Correlation between microstructural features and magnetic behavior of Fe-based metallic nanoneedles

The microstructural events that occur during the formation of metallic particles starting from undoped and Al/Y doped α-FeOOH nanoneedles have been extensively studied. We have determined that during heating in air (carried out previously to the thermal reduction) the additives act as a physical barrier to slow the dehydration rate of goethite precursors. This retardation of the dehydration rate leads first to particles that are in different stages of pore elimination and finally to metallic particles with different stability against corrosion. We have also found by comparing samples with similar microstructures and thermal histories that the nature of the additive dictates the magnetic properties exhibited by the metallic particles. It seems that yttria is better than alumina in protecting particles against corrosion. This better protection against corrosion is the basis of the enhanced values of coercivity and saturation magnetization observed in the samples containing this yttria.