Magnetic iron oxide mesocrystals have been reported to exhibit collective magnetic properties and consequently enhanced heating capabilities under alternating magnetic fields. However, there is no universal mechanism to fully explain the formation pathway that determines the particle diameter, crystal size, and shape of these mesocrystals and their evolution along with the reaction. In this work, we have analyzed the formation of cubic magnetic iron oxide mesocrystals by thermal decomposition in organic media. We have observed that a nonclassical pathway leads to mesocrystals via the attachment of crystallographically aligned primary cubic particles and grows through sintering with time to achieve a sizable single crystal. In this case, the solvent 1-octadecene and the surfactant agent biphenyl-4-carboxylic acid seem to be the key parameters to form cubic mesocrystals as intermediates of the reaction in the presence of oleic acid. Interestingly, the magnetic properties and hyperthermia efficiency of the aqueous suspensions strongly depend on the degree of aggregation of the cores forming the final particle. The highest saturation magnetization and specific absorption rate values were found for the less aggregated mesocrystals. Thus, these cubic magnetic iron oxide mesocrystals stand out as an excellent alternative for biomedical applications with their enhanced magnetic properties.

Assembled magnetite nanoparticles with oriented attachment have been prepared to study the effect of autoassembling on their heating efficiency. Their magnetic properties have been compared with those corresponding to singlecore particles, also prepared in this study. It has been found that not only particle size and shape are determinant factors on the magnetic behavior but also the interparticle interactions as well since they drastically modify the effective magnetic volume related to the magnitude of the superparamagnetic moment. The high values of the superparamagnetic moment observed for all of the samples in a broad temperature range are probably a consequence of the dipolar interactions. Moreover, the sample exhibiting both large anisotropy barriers and superparamagnetic moment at 300 K is the one presenting the largest SAR values. Therefore, the superparamagnetic behavior at room temperature together with the magnetic hardness seem to be key factors for the heat generation.

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