Influence of a metallic or oxide top layer in epitaxial magnetic bilayers containing CoFe2O4(111) tunnel barriers

We investigate the interaction of CoFe2O4(111) tunnel barriers with Co and Fe3O4 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 CoFe2O4/Co and CoFe2O4/Fe3O4 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 CoFe2O4/Fe3O4 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.