Grain growth in ultrathin films of CoPt and FePt
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Grain growth in ultrathin films of CoPt and FePt R.A. Ristau and K. Barmak Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania 18015
K.R. Coffey and J.K. Howard IBM Storage Systems Division, 5600 Cottle Road, San Jose, California (Received 17 February 1999; accepted 24 May 1999)
The microstructure of sputtered 10-nm thin films of equiatomic binary alloys of CoPt and FePt was characterized using transmission electron microscopy (TEM). Grain growth kinetics was examined using manual and digital analysis of bright-field TEM images and was seen to take two stages during annealing in these films. A rapid growth stage concurrent with the formation of a [111] fiber texture was observed to occur within the first 5–10 min of annealing, followed by a much slower growth stage after the fiber texturing was well advanced. Differences in grain growth rate and ultimate grain size were also observed to depend on heating rate.
I. INTRODUCTION
Thin film grain growth, during both vapor deposition and postdeposition annealing has been a much-studied phenomenon. A “structural zone model”1–4 for growth during deposition has been proposed in which microstructure morphology is classified into zones that are defined by the ratio of the deposition temperature to the film-melting temperature. The various zone models have been summarized in Ref. 4 and will not be discussed here. It is sufficient for our purposes to divide the microstructure of deposited films into two simple classes: those where the film has equiaxed grains with a mean grain size (d) less than the film thickness (h), and those where the grains have a columnar structure in which the film normal dimension is equal to the film thickness. In the former case, three-dimensional grain growth can occur during postdeposition growth; in the latter, only twodimensional growth is possible. Generalized theories for postdeposition grain growth have been presented, which account for the reduction of surface energy and strain energy, as well as grain boundary energy.5–7 These factors have been included in a general equation for grain boundary velocity (v) as follows5:
冋 冉 冊
v = m ␥gb
册
1 1 ⌬␥s + ⌬␥i − + ⌬W . + r¯ r h
(1)
The first term in the bracket represents curvature-driven grain growth (␥gb is the average grain boundary energy; r the average grain radius in the film; r the growing grain radius). The second term represents the surface energy contribution to grain growth (⌬␥ being the difference in surface energy of the growing grain relative to other J. Mater. Res., Vol. 14, No. 8, Aug 1999
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grains, the subscripts s and i denoting that the film freesurface and film/substrate interface must be considered separately; h is the film thickness). The last term represents the strain energy density difference between the growing grain and the grains in bulk. It should be emphasized that the strain energy driving force (⌬W) is highly dependent on
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