Spin-reorientation transitions of Co/Pd(1 1 1) magnetic thin films induced by chemisorption of molecules have been investigated by the magneto-optical Kerr effect. The magnetic hysteresis curves during the phase transitions induced by CO adsorption were recorded with the polar and longitudinal Kerr geometries. Essential differences in the variations of the coercive field and the remanent magnetization were observed between the in-plane and perpendicular magnetization processes. It was found that the larger the area of perpendicular magnetization is, the larger the perpendicular coercive field becomes. On the contrary, as the area of in-plane magnetization is smaller, the in-plane coercive field becomes larger, although the enhancement is much less significant than in the perpendicular case. This difference is explained in terms of the different character of domain walls. NO adsorption has also been investigated. Similarities and dissimilarities between CO and NO adsorption indicate that NO stabilizes perpendicular magnetization even from the initial stage of adsorption, while the stabilization of perpendicular magnetization by CO adsorption occurs only at the second stage where the atop-site adsorption is completed and the bridge-site adsorption starts.
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