Magneto-optic rotation and ellipticity of ultrathin ferromagnetic films

Magneto-optic rotation {φ}' and ellipticity {φ}'' values are reported as a function of thickness for 0--400 A{} of bcc Fe(100) epitaxially deposited on Au(100) for both s- and p-polarized He-Ne laser light. The values are derived from a formula that connects {φ}' and {φ}'' with our recently reported longitudinal Kerr-effect measurements of φₘ=({φ}'²+{φ}''²{)}1/2$ and the parameters of two optical compensators that are used to convert ellipticity to rotation. The measurements were made in situ through an ultrahigh-vacuum window; our approach eliminates the effect of the window birefringence. The dominant contribution to φₘ reverses from being {φ}'' in the ultrathin limit to being {φ}' in the thick-film limit. Also, in the 0--30 A{} range, the rotation remains near zero while the ellipticity increases linearly.