One of the main challenges in spintronics today is the generation of highly spin-polarized electron currents for application in spin-based devices. Much of the current research in this field thus revolves around the quest for new materials capable of producing such effects at room temperature. One approach, known as spin filtering, involves creating a highly polarized current via the combination of a non-polarized source with a ferro- or ferrimagnetic tunnel barrier. Efficient spin filtering at room temperature is particularly interesting for ultra sensitive detectors and spin injection into semiconductors, leading to the growth of spin-based devices.
Cobalt ferrite (CoFe2O4) is a ferrimagnetic insulator with a remarkably high Curie temperature (793 K), and thus a good candidate for spin filter tunnel barriers. We therefore investigate CoFe2O4(111) epitaxial thin films in light of their high potential for the above-mentioned applications. CoFe2O4(111)-based multilayers containing Co(0002) and Fe3O4(111) top electrodes have been grown by oxygen plasma-assisted molecular beam epitaxy. Their structural, chemical and magnetic properties are studied by a number of in situ and ex situ characterization techniques in order to obtain two systems that are optimized for integration into fully epitaxial magnetic tunnel junctions (MTJs). We pay special attention to the magnetization reversal behaviour in these two magnetic barrier/electrode systems, as this is a crucial step towards their integration into full MTJs. 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. Finally, we present the first tunnelling magnetoresistance (TMR) measurements in spin filter MTJs containing CoFe2O4(111) tunnel barriers. The spin filter capabilities of this novel material, both at low and room temperature, are discussed as is the relation between its spin-polarized transport properties and the results of our structural, chemical and magnetic investigation.
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