Nonlinear magneto-optical Kerr effect study of quantum-well states in a Au overlayer on a Co(0001) thin film

Abstract

4We have measured the polar nonlinear magneto-optical Kerr rotation and the total generated second harmonic intensity from a perpendicularly magnetized Co(0001)/Au(lll) thin film (6 ML) versus the thickness of a Au overlayer. For both experiments we find a clear oscillation with a period of about 13,5 ML. This behavior can be interpreted as arising from quantum-well states (QWSs) in the Au overlayer, though interestingly, the observed period is twice the expected one. Especially for the reflected intensity this oscillation is very pronounced: the intensity changes by a factor of 10 when the Au overlayer thickness changes from 7 to 13 ML. These strong effects make this nonlinear technique very suitable for the study of these QW oscillations. © 1.996 American Institute of Physics. [S0021 -8979(96) 19808-5] Since the discovery that magnetic films separated by a nonmagnetic spacer layer could be coupled antiferromagnetically,1 and the subsequent discovery that this coupling could oscillate between ferromagnetic and antiferromagnetic,2 there have been intense efforts to under stand these phenonema. It was shown that in ultrathin films, due to the electronic potential discontinuities experienced by electronic states at interfaces, the perpendicular component of the wave vector can become quantized, giving rise to reso nances in the density of states. Those quantum-well states (QWSs) may act as the mediator for this magnetic coupling.3 Magneto-optical Kerr effect (MOKE) and direct-and inverse photo emission experiments have shown direct evidence of such QWSs in thin noble metal films on magnetic substrates (Fe and Co),4"'6 and in a bcc Fe(100) layer on Au(100).7,8 More recently, oscillations of the Kerr angle with changes of overlayer film thickness have been observed for a Au/Co/Au system.9 These results showed the existence of quantum size effects in the thin Au overlayer and also gave an indication of possible interface contributions.Due to the fact that optical second harmonic generation (SHG) is known to be surface sensitive on an atomic scale, it seems that SHG studies might be particularly suitable for improving our knowledge of the electrodynamics of ultrathin metallic films. Though the absolute nonlinear signals are small, the nonlinear magneto-optical effects can be large: compared to the linear Kerr angle, enhancements up to a factor of 1000 have been observed.10 Wierenga et al.n were the first to report about the possibilities of this nonlinear optical technique for detecting QW oscillations. They have found very strong oscillations in the magnetization-induced SHG signal for the Cu/Co/Cu system as a function of the thickness of the Cu overlayer, which are probably related to QWSs in these films.11 These strong effects are directly re lated to the extreme interface localization of the nonlinear response.In this paper we report the unambiguous observation of QWSs in a A u (lll) overlayer on Co(0001). The oscillations are found in both the polar nonlinear MOKE as well as in the total generated second harmo...

Quantum Well States in Noble Metals: A Second Harmonic Generation Study

“…Evidently, these oscillations are a dominating feature of the curves, especially for longer wavelengths. Similar oscillatory behaviour has been found for the magnetization induced contributions to SHG, both for the polarization rotation in the polar configuration [12] transversal setup (eq. (3»; see Fig.…”

Second harmonic generation study of quantum well states in thin noble metal overlayer films

“…Evidently, these oscillations are a dominating feature o f the curves, especially at longer wavelengths. Similar oscillatory behaviour has been found for the MSHG response, both for the polarization rota tion in the polar configuration [ 14] and for the magnetization contrast in the transversal one. All observed thickness dependencies could be fitted with a formula, / SH =^exp -5) s ' n + 7 + 5.…”

Room-temperature deposition and growth of Au on clean and oxygen passivated Si(111) surfaces investigated by optical second-harmonic generation

“…Oscillating SHG with film thickness has previously been observed and discussed [9,28] for Au and Cu epitaxial films on Co. For the Au film the period was found to be 13 ML and 16 ML for p to p and s to p polarizations, respectively. Within our experimental uncertainty, estimated to be ±3 ML, the periods in the present experiments are identical to those of [9].…”