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