Structural Characterisation of Fe-Cu Multilayers of Differing Fe Thicknesses Using Transmission Electron Microscopy
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.20nm Fig. 1 Dark field image formed from the 022 reflection from multilayer C. The Cu/D interface is marked. .20nm Fig.3 Dark field image formed from the 113 reflection of the Cu capping layer. The D/Cu interface is marked.
(220)bccFe (222)fccCu
(002 )fccC (200 )fccCu
Fig.2 Diffraction pattern formed from multilayers C and D and the Cu capping layer.
Fi&.4 Dark field image formed from the 11 lfccCu and 1 10bccFe reflections. The D/Cu interface is marked.
2.4nm. Our initial approach to the calculation of the lattice plane spacing to be expected was based on using conventional elasticity theory and the assumption that the strain energy in each individual multilayer is minimised. This gives values for the in-plane (ax) and layer normal (az) spacings as in table 1, using cubic lattice parameters of 0.3615nm for Cu and an estimated value of 0.3585nm for f.c.c. Fe [5]. Multilayer
Structure
A B C
'fEc.c.' 'f.c.c.' 'f.c.c.'
D
f.c.c./b.c.c.
Experimental Fe wavelength (nm) W3anes 3.05 +±0.1 2 3.05 ± 0.1 4 . 2.90:±:0.1 7
2.90 ± 0.1
10
Cu a. az(Cu) Wdanes (nm) (n) 15 .0.3611.... 0.36.2.1 13 0.3607 0.3627 9 0.3600 0.3637
6
-
az(Fe) (nm) 0...03.55.9.. 0.3559 0.3567
0.3615 0.2866
Table 1. Parameters used for modelling the multilayer structure. 'f.c.c.' refers t structure that the close packed metals adopt in a coherent multilayer.
stored
3. TRANSFORMATION TO THE B.C.C. STRUCTURE Before we discuss the more interesting coherent multilayers we shall consider the way in which coherency is lost for multilayers with thicker Fe layers. The transformation of the Fe to the b.c.c. structure occurred in multilayer D, which had an estimated Fe thickness of 1.8nm (±0.lnm). Fig.l, a dark field image taken with the f.c.c. multilayer 022 reflection shows that coherency is lost within the first few repeats of multilayer D. Diffraction patterns such as fig.2 demonstrate that b.c.c. Fe is formed with [110] parallel to the growth direction,
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consistently with the Pitsch orientation relationship [6] which is in agreement with that observed by other workers using surface sensitive methods on thin films [7-10]. Fig.3, taken using a Cu 113 reflection shows that the f.c.c. Cu in multilayer D is locally in a common orientation with the Cu capping layer above it. The Cu capping layer was infact polycrystalline, with a grain size similar to the total multilayer thickness, and grew with [111] parallel to the growth direction, in two orientations as illustrated in fig.2. Fig 4 shows how the orientation displayed by the Cu capping layer is nucleated within the transformed multilayer D, this leading to the observed relationship between the grain size and the multilayer thickness. The behaviour of Cu growing on the transformed Fe contrasts with a previous study of the FeNi system [11] for which the original Ni orientation ([001] parallel to growth direction) was maintained above the transformed b.c.c. Fe. We now turn to the structure of the coherent 'f.c.c.' layers. 4. HREM STRUCTURAL CHARACTERISATION In order to quantify the in-plan
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