Epitaxial CoFe$_2$O$_4$(111)-based multilayers for spin filter applications*

Efficient spin filtering at room temperature has high potential for ultra sensitive detectors and spin injection into semiconductors, leading to the growth of spin-based devices. We investigate the interaction of spin filter CoFe$_2$O$_4$(111) epitaxial tunnel barriers with Co and Fe$_3$O$_4$ electrodes in light of their possible application at room temperature. 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 CoFe$_2$O$_4$/Co and CoFe$_2$O$_4$/Fe$_3$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 CoFe$_2$O$_4$/Fe$_3$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 barrier/electrode system that switches independently without the necessity of a non-magnetic spacer.