Onion-like Fe3O4/MgO/CoFe2O4 magnetic nanoparticles: new ways to control magnetic coupling between soft and hard magnetic phases

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.