We apply the theory of the magneto-optic Kerr effect (MOKE) for multilayer thin films to analyze the surface magnetic properties, which have been observed using focused MOKE, for Ni75Fe25 and Fe thin films evaporated on poly(ethylene naphthalate) (PEN) organic substrates. The calculation is performed for the thickness dependence and incident angle dependence of Kerr rotation and ellipticity. We have measured the thickness dependence of Kerr rotation at a wavelength of 405nm for both Ni 75Fe25 and Fe thin films on PEN organic substrates. These results are fitted using the theory by adjusting the values of magneto-optic constants Q's. These Q's are 0.01 exp(-i48π=180) and 0.025 exp(-i47π=180) for Ni75Fe25 and Fe thin films, respectively. These results lead to the quantitative estimation of the surface magnetic properties of thin films on organic substrates. Also, the magneto-optic constants are estimated for ferromagnetic thin films on organic substrates for the first time.
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