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

Abstract

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 coreshell 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 .

Magnetic hyperthermia investigation of cobalt ferrite nanoparticles: Comparison between experiment, linear response theory, and dynamic hysteresis simulations

“…In this case, a shell rich in Fe 3þ is formed at the nanoparticle surface, which promotes the formation of a core-shell nanoparticle (expected to be a CoFe 2 O 4 -c-Fe 2 O 3 nanostructure). 43 It is found, for this type of ferrite, that the shell is weakly magnetic and its length decreases the lower the particle size, while the core might be considered with similar properties as the bulk. 43 So, within the core-shell model, one should consider the bulk saturation magnetization of 425 emu=cm 3 in the effective anisotropy calculation (see Eq.…”

Field dependent transition to the non-linear regime in magnetic hyperthermia experiments: Comparison between maghemite, copper, zinc, nickel and cobalt ferrite nanoparticles of similar sizes

“…During this synthesis step a weakly magnetic layer is formed at the nanoparticle surface, as found by other authors. 32 Further, in a recent paper of our group, using cobalt-ferrite nanoparticles of distinct sizes, a better representation of the hyperthermia experimental data, at low field amplitudes, was found considering the core-shell model. 31 Note that the same model can be used to explain the discrepancies between the measured magnetization values and the expected bulk ones.…”

“…Note that the values found for the present samples are of the same order of others reported in the literature. 32 Also, in Table II, are the new values of other parameters when using the core-shell model. In particular, higher anisotropy values were found when compared with the homogeneous particle calculation (see Table I).…”

In-field Mossbauer study of disordered surface spins in core/shell ferrite nanoparticles

“…25 Their internal chemical composition is no longer homogeneous and they can be described as made of a stoichiometric ferrite core surrounded by a thin maghemite surface layer. 26 At room temperature the dispersion state of our magnetic nanocolloids is controlled inside a well-known phase diagram, by using small angle scattering experiments combined with electrochemical measurements which allow a fine tuning of the electrostatic repulsion. [27][28][29][30] Magnetization measurements performed on "gas-like" dilute dispersions of independent nanoparticles based on copper and manganese ferrite have recently shown that the spin structure of these particles is well accounted for by a magnetic core-shell model.…”

“…25,26 Nickel ferrite nanoparticles are prepared by hydrothermal coprecipitation of aqueous solutions of a Ni͑NO 3 ͒ 2 -FeCl 3 mixture in alkaline medium, after which the precipitate is thoroughly washed with water. In this step the mean particle size can be tuned by controlling the mixing velocity of the reagents.…”