X-ray Probes of Magnetic Multilayer Structure
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X-ray Probes of Magnetic Multilayer Structure B.K.Tanner, T.P.A.Hase, B.D.Fulthorpe, J.Clarke, G.M.Luo+, S.K.Halder*, A.S.H.Rozatian and S.B.Wilkins Department of Physics, University of Durham, South Road, Durham, DH1 3LE, U.K. + Permanent address: Institute of Physics, Chinese Academy of Sciences, Beijing, China * Permanent address: National Physical Laboratory, New Dehli, India
ABSTRACT We discuss the application of x-ray scattering and fluorescence to the problem of unravelling the relationship between the structural and magnetic properties of magnetic multilayers. Particular attention is paid to the use of grazing incidence diffuse scatter to determine the compositional gradient, out-of-plane roughness amplitude, in-plane correlation length and fractal parameter of buried interfaces. Anomalous scattering provides information on the local environment of specific atoms and grazing incidence fluorescence is a depth -sensitive probe of chemical composition. We present examples indicating the sensitivity limits and the reproducibility of the techniques, all from multiple layer structures of magnetic metals. INTRODUCTION In order to understand the physical processes that give rise to the unique magnetic properties of thin films and multilayers, a detailed knowledge of the structure of these films and the interface between layers is essential. There are too many examples in the literature, which we will diplomatically not cite, where the wrong conclusions are reached because there has been inadequate knowledge of the material structure. In particular it has not always been recognised that changing sputter growth conditions often results in both changes in crystallographic texture as well as interface morphology. An excellent example is in sputter growth of Co/Cu multilayers on ion beam etched substrates. As the ion beam voltage is increased there is a catastrophic loss of coupling and giant magnetoresistance (GMR). There is, however, no change in interface roughness, contrary to what might be expected intuiti vely. What does change is the crystallographic texture; increase in voltage results in improved {111} texture [1]. In addition to the crystallographic texture and perfection, the layer thickness and interface structure is critical to the resulting magnetic properties. While scanning probe techniques are superb for the study of the structure of the top surface, very few techniques are capable of analysing the structure of buried interfaces. Transmission electron microscopy is destructive and gives only a very limited field of view. On the other hand, grazing incidence x-ray scattering and fluorescence techniques provide data from buried layers with a sub-monolayer depth sensitivity. While they do average over the sample surface, analysis of diffuse scatter enables lateral length scales down to the order of nanometers to be determined quantitatively. In this paper we review these grazing incidence techniques and illustrate their application to a range of magnetic multilayer systems.
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