Magneto-optical Kerr hysteresis loops and their energy spectra are mea
sured for ultrathin Co films. In Au/20 angstrom Co/Au sandwiches, the
spectra shape of the Kerr rotation as well as the Kerr ellipticity agr
ees with those calculated from bulk data. With decreasing Co thickness
, the spectral shape becomes different from the calculated one. The im
aginary part of off-diagonal conductivity was also estimated for ultra
thin Co films.