Marc Respaud

Abstract Progress in the prediction and optimization of the heating of magnetic nanoparticles in an alternating magnetic field is highly desirable for their application in magnetic hyperthermia. Here, a model system consisting of metallic iron nanoparticles with a size ranging from 5.5 to 28 nm is extensively studied. Their properties depend strongly on their size: behaviors typical of single‐domain particles in the superparamagnetic regime, in the ferromagnetic regime, and of multi‐domain particles are observed. Ferromagnetic single‐domain nanoparticles are the best candidates and display the highest specific losses reported in the literature so far (11.2 ± 1 mJ g −1 ). Measurements are analysed using recently a demonstrated analytical formula and numerical simulations of the hysteresis loops. Several features expected theoretically are observed for the first time experimentally: i) the correlation between the nanoparticle diameter and their coercive field, ii) the correlation between the amplitude of the coercive field and the losses, and iii) the variation of the optimal size with the amplitude of the magnetic field. None of these features are predicted by the linear response theory – generally used to interpret hyperthermia experiments – but are a natural consequence of theories deriving from the Stoner–Wohlfarth model; they also appear clearly in numerical simulations. These results open the path to a more accurate description, prediction, and analysis of magnetic hyperthermia.

Coalescence of initially produced nanospheres inside 3D super-lattices may be the reason for the formation of ferromagnetic cobalt nanorods, monodisperse in length and diameter, from [Co(η3-C8H13)(η4-C8H12)] in the presence of stearic acid and hexadecylamine. The self-organization of these nanorods into unprecedented hexagonal 2D and 3D super-lattices has been studied by transmission electron microscopy (see picture). Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2003/z52090_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

The reaction of Ni(COD)2 with H2 (3 bar) in CH2Cl2 in the presence of poly(vinylpyrrolidone) (PVP) K 30 leads to air-stable agglomerates of ≈30 nm, composed of individual 3−4 nm fcc Ni particles and displaying a ferromagnetic behavior whereas the same reaction in the presence of PVP K 90 leads to 4 nm, well-dispersed fcc Ni particles displaying a superparamagnetic behavior. In both cases the magnetic moment per atom is similar to the value found for bulk nickel, hence demonstrating the absence of oxidation of the surface of the particles.

When magnetic nanoparticles (MNPs) are single domain and magnetically independent, their magnetic properties and the conditions to optimize their efficiency in magnetic hyperthermia applications are now well understood. However, the influence of magnetic interactions on magnetic hyperthermia properties is still unclear. Here, we report hyperthermia and high-frequency hysteresis loop measurements on a model system consisting of MNPs with the same size but a varying anisotropy, which is an interesting way to tune the relative strength of magnetic interactions. A clear correlation between the MNP anisotropy and the squareness of their hysteresis loop in colloidal solution is observed: the larger the anisotropy, the smaller the squareness. Since low anisotropy MNPs display a squareness higher than the one of magnetically independent nanoparticles, magnetic interactions enhance their heating power in this case. Hysteresis loop calculations of independent and coupled MNPs are compared to experimental results. It is shown that the observed features are a natural consequence of the formation of chains and columns of MNPs during hyperthermia experiments: in these structures, when the MNP magnetocristalline anisotropy is small enough to be dominated by magnetic interactions, the hysteresis loop shape tends to be rectangular, which enhances their efficiency. On the contrary, when MNPs do not form chains and columns, magnetic interactions reduce the hysteresis loop squareness and the efficiency of MNPs compared to independent ones. Our finding can thus explain contradictory results in the literature on the influence of magnetic interactions on magnetic hyperthermia. It also provides an alternate explanation to some experiments where an enhanced specific absorption rate for MNPs in liquids has been found compared to the one of MNPs in gels, usually interpreted with some contribution of the brownian motion. The present work should improve the understanding and interpretation of magnetic hyperthermia experiments.