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, Mössbauer,
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.