We report photoemission studies of ultrathin films of the minority-spin conductor, Fe3O4 on a Cu(001) substrate. Fe films are deposited at room temperature and oxidized at 810 K in 10−6 Torr O2. For Fe films less than 2 ML thick, LEED and STM measurements show that oxidation produces mostly an FeO(111) while thicker Fe layers give Fe3O4(111). Photoemission reveals that there is a distinct electronic structure for these two phases. There are also significant differences between the electronic structures of ultrathin oxide films and those of corresponding bulk oxides.
The structural and magnetic properties of NiMnSb films, 5-120 nm thick, grown on InGaAs/ InP͑001͒ substrates by molecular-beam epitaxy, were studied by x-ray diffraction, transmission electron microscopy ͑TEM͒, and ferromagnetic resonance ͑FMR͒ techniques. X-ray diffraction and TEM studies show that the NiMnSb films had the expected half-Heusler structure, and films up to 120 nm were pseudomorphically strained at the interface, greater than the critical thickness for this system, about 70 nm ͑0.6% mismatch to InP͒. No interfacial misfit dislocations were detected up to 85 nm, however, relaxation in the surface regions of films thicker than 40 nm was evident in x-ray reciprocal space maps. TEM investigations show that bulk, planar defects are present beginning in the thinnest film ͑10 nm͒. Their density remains constant but they gradually increase in size with increasing film thickness. By 40 nm these defects have overlapped to form a quasicontinuous network aligned closely with ͗100͘ in-plane directions. The associated strain fields and or compositional ordering from these defects introduced a reduction in crystal symmetry that influenced the magnetic properties. The in-plane and perpendicular FMR anisotropies are not well described by bulk and interface contributions. In thick films, the in-plane uniaxial and fourfold anisotropies increased with increasing film thickness. The lattice defects resulted in a large extrinsic magnetic damping caused by two-magnon scattering, an increase in the coersive field with increasing film thickness, and a lower magnetic moment ͑3.6 Bohr magnetons͒ compared to the expected value for the bulk crystals ͑4 Bohr magnetons͒.
We compare the strain relaxation of In0.08Ga0.92As and In0.12Ga0.88As0.99N0.01 epitaxial thin films grown on GaAs (001) by elemental-source molecular-beam epitaxy. The epilayers we studied were essentially identical in their compressive lattice mismatch (0.62±0.02%), and thickness (600 nm). The strain state of the samples was determined by in situ substrate curvature monitoring, and by ex situ x-ray diffraction and plan-view transmission electron microscopy. We observe a slower rate of strain relaxation, and a 25% higher residual strain in the nitride. This is attributed to the presence of nitrogen interstitials in the InGaAsN epilayers and/or to the higher nitrogen bond strengths.
The Fermi surface of tetragonally distorted fcc Co films grown on Cu(001) has been investigated with first-principles calculations and compared with an experimental determination using angle-resolved photoemission. The angular distributions for hnu=21-45 eV are dominated by the structure of the final states rather than by the shape of the Fermi surface. Theoretical estimates of the photoemission matrix elements support this observation. This suggests that photoemission can have limitations in mapping Fermi surfaces, especially for materials that exhibit flat, closely spaced valence bands.
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