Thermally Driven Shape Instability of Multilayer Structures
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were composed of 0.5 up to 5 rum thick intercalated compound (Fe or Nb) and a 10 rnm thick matrix layer of Au, Ag, and Cu, respectively. The investigations of the microstructure were performed with samples composed of 2 nm Fe or Nb and 10 rim Au, Ag, and Cu respectively. The number of bilayers amounted to 100 for transmission electron microscopy (TEM) measurements and to 10 for X-ray microscopy. In the latter case the number of bilayers was reduced because films of this thickness can be transmitted by X-ray without any additional thinning of the film; only the substrate has to be removed by chemical etching. Beside room temperature investigations of in-situ annealed samples, XRD measurements were performed in the hot stage of a X-ray diffractometer under a vacuum of better than 10 -5mbar. X-ray scans were recorded as a function of annealing time (in 5-minutes steps) at a fixed temperature or at a fixed heating rate of 0.2 K/min. The lattice constant of the compounds and the grain size were determined from these scans. The microstructure of the samples was investigated by means of TEM and XRM. For the cross-sectional TEM investigations the samples were ground, dimple-thinned and finally ionmilled. The experimental set up of the X-ray microscope is similar to an optical microscope. The samples are irradiated by synchrotron radiation, which is focused by a zone plate working as a fresnel lens. Due to the strong wavelength dependence of the focal length, the radiation becomes monochromatic by employing a monochromator pinhole. The camera in the picture field can detect either intensity contrasts or phase contrasts. The micrographs presented in this paper are taken by using an amplitude contrast. RESULTS The disintegration process of multilayered systems can be easily detected by X-ray diffraction during a heat treatment, observing the change of the superstructure diffraction pattern. In multilayered systems satellite reflexes due to the periodicity of the multilayer structure are also visible, modifying the common Bragg pattern. A model proposed by [6] was used to analyze the degree of periodicity, the roughness, and the intermixing from the diffraction pattern. Therefore, the change of this pattern reflects the change of the microstructure. In particular, the disappear-
ing superstructure reflections indicate the disintegration of the layered structure. Fig. I depicts the development of the diffraction pattern of an Fe/Au multilayer (1.25 nm / 10 nm) during a heat treatment with increasing temperature up to 330'C. The diffraction pattern of the as-prepared samples results from both the atomic Au lattice Au (111) and the multilayer superlattice. The Fe does not cause Fe (110) "anydiffraction pattern due *
_____280oC
to its small atomic form
factor and its strongly disstructure. The latter been confirmed by C ___TEM investigations, which ... .320'"C reveal no lattice fringes in _ _Fe (Fig. 4). During the heat treatment the sample lost its 41 42 43 44 45 146 47 51 52 53 54 periodicity at about 320°C 2 Theta [°] the F
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