  The effects of cobalt incorporation in spherical heterostructured iron oxide
nanocrystals (NCs) of sub-critical size have been explored by colloidal
chemistry methods. Synchrotron X-ray total scattering methods suggest that
cobalt (Co) substitution in rock salt iron oxide NCs tends to remedy its vacant
iron sites, offering a higher degree of resistance to oxidative conversion.
Self-passivation still creates a spinel-like shell, but with higher volume
fraction of the rock salt Co-containing phase in the core. The higher divalent
metal stoichiometry in the rock salt phase, with increasing Co content, results
in a population of unoccupied tetrahedral metal sites in the spinel part,
likely through oxidative shell creation, involving an ordered defect-clustering
mechanism, directly correlated to the core stabilization. To shed light on the
effects of Co-substitution and atomic-scale defects (vacant sites), Monte Carlo
simulations suggest that designed NCs, with desirable, enhanced magnetic
properties (cf. exchange bias and coercivity), are developed with
magnetocrystalline anisotropy raised at relatively low content of Co ions in
the lattice. Growth of optimally performing candidates combines also a strongly
exchange-coupled system, secured through a high volumetric ratio rock salt
phase, interfaced by a not so defective spinel shell. In view of these
requirements, Specific Absorption Rate (SAR) calculations demonstrate that the
sufficiently protected from oxidation rock salt core and preserved over time
heterostructure, play a key role in magnetically-mediated heating efficacies,
for potential use of such NCs in magnetic hyperthermia applications.
