We have observed a longitudinal nonlinear (second-harmonic) Kerr rotation up to 22° for an Fe(2 nm)/Cr(2 nm) multilayer on an S i0 2 substrate that shows a linear Kerr rotation o f only a few hundreths o f a degree. The latter value excludes any linearly induced mechanism and confirms the interface sensitivity o f this nonlinear Kerr technique. The enormous enhancement is shown to be in good agreement with recent predictions, based on spin dependent band structure calculations. The local optical field effects are shown to play an important role for the additional enhancements for such multilayers on a substrate. Besides their fundamental interest, these systems are also interesting for their potential in practical applications.One o f the most widely used techniques to study the magnetic properties o f magnetic thin films is the magneto optical Kerr effect (MOKE). In addition, MOKE is widely used for magnetic imaging. In this context, and also to study very thin films, large Kerr rotation angles are favorable.Recently, it has been pointed out by Pustogowa, Hiibner, and Bennemann that in nonlinear optical secondharmonic generation (SHG) from magnetic materials enormous enhancements of the Kerr angles can be expected [ In this paper we present experimental nonlinear Kerr data for a magnetic Fe/Cr multilayer in the longitudinal configuration. We show that extremely large nonlinear Kerr angles are found for this very thin magnetic layer, for which the linear magneto-optical Kerr rotation is (vanishingly) small. Experimentally is found to be largest for the ^-polarized input, with values between 5° and 2 2° for angles of incidence between 15° and 70°. For the p -input configuration we measure a rotation angle of 1° to 5° for incident angles between 15° and 7 0°. This is shown to be in good agreement with the results of band structure calculations on (bulk) Fe surfaces by Pustogowa, Hiibner, and Bennemann [9]. We will also show how the substrate, through local field effects, can influence the magnitude of the observed 4>k\The sample consisted of a thin Fe film (thickness 2 nm), covered with a 2 nm Cr film deposited by rf diode and dc magnetron sputtering, respectively. As a substrate we used a (100) silicon wafer, with a thermal oxide layer of about 525 nm. The substrate was on a rotating table, which moved with a velocity of 0 .9 6 and 3 .9 7 m /m in underneath the Fe and Cr targets, respectively. Both targets were equipped with screens for getting uniformity of the layer thickness better than 1%. The base pressure was 2 X 10~7 Torr and the Ar pressure was 5 mTorr. For the second harmonic experiments we used the 7 7 0 nm output of a mode-locked (8 0 MHz) Ti-sapphire laser. The pulse width was 7 0 fs and the input power was 100 mW focused on a spot diameter of 100 jmm. The experiments were done in the longitudinal configuration, i.e., the magnetization M was in the plane of the sample and in the optical plane of incidence (see inset, Fig. 1). Figure 1 shows the polarization dependence of the SH signal for an ^-polariz...

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Second harmonic generation from magnetic materials is shown to lead to a nonlinear magneto-optical Kerr effect that can be orders of magnitude larger than its linear equivalent. The origin ot this effect can be found in the differences between the linear and nonlinear solutions of the optical wave equations and in the symmetry properties of the corresponding optical tensors. Applications for the study of magnetic surfaces, thin films, and multilayers will be discussed.

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...

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