In a previous study , we considered the iron oxide Fe3O4 nanoparticles and their dispersion in a supporting material, to obtain a composite which can better respond to the electromagnetic (EM) fields by absorbing a part of their energy. The use of magnetic nanoparticles (NPs) aims to improve the EM interference shielding effectiveness (EMI-SE) of the composites and, at the same time, to reduce their reflection loss (RL) . The iron oxide nanoparticles are sometimes mentioned as being MIONs (Monocrystalline Iron Oxide Nanoparticles) or designed as belonging to the family of IONPs, iron oxide NPs. Because of their superparamagnetic (SPM) behavior, the Fe3O4 particles are SPM IONPs or SPIONs . Dispersed in a supporting material, IONPs can respond to an external magnetic field . About IONPs, a review was proposed by . The review illustrates the "strategies in the preparation, microstructure, and magnetic properties of bare and surface functionalized" IONPs . Biological applications are addressed in the review starting from the magnetic IONPs used "as a contrast agent for in vitro diagnostics", in methods "practiced for nearly half a century" (Wu et al. and references therein). More recently, magnetic IONPs has been largely investigated for the targeted drug delivery and magnetic resonance imaging (MRI), besides other biological applications (see . Among the technological applications, we focused on the EMI-SE increase obtained by means of Fe3O4 nanoparticles, but catalysis is also largely employing them (reviews by . , tell that eight iron oxides are known, and that the most popular are hematite (α-Fe2O3), magnetite (Fe3O4) and maghemite (γ-Fe2O3), which are possessing "polymorphism, involving temperature-induced phase transition". "Each of these three iron oxides has unique biochemical, magnetic, catalytic, and other properties which provide suitability for specific technical and biomedical applications" . The set of the eight oxides is made of the Fe II oxide (FeO, iron(II) oxide, wüstite), the mixed oxides of Fe II and Fe III , among which we have Fe3O4, Iron(II,III) oxide, that is the magnetite, and of the oxide of Fe III , Fe2O3, in the form of hematite, maghemite and other phases. Besides oxides, we have hydroxides and oxyhydroxides. "All together, there are sixteen known iron oxides and oxyhydroxides with different mineral structures" . Let us stress that Fe3O4 (magnetite) is Fe II Fe2 III O4, containing the divalent form of iron . , are calling all these sixteen chemical compounds as "iron oxides". We will do the same.