Onion-like Fe$_3$O$_4$/MgO/CoFe$_2$O$4$ magnetic nanoparticles: new ways to control magnetic coupling between soft and hard magnetic phases

The control of magnetization inversion dynamics is one of the main challenges driving the design of new nanostructured magnetic materials for magnetoelectronic applications. Nanoparticles with an onion-like architecture offer a unique opportunity to expand the possibilities of combining different phases at the nanoscale and also modulating the coupling between magnetic phases by introducing spacers into the same structure. Here, we report the fabrication, by a three-step high temperature decomposition method, of Fe$_3$O$_4$/MgO/CoFe$_2$O$_4$ onion-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 an overall size of (29 ± 6) nm. These nanoparticles consist of a cubic iron oxide core of 22 ± 4nm in size covered by two shells, the inner of magnesium oxide and the outer of cobalt ferrite of thicknesses ∼1 and ∼2.5 nm, respectively. The magnetization measurements show a single reversion magnetization curve and the enhancement of the coercivity field, from H$_C$ ∼ 608 Oe for the Fe$_3$O$_4$/MgO to H$_C$ ∼ 5890 Oe for the Fe$_3$O$_4$/MgO/CoFe$_2$O$_4$ nanoparticles at T = 5 K, ascribed to the coupling between both ferrimagnetic phases with a coupling constant of J$_c$ = 2 erg cm$^{−2}$. ... mehrThe system also exhibits an exchange bias effect, where the exchange bias field increases up to H$_{EB}$ ∼ 2850 Oe at 5 K accompanied by the broadening of the magnetization loop of H$_C$ ∼ 6650 Oe. This exchange bias effect originates from the freezing of the surface spins below the freezing temperature T$_F$ = 32 K that pinned the magnetic moment of the cobalt ferrite shell.