High-Resolution Electron Microscopy Studies of Co/Cr Multilayers
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HIGH-RESOLUTION ELECTRON MICROSCOPY STUDIES OF Co/Cr MULTILAYERS Shu-Chen Y. Tsen, Mary Beth Stearns and David J. Smith* Department of Physics, Arizona State University, Tempe, AZ 85287 ABSTRACT The growth characteristics of Co/Cr multilayers have been studied in cross-section with a 400kV high-resolution electron microscope. The growth
directions observed
for bcc Cr were predominantly while
crystallite
fringes
were
mainly
and
the axes of the
.
The
hcp Co
crystallites grew predominantly in the direction while the areas with strong fringes were in agreement with scattering spectra. layers growing right
along a projection. This observed structure was that obtained from analysis of large angle X-ray The columnar structure was found to be present with the through the columns.
INTRODUCTION Because
of their fundamental
magnetic
properties (31 and high potential as a medium
behavior
[1,2],
electronic
for perpendicular magnetic
recording, it is important to understand the growth characteristics of Co/Cr multilayers (ML). Large angle X-ray scattering (LAXS) spectra of
Co/Cr ML with a total thickness
of about 3000A and different bilayer
thicknesses have previously been analyzed to deduce the bilayer thicknesses, the coherent length of multilayer growth and the sharpness of the interface [4,51. In this study, a 400kV high-resolution electron microscope (HREM) was used to obtain atomic resolution images of specimens prepared in cross-
section.
Details of the growth of bcc Cr and hcp Co crystallites within
each layer, as well as the columnar structure and coherent growth of Co/Cr ML, have been investigated. In order to obtain further quantitative information, the HREM images were analyzed using a sophisticated image processing system "SEMPER" [61, and the results were found to be consistent with those obtained by LAXS. Since the X-ray studies give an average structure which is strongly weighted by the crystallite size, while the features that stand out in the TEM images are projections along highly symmetric directions, the two methods are complementary. Thus, some crystallite alignment orientations may be seen in TEM that are not evident in LAXS. EXPERIMENTAL TECHNIQUES Multilayer materials with different bilayer thicknesses (14A - 400A) were prepared using a dual e-beam UHV system having a base pressure of around 5xlO-10 torr. The growth pressure was typically = 10-0 torr at a deposition rate of 1 A/s. The substrates were p-type (100)-oriented Si crystals with oxidized surfaces. Cross-sectional TEM specimens were prepared by a method similar to that described by Bravman and Sinclair [7]. After mechanically thinning to a thickness of about 15 microns, the specimens were cooled to liquid nitrogen temperature and ion-milled with Ar+ ions. A JEM-4000EX HREM having a point-to-point resolution of better than 1.7A was used to obtain the atomic resolution images. Micrographs recorded at 400 KV and the optimum defocus condition were digitized for analysis with the SEMPER image processing programs in order
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