Exchange bias at the organic/ferromagnet interface may not be a spinterface effect

Exchange bias is a physical effect that is used in many spintronic devices like magnetic read heads, magnetic random access memories, and most kinds of magnetic sensors. For the next generation of fully organic devices, molecular exchange bias, if existing, could have a huge impact for developing mechanically soft and environment friendly devices. The observation of molecular exchange bias has been reported recently in hybrid systems where a metallic ferromagnet is exchanged biased by an organic film, and it is considered to be a spinterface effect. To understand this effect, we investigate if the molecular exchange bias exists in Co/metal tetra-phenyl porphyrin hybrid bilayer systems. The molecular exchange bias is never observed when the samples are properly encapsulated, and when the exchange bias is eventually observed, it is not a spinterface effect, but it results from air-driven partial oxidation of the cobalt film transforming part of the metallic cobalt into a cobalt oxide that is well known to induce exchange bias effects. Surprisingly, oxidation is very difficult to prevent even by using very thick metallic encapsulating layers. A similar effect is observed in the Co/metal-phthalocyanine bilayer system, showing that the molecular exchange bias is not a spinterface effect also in the hybrid system in which this effect was originally discovered.