Z. Phys. B 100, 335–341 (1996) ZEITSCHRIFT F ¨ UR PHYSIK B c  Springer-Verlag 1996 X-ray magnetic circular dichroism study of the induced spin polarization of Cu in Co/Cu and Fe/Cu multilayers G. A. Held 1 , M. G. Samant 2 , J. St¨ ohr 2 , S. S. P. Parkin 2 , B. D. Hermsmeier 3 , M. van Schilfgaarde 4 , R. Nakajima 5 1 IBM Research Division, TJ Watson Research Center, PO Box 218, Yorktown Heights, NY 10598, USA 2 IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120, USA 3 IBM San Jose, 5600 Cottle Road, San Jose, CA 95153, USA 4 SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025, USA 5 Department of Materials Science, Stanford University, Stanford, CA 94305, USA Received: 18 May 1995 Abstract. We present an x-ray magnetic circular dichroism (XMCD) study of Co/Cu and Fe/Cu multilayers, finding that the Cu atoms in these structures exhibit an induced mag- netic moment in the d shell. The average Cu spin moment is shown to fall-off inversely with the thickness of the Cu layer. Further, for comparable Cu layer thicknesses, the Cu moments in Fe/Cu and Co/Cu multilayers are found to be nearly equal, despite the fact that the Cu layers in the Co/Cu multilayers are shown to be fcc while those in the Fe/Cu structures are bcc. These observations suggest that the in- duced moment is primarily situated at the Co/Cu and Fe/Cu interfaces and is resultant from short range chemical hy- bridization between the ferromagnetic and Cu atoms. Results from a local spin density functional theory are presented and found to be in excellent agreement with experimental obser- vations. These results indicate that the Cu d electrons play a central role in mediating the exchange coupling between successive ferromagnetic layers. PACS: 78.70.Dm; 75.50.Rr; 78.20.Ls 1. Introduction The discovery that the coupling between successive 3d tran- sition metal ferromagnetic layers, separated by non-magnetic metallic layers, oscillates sign as a function of the thickness of the non-magnetic spacer layers [1, 2] has lead to nu- merous experimental and theoretical studies which attempt to characterize the nature of this coupling. Principally, the coupling has been modeled as an RKKY interaction; the pe- riodicity of the oscillations has been shown to be consistent with a simple RKKY interaction adapted to a planar geom- etry [3, 4], as well as with the RKKY oscillations expected for conduction electrons which are quantum confined within the non-magnetic layers [5, 6]. A model in which the oscilla- tions arise purely as a consequence of quantum confinement has also been proposed [7, 8], and inverse photoemission measurements have unambiguously identified quantum well states in thin, non-magnetic films adjacent to magnetic lay- ers [9, 10]. Subsequent spin resolved photoemission studies [11–13] have shown these quantum well states to be spin polarized. In each of the models based on RKKY inter- actions, the spin-spin coupling is conveyed across the the spacer layer by delocalized sp-shell electrons. However, re- cent x-ray magnetic circular dichroism (XMCD) [14] and spin polarized photoemission [12] studies have identified a strong spin polarization in the d-shell electrons within the non-magnetic layers. The polarized nature of these d-band electrons is indicative of strong hybridization between the sp and d-band electrons at the spacer layer boundaries. Such a hybridization is, in fact, predicted by detailed band structure calculations of these multilayer materials [12, 15]. Nonethe- less, the precise nature of the interplanar coupling remains controversial and the relationship between the magnetic po- larization and the exchange coupling continues to be imper- fectly understood [16]. Observations of spin polarization in non-magnetic spacer layers has been reported for Fe/Cr/Fe [17–19] and Fe/Ru/Fe [20] sandwiches. The polarization of Pt dissolved in a fer- romagnetic Fe host has also been observed [21]. In each of these systems the spacer layer is a transition metal; the cou- pling between the sp and d-bands near the Fermi surface follows directly from the bulk band structure. However, for the case of noble metal spacer layers (Cu, Ag, and Au), the mechanism of spin coupling between the sp and d electrons is less clear. We present here evidence for an induced magnetic mo- ment of the Cu d-band electrons in both Co/Cu and Fe/Cu multilayer structures. These induced magnetic moments are observed using XMCD absorption spectroscopy [22–25]. This technique allows us to unambiguously establish the ex- istence of a magnetic moment. Further, through the applica- tion of approximate sum rules [23, 24, 26–29], it enables us to obtain quantitative estimates of the orbital and spin mag- netic moments of the conduction electrons at elementally specific core sites. We find that these moments are aligned along the direction of the Co (Fe) moments and are almost entirely of spin character (ie, no detectable orbital compo- nent). In both Co/Cu and Fe/Cu multilayers, the Cu spin moment is found to fall-off inversely with the spacer layer thickness – consistent with an sp–d hybridization situated