Figure 1.

Three classes of magnetic nanocrystals are investigated: (a, e) homogeneous spherical magnetite (Fe₃O₄) nanocrystals, (b, f) gold/magnetite core/shell nanocrystals, and (c, g) Wüstite (FeO)/magnetite core/shell nanocrystals. (a–c) TEM images of the starting materials. (d) Scheme of the cation exchange procedure in the three different nanocrystal types with inverse spinel or rock-salt crystal structure, (blue balls represent oxygen atoms; red balls, Fe³⁺; orange, Fe²⁺; green, Co²⁺; gold, Au atoms). (e, f) are high-angle annular dark field STEM images of nanocrystals after the Fe²⁺ to Co²⁺ cation exchange procedure is performed. The ion concentration distributions for Fe (red) and Co (green), and Au (golden) across the nanocrystals, shown by the EDX line scans (obtained for transitions from the K or M shell respectively), evidence the Co doping after the cation exchange treatment.