Magnetic properties of bi-magnetic core/shell nanoparticles: The case of thin shells

Abstract

Bi-magnetic core/shell nanoparticles were synthesized by a two-step high-temperature decomposition method of metal acetylacetonate salts. Transmission electron microscopy confirmed the formation of an ultrathin shell (~0.6 nm) of NiO and NiFe2 O4 around the magnetically hard 8 nm CoFe2 O4 core nanoparticle. Magnetization measurements showed an increase in the coercivity of the single-phase CoFe2 O4 seed nanoparticles from ~1.2 T to ~1.5 T and to ~2.0 T for CoFe2 O4 /NiFe2 O4 and CoFe2 O4 /NiO, respectively. The NiFe2 O4 shell also increases the magnetic volume of particles and the dipolar interparticle interactions. In contrast, the NiO shell prevents such interactions and keeps the magnetic volume almost unchanged.