New Insights into the Magnetic Properties of CoFe 2 O 4 @SiO 2 @Au Magnetoplasmonic Nanoparticles.

We report the successful synthesis and a complete magnetic characterization of CoFe 2 O 4 @SiO 2 @Au magnetoplasmonic nanoparticles. The CoFe 2 O 4 magnetic nanoparticles were prepared using the hydrothermal method. A subsequent SiO 2 shell followed by a plasmonic Au shell were deposited on the magnetic core creating magnetoplasmonic nanoparticles with a core-shell architecture. A spin-glass-type magnetism was shown at the surface of the CoFe 2 O 4 nanograins. Depending on the external magnetic field, two types of spin-glass were identified and analyzed in correlation with the exchange field acting on octahedral and tetrahedral iron sites. The magnetization per formula unit of the CoFe 2 O 4 core is not changed in the case of CoFe 2 O 4 @SiO 2 @Au nanocomposites. The gold nanoparticles creating the plasmonic shell show a giant diamagnetic susceptibility, dependent on their crystallite sizes.