Initial Growth and Morphology of Ultrathin Magnetic Films Studied Using Scanning Tunneling Microscopy
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INITIAL GROWTH AND MORPHOLOGY OF ULTRATHIN MAGNETIC FILMS STUDIED USING SCANNING TUNNELING MICROSCOPY. DAVID 1). CIIAMBI.ISS, K.E. JOIINSON, K. KALKI, S. CIIIAN(; AND R.J.WILSON IBM Research Division, Almaden Research Center, 650 Harry Road, San .lose, California 95120-6099 ABSTRACT The room-temperature growth of Fe on Cu(100) has been studied using the scanning tunneling microscope (STM) to determine low-coverage growth mode and local structures related to the FCC-BCC structural transformation. Results for submonolayer deposition demonstrate an initial interchange of deposited Fe atoms with substrate Cu. This leads to a highly rough Fe-Cu interface and growth characteristics that for different experimental techniques can resemble 3-D island growth or layer-by-layer growth. For a thickness -14 monolayers, the FCC-BCC transition is observed to occur via the formation of fairly large martensitic grains, rather than by a change in atomic aggregation. The implications of the instability of FCC-Fe, as evident in both low- and high-coverage data, are considered. INTRODUCTION A great deal of effort is being devoted to understanding the magnetic properties of ultrathin metal films and layered structures, with layers typically less than 10 atoms thick. A serious hindrance to the complete understanding of these properties has been uncertainty about the actual structure of the films. Of obvious importance is the film morphology, since films of a uniform thickness can differ markedly in magnetic properties from films whose thickness varies by only a few atomic diameters over length scales of several nanometers. Equally interesting are local or global variations in atomic structure in the films such as strain and defects. In recent years it has become increasingly clear that the scanning tunneling microscope (STM) is an excellent tool for investigating these questions. Fe grown epitaxially on Cu(100), which is the subject of this paper, is a magnetically important system that illustrates the richness of structural variation possible in these very thin films. This is ironic, since early hopes were that one could determine the physics of a simple monolayer of face-centered cubic Fe readily. Soon after the growth of face-centered cubic (FCC) Fe on Cu(100) was reported,l contradictions began to appear in the literature Auger electron spectroscopy (AES) regarding its room-temperature growth mode. 2 and later as breakpoints were interpreted initially as indicating layer-by-layer (XPS) intensities growth. Anisotropies of AES and x-ray photoemission bilayer-by-bilayer indicated that a significant fraction of Fe atoms in a submonolayer deposit were4 buried beneath other atoms, which seemed to indicate the growth of bilayer or 3-I) islands. , S The variations in diffracted intensity observed using reflection high-energy electron diffraction 7 6 (RIIEED) , medium-energy electron diffraction (MF.F.I)) , and helium atom scattering (hIAS) 8 . 9 have been interpreted as indicating an initial 3-1) growth followed by island of only monolayer-hi
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