Thickness and polarization dependence of the magnetooptic signal from ultrathin ferromagnetic films

Longitudinal Kerr-effect measurements using both s- and p-polarized light are presented as a function of thickness for bcc Fe films grown epitaxially on Au(100) in ultrahigh vacuum (UHV). The motivation is to explore the origins of the magnetooptic response at Fe thicknesses comparable to the depth penetration of light. Special efforts were taken to optically compensate for the birefringence of the UHV window. The compensated measurements yield the magnitude of the complex rotation ${\phi }_m$=(φ${\mathcal{’}}^2$+φ’ ${\mathcal{’}}^2$ ${)}^{1/2}$, where φ’ and φ’ ’ are the real magnetooptic rotation and the ellipticity, respectively. The results show linear initial increase of ${\phi }_m$ with thickness (up to the optical penetration depth) followed by a shallow peak and leveling off to a saturation value of the thick-film Kerr rotation. A review of the relevant theory includes the Faraday and Kerr contributions to the magnetooptic response. The Faraday contribution arises from metallic reflection from the substrate and passage back through the iron overlayer. We argue that the Faraday effect dominates the response in the ultrathin limit, while the Kerr effect controls the thick-film regime.