Summary: | We report the successful synthesis and a complete magnetic characterization of CoFe<sub>2</sub>O<sub>4</sub>@SiO<sub>2</sub>@Au magnetoplasmonic nanoparticles. The CoFe<sub>2</sub>O<sub>4</sub> magnetic nanoparticles were prepared using the hydrothermal method. A subsequent SiO<sub>2</sub> 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<sub>2</sub>O<sub>4</sub> 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<sub>2</sub>O<sub>4</sub> core is not changed in the case of CoFe<sub>2</sub>O<sub>4</sub>@SiO<sub>2</sub>@Au nanocomposites. The gold nanoparticles creating the plasmonic shell show a giant diamagnetic susceptibility, dependent on their crystallite sizes.
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