The effect of surface spin disorder on magnetic properties of Fe/FexOy core-shell nanoparticles

Fe-oxide nanoparticles are of considerable interest nowadays because of their unique characteristics, such as superparamagnetism, high saturation fields, and extra anisotropy contributions, which arise from the effects of finite size and large surface area. Usually they are obtained by chemical methods, but more recently some groups reported on their successful preparation by wet high-energy ball-milling. It is also well known that as the size of the nanoparticles decreases, surface effects would become more significant due to the increasing surface relative to their volume. We report here our recent results on the effect of ligands on the induced surface anisotropies and magnetic properties of Fe/Fe
₂
O
₃
and Fe/Fe
₃
O
₄
core-shell nanoparticles functionalized with 3-aminopropyltriethoxysilane (APTS) for biomedical applications (image contrast agents in magnetic resonance imaging (MRI) and magnetic carriers for drug delivery). Core-shell nanoparticles have been prepared by high-energy ball milling. In the presence of air or Ar, the Fe core was progressively covered with a Fe
₂
O
₃
shell, and the obtained Fe/ Fe
₂
O
₃
nanoparticles have diameters of 200-300 nm after 68 h of milling. Fe/ Fe
₃
O
₄
nanoparticles of 20-60 nm were obtained by wet milling of Fe microparticles for 42 h. For milling times larger than 42 h the whole amount of Fe is transformed into Fe
₃
O
₄
, and the resulting magnetite nanoparticles have diameters ranging from 15 to 50 nm (Fig. 1). The magnetic properties of Fe/ Fe
_x
O
_y
core-shell nanoparticles can be tailored from ferromagnetic Fe/Fe
₂
O
₃
to weak ferromagnetic Fe/Fe
₃
O
₄
and superparamagnetic Fe
₃
O
₄
(Fig. 2). By choosing the appropriate milling conditions and starting materials is possible to tune the magnetic properties and make the Fe/FeO
_x
core-shell NPs suitable for different biomedical applications. The main advantage of such coreshell nanoparticles in biomedical applications, compared with simple Fe-oxides nanoparticles, resides in their easier use and manipulation for specific applications. To understand the surface spin disorder and its influence on the magnetic properties of Fe/Fe-oxide core-shell nanoparticles, their surface was systematically modified with APTS, by increasing progressively the concentration of the ligand. APTS was chosen as ligand because the bonding with the magnetic NPs is made through Si-O, NH
₂
remaining free for bonding with different types of biomolecules. Low temperature magnetization measurements and ZFC/FC curves indicate a strong influence of the ligand on the magnetic properties. The change of the magnetic properties of nanoparticles also correlates with the specific coordinating functional group bound on the nanoparticles surface. The correlation suggests the decrease in spin-orbital coupling and surface anisotropy of magnetic nanoparticles due to the surface coordination. Because of the high saturation magnetization, these Fe/FeO
_x
core-shell NPs have a higher Specific Absorption Rate (SAR), making them suitable for hyperthermia applications. They can be also visualized by Magnetic Resonance Imaging because of the Fe-oxides shell. This work was financially suported by the 3MAP NUCLEU Programme (2018).