Fe-Doping-Induced Magnetism in Nano-Hydroxyapatites

Doping of biocompatible nanomaterials with magnetic phases is currently one of the most promising strategies for the development of advanced magnetic biomaterials. However, especially in the case of iron-doped magnetic hydroxyapatites, it is not clear if the magnetic features come merely from the magnetic phases/ions used as dopants or from complex mechanisms involving interactions at the nanoscale. Here, we report an extensive chemical physical and magnetic investigation of three hydroxyapatite nanocrystals doped with different iron species and containing small or no amounts of maghemite as a secondary phase. The association of several investigation techniques such as X-ray absorption spectroscopy, Mossbauer, magnetometry, and TEM allowed us to determine that the unusual magnetic properties of Fe2+/3+-doped hydroxyapatites (FeHA) occur by a synergy of two different phenomena: i.e., (i) interacting superparamagnetism due to the interplay between iron-doped apatite and iron oxide nanoparticles as well as to the occurrence of dipolar interactions and (ii) interacting paramagnetism due to Fe3+ ions present in the superficial hydrated layer of the apatite nanophase and, to a lesser extent, paramagnetism due to isolated Fe3+ ions in the apatite lattice. We also show that a major player in the activation of the above phenomena is the oxidation of Fe2+ into Fe3+, as induced by the synthesis process, and their consequent specific positioning in the FeHA structure.