Correlations, Coherence, and Complexity

With the availability of high brightness soft and hard x-ray sources, a new suite of applications of synchrotron radiation based on transverse coherence has emerged. Our own interest has been to combine the benefits of full transverse coherence with the well-established charge and magnetic contrast available near x-ray absorption edges, particularly in the soft x-ray regime, to probe complex magnetic materials and structures. Scattering a coherent beam off material inhomogeneities produces a speckle/diffraction pattern of the specific illuminated region. Such speckle patterns have been used in various optical metrology measurements and also form the basis for dynamic laser light scattering. In both of these applications, a system is compared to itself, either at a later time or after application of an external stress or field. With appropriate processing, such measurements provide a statistical measure of how a system evolves under these influences. We have adapted these approaches to the soft x-ray regime to study magnetic materials that spontaneously form complex nanoscale structures. I will discuss how we have used these techniques to learn how to control the microscopic memory of the domain structure of exchange-biased magnetic films and to probe the fluctuations of orbital domains in manganite crystals.