A magnetic phase diagram of the ultrathin Co/Si(111) film deposited at 300 K has been established by the surface magneto-optic Kerr effect technique. The temperature, where ferromagnetism vanishes, increases from 375 to 625 K as the coverage of the Co film increases from 3.5 to 16 monolayers. A quantitative calculation of the normalized Auger signal of CoSi2 shows that the calculated values lie between the experimental measured Auger signals before and after ferromagnetism vanishes for films with coverage between 3.5 and 9.1 monolayers. For samples with higher coverage, the experimental data are smaller than that by calculation. This may be qualitatively explained by Co atoms escaping from the CoSi2 phase to diffuse into the Si substrate. The disappearance of ferromagnetism is mainly attributed to suicide formation.
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