Shell structure, magnetic and magnetodynamic properties of oxidized iron nanoparticles with partial gold coating

Abstract

Magnetic nanoparticles, consisting of a metallic iron core with a shell of gold and iron oxides, were synthesized by ultra-high vacuum surface nucleation on a water-soluble NaCl substrate followed by oxidation in water. Using the methods of electron microscopy, electron diffraction, photoelectron spectroscopy and calculation of the specific density of iron in the oxidized shell, the oxides γ-Fe₂O₃, Fe₃O₄ and oxyhydroxide α-FeO(OH) were discovered. A non-uniform distribution of the Fe³⁺/Fe²⁺ ion ratio over the particle depth and a predominance of α-FeO(OH) in the contrast shell of nanoparticles, directly observed in transmission electron microscopy, were discovered. Comparison of the magnetic properties of partially gold-coated particles with similar Fe particles without gold showed a larger residual volume of unoxidized metal core with partial gold coating. This points to the anisotropy of the surface chemical properties associated with the Janus-like structure. For the first time, the magnetodynamic properties of partially gold-coated Fe nanoparticles were assessed by ellipsometric measurements of the surface of a colloidal solution in a gradient magnetic field.