, induced by the dealloying process, affect the mechanical properties of the sample at the nanoscale, as detected by the AFM force curves. Specifically, nano-porous systems turn out to display an increased plasticity, with a complex evolution related to the dealloying time. These effects are particularly evident for thin films obtained by sputtering method, which present already in the as-prepared state a microstructure with a certain fraction of free volume, lacking of density. Conversely, electrodeposited samples, being much more compact in their as-prepared state, turn out to be less prone to dealloying and to variations in their mechanical properties. To summarize, mechanical and magnetic investigations have been carried out in order to provide a deep insight into the fundamental properties of those nanostructured thin films. This thesis has been executed in collaboration with INRiM laboratories.