Atomic Scale Engineering of Superlattices and Magnetic Wires
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Introduction.
In the past years artificially-structured materials have been grown with an increasing degree of sophistication due to steady progress in our ability to control growth processes down to the atomic level. These materials have yielded new physical properties due to the confinement of electrons in less than three dimensions. Thus, the confinement of electrons in two-dimensional (2D) metallic superlattices has resulted in oscillatory magnetic coupling with an associated oscillatory giant magnetoresistance (GMR). New properties are expected when the electrons are further confined to one dimension (11D) of free motion in the structures known as quantum wires. In this report we briefly describe two recent examples of atomic-scale engineering of materials. In the first case a surfactant is used to purposely modify the structure of magnetic/non magnetic superlattices. The second
example illustrates a further reduction in dimensionality obtained by modifying the substrate onto which the growth takes place: the fabrication of ID magnetic quantum wires on vicinal surfaces.
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Experimental.
The experiments have been carried out in three different Ultra High Vacuum (UHV) chambers: a) A Molecular Beam Epitaxy (MBE) system equipped with Thermal Energy Atom Scattering (TEAS) and Low Energy Electron Diffraction (LEED), b) a system equipped with Scanning Tunneling Microscopy (STM) and LEED and c) a six-circle Xray diffractometer placed at the ID3 beamline of the European Synchrotron Radiation Facility (ESRF) at Grenoble. The samples were Cu(111) flat and vicinal surfaces, cleaned in-situ by ion sputtering and annealing. In all cases, prior to the growth, the cleanliness was checked by Auger Electron Spectroscopy (AES) and the crystalline perfection by LEED, TEAS and STM respectively. The metals evaporated, Pb, Co, Cu and Fe were deposited from different electron bombardment and thermal sources at low rates (A/min) under residual pressures of the order of 10-" Torr.
49 Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
3 3.1
Results and Discussion. Surfactant effects on the growth of {Co/Cu}(111) superlattices.
It has been shown that Co can be grown by MBE at room temperature (RT) in the metastable fct phase on an adequate substrate such as Cu(100) [1, 2]. The lattice mismatch being moderate (,- 2%), the Cu surface provides a template onto which the Co atoms lock, accommodating the forced lateral expansion with a contraction of their interlayer spacing. This structure can be maintained up to rather large thicknesses because the square symmetry of the Cu(100) face is very different from the hexagonal one of bulk hcp-Co, thus making the transition very unlikely [1]. Accordingly, Co/Cu superlattices grown along the (100) direction are strictly fcc and have shown antiferromagnetic (AF) coupling [3] and oscillations in the coupling as a function of the Cu spacer [4], reproduced by several groups [5, 6]. On the contrary, experimental studies of epitaxial films of Co grown onl Cu( 111) and (111)-orien
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