Oxidation processes at the metal/oxide interface in CoFe2/CoFe2O4 bilayers deposited by pulsed laser deposition

CoFe2/CoFe2O4 bilayers were made by pulsed laser ablation of a CoFe2 target on Si(1 0 0) substrates. The metallic layer was deposited first, in vacuum. The oxide was then deposited in an oxidizing O2:N2 (20:80) atmosphere. Two different procedures were used for the introduction of the oxidizing atmosphere in the deposition chamber: the laser ablation of the target was either stopped (discontinuous deposition process) or maintained (continuous deposition process) during the 20 min necessary for the establishment of the desired O2:N2 pressure. In both cases, the different electronegativities of Fe and Co cause an important modification of the Fe/Co ratio at the metal/oxide interface, with a depletion of Fe in the metal region and of Co in the oxide region. In the continuous procedure, the combination of the kinetic energy given by the ablation process to the Fe and Co adatoms with the one they get from their different affinity towards oxidation allows the formation of a low roughness metal/oxide interface with a high (1 1 1) preferred orientation of the CoFe2O4 layer, an induced re-crystallisation of the metal layer underneath and an unusual antiferromagnetic metal/oxide magnetic coupling.