Evolution of Gd Thin Film Structure Due to Amorphization by Co Deposition
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EVOLUTION OF Gd THIN FILM STRUCTURE DUE TO AMORPHIZATION BY Co DEPOSITION T.C. HUFNAGEL, G.A. BERTERO, R. SINCLAIR and B.M. CLEMENS Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305-2205
ABSTRACT Multilayers of Gd/Co produced by sputter deposition are observed to be amorphous for a wide range of bilayer periods. This structure is largely the result of an amorphization reaction during the deposition between crystalline Gd and newly arriving Co atoms. We explore this reaction by producing multilayers of Gd/Co/Cr, and examining the resulting microstructure. We observe both interfacial disordering due to a layer thickening mechanism and evidence for growth of the amorphous phase from Gd grain boundaries. The reaction is enhanced by the presence of a large number of crystalline defects in the Gd layers.
INTRODUCTION The development of interfacial structure plays an important role in determining the bulk structure and properties of metallic thin films and multilayers. A range of behaviors are observed, ranging from complete structural coherency at the interface at one extreme, to the development of extensive disordered interfaces at the other. In systems which form disordered interfaces, multilayers with a bilayer period below some critical value are observed to be compositionally modulated amorphous alloys. Recent work on the Gd/Co system indicates that at least two distinct processes can result in the formation of disordered interfaces[1,2J. First, deposition of Co onto crystalline Gd results in diffusion of Co into the Gd, accompanied by a solid-state amorphization reaction. Second, the presence of Co in an underlayer inhibits the nucleation and growth of crystalline Gd. The result of these two processes is that Gd/Co multilayers are amorphous for all bilayer periods up to approximately 200 A; only at larger bilayer periods are significant amounts of crystalline Gd and Co observed. In the present work, we examine the first of these two processes, the amorphization of crystalline Gd layers by the subsequent deposition of Co. To separate the effects of the two reactions, we deposit multilayers of Gd/Co/Cr, wherein the thicknesses of Gd and Cr are kept constant and the thickness of Co varied between samples. The Cr interlayer is intended to act as a diffusion barrier between the Co and subsequently deposited Gd; however, as will be seen later, there appears to be a significant amount of Co dissolved in Cr in these samples. In situ x-ray diffraction observations during deposition have shown that the amorphization reaction occurs very rapidly at room temperature[1]. If the reaction proceeds by layer thickening from the surface of the growing film, then a very high diffusivity of Co through Mat. Res. Soc. Symp. Proc. Vol. 311. @1993 Materials Research Society
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the growing amorphous phase is required to maintain the rate of the reaction. This led to speculation that high diffusivity paths through the Gd layer play an important role in the reaction. If correct, such paths would ma
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