Magnetisatization-induced optical second-harmonic generation: A probe for interface magnetism.

The magnetic properties of interfaces in multilayered systems containing ferromagnetic material is a subject of great current interest.1 It has been shown th at the spin behavior at clean surfaces can be very different from the bulk and in general interface magnetism is likely to differ from bulk behavior.2’3 Systems of alternating layers of magnetic and nonmagnetic materials are of fundamental interest, because of the correlation between the magnetic layers.4-6 Prom a practical point of view the switching characteristics of these systems make them very attrac­ tive for magnetic data recording.1 Since interface quality is a m ajor influence, a study of their magnetic properties is desirable.6 There exist several techniques (e.g., spin polarized pho­ toemission spectroscopy,7 spin polarized electron energy loss spectroscopy,8 and spin polarized low energy electron diffraction9) to study the magnetic properties of clean surfaces. Unfortunately (polarized) electrons are difficult to use for studying buried interfaces due to their short mean free path. Since interfaces between thin metallic films are accessible by light, an optical technique would have significant advantage. The magneto-optical Kerr effect (MOKE) is well known and frequently used. This linear optical technique studies the changes in the lin­ ear susceptibility as a function of the applied magnetic field. As it is based on the rotation of the polarization of light traveling through a magnetic material, it probes the bulk magnetization. Though very sensitive and even applicable to monolayers, MOKE is not interface specific. Second-harmonic generation (SHG) is a nonlinear op­ tical technique th at derives its interface sensitivity from the breaking of symmetry at boundaries between centrosymmetric media.10 On theoretical grounds it has been shown th at m agnetization dependent effects should be detectable with SHG,11,12 and first experimental in­ dications of magnetization-induced SHG (MSHG) were recently given by Reif et al.13 and Spierings et al 14 In this paper we discuss the influence of the magnetization on the second-harmonic signal from C o/A u multilayers. In order to prove interface sensitivity we vary the num­ ber of C o/A u interfaces. Experiments at near normal incidence show th at this has a pronounced effect on the observed magnetic field dependence of the second har­ monic. The results are interpreted quantitatively in a model th at describes the magnetic effects in terms of pure interface contributions, taking into account the multiple reflections between the various interfaces. There is no relation between the amount of bulk Co or Au and the observed effects. This demonstrates th at MSHG is a vi­ able tool for probing the magnetic properties of buried interfaces. SHG arises from the nonlinear polarization P(2u;) in­ duced by an incident laser field E (cj). This polarization can be w ritten as an expansion in E(a>):