Structural characterization of Fe/Pd/Fe/GaAs(001) thin films by x-ray absorption fine structure spectroscopy

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Iron palladium
Ultrathin magnetic film

The small lateral dimensions of spintronic devices and high density memory bits require the employment of magnetic ultrathin metallic film structures. Understanding the structure of such films is a critical component of developing the correct interpretation of their magnetic behaviour. The Molecular Beam Epitaxy facility, MBE-1, was developed for use on the undulator beamline of the Pacific Northwest Consortium Collaborative Access Team at the Advanced Photon Source to permit in situ epitaxial growth of metal films and their structural characterization by synchrotron radiation techniques. In this thesis it was used to characterize the trilayer system iron/palladium/iron on GaAs(001). Three categories of samples were prepared and examined in situ. Iron films were deposited on the 4x6-reconstructed surface of GaAs(001) with thicknesses ranging from 0.5 to 38.5 monolayers. Palladium films were deposited on iron as follows: 1 monolayer palladium on 9 monolayers of iron, and 3.5 and 7 monolayers palladium on 38.5 monolayers of iron. Finally, iron films 4 and 10 monolayers thick were deposited on 7 monolayers of palladium on 38.5 monolayers of iron. The polarization-dependent X-ray absorption fine structure (XAFS) technique in total reflection mode was employed to examine the samples and compare in-plane to out-of-plane structure in these films. This technique allows extracting identities of nearest neighbours, nearest neighbour radial distances, coordination numbers, and mean square relative displacement to characterize the probed system. Iron and palladium K-edge spectra were obtained both above and below the critical angle for total reflection. Near 4 monolayers for iron films on GaAs(001), a transition from island to layer-by-layer growth modes is accompanied by the observation of a body-centered tetragonal structure with a $c/a$ ratio of 1.030(8), with no thickness dependence observed up to 38.5 monolayers. The intermediate palladium layer shows a face-centered tetragonal structure. Alloying at the interface with the underlying iron is restricted to a depth of 0.5-1.0 monolayers. The upper layer of iron shows tetragonal distortion similar to the layers of iron grown on GaAs(001)-4x6. There is also evidence of alloy formation at the interface involving the underlying palladium with the thickness of the alloy region being 1.5-2.0 monolayers.

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E. Daryl Crozier
Science: Department of Physics
Thesis type: 
(Thesis) Ph.D.