We have reinvestigated the Cu capping effect for Ni films on clean Cu(0 0 1) by means of polar and longitudinal magneto-optical Kerr effect measurements in order to account for previous discrepancies among the results regarding whether the Cu capping stabilizes perpendicular or in-plane magnetization. We concluded that Cu capping stabilizes perpendicular magnetization. We also found that the previous erroneous observation of in-plane stabilization could be accounted for by the presence of a small amount of C contamination, which was revealed to stabilize perpendicular magnetization surprisingly. The C atoms were found to act as surfactants and were always located at the top surface. As regards perpendicularly magnetized films on clean Cu(0 0 1), the enhancement of the coercivity with Cu capping was observed. This finding indicates that Cu does not act as a simple magnetism killer but effectively suppresses the surface anisotropy that favors in-plane magnetization.
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