Influence of a metallic or oxide top layer in epitaxial magnetic bilayers containingCoFe2O4(111)tunnel barriers

We investigate the interaction of $\mathrm{Co}{\mathrm{Fe}}_2{\mathrm{O}}_4\left(111\right)$ tunnel barriers with Co and ${\mathrm{Fe}}_3{\mathrm{O}}_4$ electrodes in light of their potential as room temperature spin filters. The question of the exchange coupling that often prohibits the independent switching between a magnetic tunnel barrier and its magnetic electrode is addressed, as is the difference between an oxide/metal and oxide/oxide system. Our study of the magnetic reversal in the $\mathrm{Co}{\mathrm{Fe}}_2{\mathrm{O}}_4∕\mathrm{Co}$ and $\mathrm{Co}{\mathrm{Fe}}_2{\mathrm{O}}_4∕{\mathrm{Fe}}_3{\mathrm{O}}_4$ bilayers, supported by a detailed structural and chemical analysis of the samples and their interfaces, clearly evidences the effect of a metallic or an oxide interface. An unusual exchange spring magnet behavior arises in the case of the $\mathrm{Co}{\mathrm{Fe}}_2{\mathrm{O}}_4∕{\mathrm{Fe}}_3{\mathrm{O}}_4$ samples due to the superexchange interactions found in these ferrimagnetic oxides. This unique exchange phenomenon at the oxide-oxide interface ultimately leads to a bilayer system displaying a two-phase magnetization loop without the insertion of a nonmagnetic spacer.