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.

Abstract:When magnetic n anop articles (MNPs) are single-domain and magnetically indep endent, their magnetic p rop erties and the conditions to optimize their efficiency in magnetic hyp erthermia app lications are now well-understood. However, the influence of magn etic interactions on magnetic hy p erthermia prop erties is still unclear. Here, we rep ort hyp erthermia and highfrequency hy steresis loop measurements on a model system consisting of M NPs with the same size but a vary ing anisotropy , which is an interesting way to tune the relative strength of magnetic interactions. A clear correlation between the M NP anisotropy and the squareness of their hy steresis loop in colloidal solution is observed : the larger the anisotropy, the smaller the squareness. Sin ce low anisotropy MNPs disp lay a squareness high er than the one of magnetically indep endent nanoparticles, magnetic interactions enhance their heatin g p ower in this case. Hysteresis loop calculations of indep endent and coup led M NPs are comp ared to exp erimental results. It is shown that the observed features are a natural consequence of the formation of chains and colu mns of MNPs during hyp erthermia exp eriments: in these structures, when the MNP magnetocristallin e anisotropy is small enough to be dominated by magnetic interactions, the hy steresis loop shap e tends to be rectan gular, which enhan ce their efficien cy. On the contrary , when MNPs do not form chains and columns, magn etic interactions reduces the hy steresis loop squareness and the efficiency of M NPs comp ared to indep endent ones. Our finding can thus exp lain contradictory results in the literature on the influence of magnetic interactions on magnetic hyp erthermia. It also p rovides an alternative exp lanation to some experiments where an enhanced specific absorp tion rate for M NPs in liqu ids has b een found comp ared to the one of MNPs in gels, usually interp reted with some contribution of the brownian motion. The p resent work should improve the understanding and interp retation of magnetic hyp erthermia exp eriments.