Effect of oxygen exposure on the magnetic properties of ultrathin Co/Si(1 1 1)-7×7 films have been studied. In ultrahigh vacuum environment, Auger electron spectroscopy (AES) analysis shows that no oxygen adsorption occurs on Si(1 1 1)-7×7 surface and Co-Si compound interfaces. As the thickness of Co films increases above 5 monolayers (ML), pure cobalt islands form on the surface and the amount of oxygen on the surface layers increases with increasing the oxygen exposure time. From the results of slight chemical shift and depth profiling measurements, the oxygen is weakly adsorbed on the topmost layer of 15 ML Co/Si(1 1 1) films. The adsorbed oxygen influences the electronic density of states of Co and leads to the changes of the magnetic properties. The appearance of the O/Co interface could modify the stress anisotropy, as a result, the coercivity of ultrathin Co/Si(1 1 1) films are enhanced. As an example for 15 ML Co/Si(1 1 1), the coercivity increases from 140 to 360 Oe with 5000 Langmuir of oxygen exposure.
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