Engineering Magnetic Properties of Ni Nanoparticles by Non-Magnetic Cores

This work focuses on the magnetism-control study of nickel (Ni) nanoparticles (NPs) with non-magnetic cores of noble metals (M = Ag, Au). Monodispersed M NPs used as templates were synthesized by reducing their salt with a weak reductant-oleylamine (OA). The OA acted as surfactant and non-coordinating, reagent in the preparation of Ni NPs with a narrow size distribution. Then Ni atoms were controlled to grow upon the M NPs by controlling the decomposition of nickel acetylacetonate in OA. The as-prepared M and core/shell M/Ni NPs exhibit spherical shape, as characterized by transmission electron microscopy. The systematic study indicated that the Ni shell exhibits face-centered cubic crystal structure and is relatively stable in air. The hydrophobic OA molecules on the surface could be exchanged by hydrophilic imidazole to make the samples water-soluble. Magnetic study of core/shell NPs indicated that their blocking temperatures could be tuned by modulating either core size or shell thickness The novel magnetic properties of the core/shell structured NPs are introduced by the size effect, exchange interaction, and reduced magnetic ordering. The novel core/shell M/Ni NPs are expected to have a significant potential for biomolecule separation, magnetic imaging, and optoelectronics.