On the top of Co/Ge(111) films, cobalt oxides are prepared by evaporating Co atoms in an oxygen atmosphere. Depth profiling measurements show a layered structure of a pure Co layer covered by a CoO overlayer. As the CoO thickness increases, the Auger intensity ratio of O KL2L2 and Co L3M45M45 Auger signals increases until reaching a saturated value which shows a CoO layer with a concentration ratio of Co and O close to 1:1. After introduction of CoO overlayers on Co/Ge(111), hysteresis occurs in the longitudinal configuration and CoO/Co/Ge(111) exhibits in-plane anisotropy. A slight reduction of the Kerr intensity occurs due to the oxidation of cobalt at the CoO/Co interface while an enhanced coercive force is observed owing to the imperfection introduced by oxygen to impede the magnetization reversal. Under conditions of cooling in a magnetic field, exchange bias field increases as the sample temperature decreases resulting from the formation of an antiferromagnetic/ferromagnetic interface.
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