Magneto-optical-Kerr-effect study of spin-polarized quantum-well states in a Au overlayer on a Co(0001) ultrathin film

We have precisely measured in ultrahigh vacuum the polar magneto-optical Kerr rotation ${\mathrm{\theta }}_{\mathit{K}}$ of a perpendicularly magnetized Co(0001) ultrathin film on Au(111), versus the thickness ${\mathit{t}}_{\mathrm{Au}}$ of a Au overlayer. Comparative measurements have been made at λ=632.8 and 543.5 nm. At both photon wavelengths, above four atomic layers (AL’s) of Au coverage we observe clear oscillations of ${\mathrm{\theta }}_{\mathit{K}}$, superimposed to the usual slow decrease with ${\mathit{t}}_{\mathrm{Au}}$. While the period (around 7.7 AL’s) does not change appreciably, the phase of the oscillations depends drastically on λ. Moreover, we could not detect any dependence of the oscillation parameters with the Co thickness in the range 3–6 AL’s. This behavior can be interpreted as arising from strongly confined spin-polarized quantum-well states in the Au overlayer.