We have precisely measured in ultrahigh vacuum the polar magneto-optical Kerr rotation Oz of a perpendicularly magnetized Co(0001) ultrathin film on An(111), versus the thickness t~"ofa Au overlayer. Comparative measurements have been made at ) =632.8 and 543.5 nm. At both photon wavelengths, above four atomic layers (AL's) of Au coverage we observe clear oscillations of 8&, superimposed to the usual slow decrease with r~".While the period (around 7.7 AL's) does not change appreciably, the phase of the oscillations depends drastically on X. 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 conAned spin-polarized quantum-well states in the Au overlayer.In ultrathin films, due to the electronic potential discontinuities experienced by electron states at interfaces, the perpendicular wave vector can be quantized, giving rise to resonances in the density of electronic states usually called quantum-well states (QWS's). Those QWS's are, for instance, at the origin of the oscillating coupling observed between ferromagnetic layers through a nonmagnetic metallic spacer layer. ' Directand inverse-photoemission experiments allow one to directly probe QWS's in many systems such as uncovered noble-metal (100) or (111)films on ferromagnetic metals Fe or Co, ' or in a bcc Fe (100) layer on Au(100). Spectroscopic magneto-optical Kerr effect (MOKE) studies of Fe(100) on Au(100) (Refs. 7 and 8) have also been used to evidence QWS's and their dispersion in the Fe layers. More recently, it was clearly shown by photoemission experiments ' that the QWS's in a Cu(100) overlayer on Co(100) are spin polarized, as expected from theoretical predictions. As emphasized by Shoenes, ' one fundamental characteristic of MOKE is that the relevant magnetization is not the total net magnetization of the layer, but the spin polarizations of the initial and final electronic states of the optical transitions involved. Moreover, MOKE is sensitive to the density of those states through the transition probability. Thus, in principle, MOKE can detect spin-polarized QWS's in a nonmagnetic layer. Oscillations of the Kerr rotation with the nonmagnetic spacer-layer thickness have indeed been observed in fcc(100)Fe/Cu/Fe (Ref. 12) or Fe(100)/(Au or Ag)/Fe (Ref. 13) trilayers, besides the coupling oscillations present in these structures. However, these two works remain incomplete, with, for instance, no study of the dependence on the ferromagnetic layer thickness, which prevents an unambiguous interpretation. Last year, we reported the existence of clear coupling oscillations in Co/Au(111)/Co trilayer structures. We report here the unambiguous observation by MOKE of QWS's in a Au(111) overlayer on Co(0001).All experiments have been performed in an ultrahighvacuum unit with base pressure around 10 mbar (below 5 X 10 mbar in the sample chamber during evaporation). The unit is equipped for in situ reflection high-energy electron-diffrac...
