SEMPA Studies of Exchange Coupling in Magnetic Multilayers
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SEMPA
R.J. Celotta, D.T. Pierce, and J. Unguris Introduction In the late 1980s, a number of exciting yet puzzling observations resulted from experiments investigating the coupling between two ferromagnetic layers separated by a nonferromagnetic spacer layer. A pioneering experiment by Griinberg et al.1 showed that Fe layers separated by a thin Cr spacer aligned with antiparallel magnetization, but with Au as the spacer layer, a parallel alignment occurred. The long-range magnetic dipole from each layer would tend to explain antiparallel alignment; small pinholes in the spacer layer would produce parallel alignment. Alternatively, the layers might be coupled through the spacer-layer conduction electrons by the Ruder man-Kit tel-Kasuya-Yosida (RKKY) effect. This was expected to produce an oscillation in coupling as the spacer thickness increased, that is, an oscillation between parallel and antiparallel alignment. Oscillatory coupling was first observed by Parkin et al.2 Researchers had also found34 that, at spacer thicknesses where antiparallel alignment occurred, the Fe/Cr/Fe system can exhibit a giant magnetoresistance (GMR) effect, that is, an anomalously large change in resistance when a magnetic field is applied. The potential technological importance of the GMR effect to magnetic sensing and magnetic information storage added further impetus to the already rapidly growing area of research in magnetic multilayers. Subsequent research greatly extended the range of systems studied and also added to the outstanding questions: What determines the period of the oscillation in the coupling? Does it depend in detail on the spacer-layer material, or is it
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roughly the same for most materials? What role, if any, does the RKKY effect play and could it account for the observed phenomena? Answering these 20
questions proved difficult, largely because meaningful comparisons with theory required layer-by-layer growth with sharp interfaces in a variety of new epitaxial systems. Further, mapping out the systematic variation of coupling with spacer-layer thickness required the production of a series of multilayer systems differing, in an accurately known way, only by the thickness of the spacer layer. We chose to study5 the problem of these coupled magnetic layers by using scanning electron microscopy with polarization analysis (SEMPA). The SEMPA Fe/Cr/Fe Experiment The SEMPA method6 is an extension of SEM. In a conventional scanning electron microscope, the number of secondary electrons produced by the highly focused probe beam at the surface is taken as a measure of the topography. In SEMPA, the degree of spin polarization of those same secondary electrons provides a measure of the surface magnetization under the probe beam. SEMPA
Cr Thickness (layers) 40 1
60
80
mi limn 1-2 nm
0-15nm
Figure 1. At the bottom, a schematic representation of the fabricated sample showing a single-crystal Fe whisker with two domains, a film of Cr grown in a wedge shape to provide a spacer-layer thickness variation, and an epitaxial ov
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