Synthesis of Core−Shell Ferrite Nanoparticles for Ferrofluids: Chemical and Magnetic Analysis

A chemical core−shell strategy is developed here for the synthesis of ferrofluids based on nanoparticles of different ferrites with different mean sizes. A heterogeneity of chemical composition, associated with a superficial enrichment of iron, allows to obtain chemically stable ionic colloids. We propose here a core−shell model to describe the synthesized nanoparticles, which is tested by chemical and magnetic measurements performed at the various steps of the synthesis. The thickness of the superficial layer, rich in iron, is ranging between 0.4 and 1.3 nm, depending on the nanoparticle size and on the underlying ferrite. Its density is found close to that of maghemite, and its magnetization depends on the core ferrite. It is low with a cobalt ferrite core and larger for the three other ferrites investigated here (NiFe 2 O 4, CuFe 2 O 4, and ZnFe 2 O 4 ). Magnetic measurements prove that there is a strong redistribution of Zn 2+ ions inside the core of the synthesized nanoparticles based on ZnFe 2 O 4 .