Structure of Ordered and Disordered Systems

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My principal interest is the structure of magnetic nanostructures engineered from ferromagnetic/ non-ferro- magnetic layers epitaxially grown on semiconductors.  The correct interpretation of the magnetic behaviour in such systems- the magnetic anisotropy and interlayer coupling- is strongly dependent on the nature of the interface between layers, the strain in the layers and the extent of any interdiffusion or reaction between layers and substrate. The same factors are important to improving spintronic devices which depend upon the efficiency with which an electron carries its spin through a metal-semiconductor interface.   We examine these factors with synchrotron radiation structural techniques applied in situ in two molecular beam epitaxy (MBE) facilities which we constructed at the PNC/XOR (Pacific Northwest Consortium/ X-ray Optics and Research) beamlines, Advanced Photon Source, IL. We study metallic films as they are grown epitaxially, layer by layer. Our structural techniques include XAFS (x-ray absorption fine structure), specular and diffuse x-ray reflectivity, x-ray standing waves and surface x-ray diffraction. We have examined extensively Fe/GaAs, Pd/Fe/GaAs and Fe/Pd/Fe/GaAs and will continue with related systems.    A persistent problem is the origin of the uniaxial  in-plane magnetic crystalline anisotropy at the  Fe-GaAs interface. We have found an in-plane structural anisotropy for 2 ML Fe but not for 4 ML and thicker films. We will determine if the in-plane structural anisotropy is a universal feature, independent of the surface reconstruction of GaAs. At the submonolayer level we will attempt to determine if a Fe-GaAs complex forms at the interface and what is its "crystal" structure. We will also look for Fe complexes at the Fe/AlGaAs interface in Fe/AlGaAs/GaAs which has a high electron spin-injection efficiency.    We are examining spin dynamics in ferrimagnetic amorphous GdCo alloys. In a sub-nanosecond  time- resolved experiment we pulse the sample temperature with the PNC/XOR 200 femtosecond laser and use x-ray magnetic circular dischroism to follow the magnetization as the Gd and Co sub-lattices re-orient.