Chemically synthesized Au–Fe₃O₄ nanostructures with controlled optical and magnetic properties

Au–Fe₃O₄ dumbbell-like, flower-like and core-shell nanostructures with different sizes ranging between 4–15 nm have been synthesized via the thermal decomposition of iron pentacarbonyl and reduction of gold salts. By means of the variation of the precursors’ molar ratio, the injection temperature and the reaction time, the optical and magnetic properties have been tailored. A strong dependence on the dielectric function of the present Fe₃O₄ nanoparticles (NPs) was detected. The gold characteristic plasmon peak shifts from 525 nm (pure gold NPs) to close to 600 nm showing an electron damping in the Au NPs surrounded by Fe₃O₄. These NPs are ferromagnetic at low temperature, exhibiting a coercive field ranging between 85–500 Oe, whereas they behave as a superparamagnetic system above the blocking temperature, which is found to be between 18–110 K. A high magnetic anisotropy is generated in the interface of Au–Fe₃O₄ NPs. (paper)