  The control of the magnetization inversion dynamics is one of the main
challenges driving the design of new nanostructured magnetic materials for
magnetoelectronic applications. Nanoparticles with onion-like architecture
offer a unique opportunity to expand the possibilities allowing to combine
different phases at the nanoscale and also modulate the coupling between
magnetic phases by introducing spacers in the same structure. Here we report
the fabrication, by a three-step high temperature decomposition method, of
Fe3O4/MgO/CoFe2O4 onio-like nanoparticles and their detailed structural
analysis, elemental compositional maps and magnetic response. The
core/shell/shell nanoparticles present epitaxial growth and cubic shape with
overall size of (29+-6) nm. These nanoparticles are formed by cubic iron oxide
core of (22+-4) nm covered by two shells, the inner of magnesium oxide and the
outer of cobalt ferrite of ~1 and ~2.5 nm of thickness, respectively. The
magnetization measurements show a single reversion magnetization curve and the
enhancement of the coercivity field, from HC~608 Oe for the Fe3O4/MgO to
HC~5890 Oe to the Fe3O4/MgO/CoFe2O4 nanoparticles at T=5 K, ascribed to the
coupling between both ferrimagnetic phases with a coupling constant of =2
erg/cm2. The system also exhibits exchange bias effect, where the exchange bias
field increases up to HEB~2850 Oe at 5 K accompanied with the broadening of the
magnetization loop of HC~6650 Oe. This exchange bias effect originates from the
freezing of the surface spins below the freezing temperature TF=32 K that
pinned the magnetic moment of the cobalt ferrite shell.
