Magnetic ordering contribution on diffusion process forming hollow materials

Abstract

Abstract We have experimentally observed the influence of a magnetic phase transition on the morphological transformation during a thermal oxidation reaction in air due to ionic diffusion process. A Fe ferromagnetic metallic microwire turns into a semiconducting iron oxide microtube when heat treated above the Curie temperature. Electrical transport behavior of Fe microwires measures changes in ionic transport across magnetic transition. Scanning electron microscopy images and x-ray diffraction measurements revealed that the formed microtubes are comprised of a Fe2O3 outer and Fe3O4 inner shells when the sample is heated above the Fe magnetic ordering temperature. This result is closely related to the higher atomic mobility in the paramagnetic state. We discuss the influence of magnetic spin interaction on Fe vacancy formation and the ionic mass transport. Numerical calculations of the vacancy formation energy in both ferromagnetic and paramagnetic states support our experimental observation that enhanced ionic mobility, leaving vacancies behind, is correlated to the vanishing of magnetic spin interactions above TC.