Looking for quantum effects in magnetic nanoparticles using the molecular nanomagnet approach

In an effort to build a unitary view of the properties of magnetic nanoparticles (MNPs) and molecular nanomagnets (MNMs), we investigated with different techniques MNPs of spinel-type iron oxide of approximately 3.5 and 8 nm mineralized in the internal cavity of ferritin-type proteins. In particular, we used electron magnetic resonance (EMR) and static and dynamic magnetic measurements and took advantage of the capacity of the protein shells to control the size of the MNPs. A signal at half-field in the EMR spectrum is observed for MNPs as big as 8 nm mineralized in protein cavities. This feature has been interpreted as the signature of the discrete structure of the energy levels and, therefore, of the quantum nature of the system. The EMR behavior of the MNPs is compared with that of two large MNMs: one containing 19 Fe(III) and a second containing 19 Mn(II) centers. The ideal structure of the latter is used as a model to show the structuring of the energy levels. In particular, the analysis of MNP behavior based on the MNM approach helps to shed light on the role of the different energy terms that govern the MNP properties.