Microstructural Characterization of Longitudinal Magnetic Recording Media
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Mat. Res. Soc. Symp. Proc. Vol. 589 0 2001 Materials Research Society
lel to either the [011 ] or [011] Cr directions. This results in a cobalt grain size usually smaller than that of the chromium, with orthogonal orientations growing on the same underlying Cr grains (the so-called "bicrystal structure") as shown in Fig. 1. Further explanations of this detail can be found elsewhere [1,2]. Discs described here were manufactured in a standard fashion at HMT Technology, Seagate Technology and Komag Corporation. Specimens for TEM analysis are prepared by conventional means. 3 mm diameter discs may be cut from the (larger) hard discs, mechanically dimpled from one side to less than 10 gtm thickness and finally ion-beam milled to perforation. The final ion-milling step can also be refined so that either the cobalt or the chromium film could be examined preferentially. Bright field, dark field, high-resolution images and their associated diffraction patterns were obtained in regular TEM's (Philips EM430 or CM20), and nanoprobe analysis for X-ray energy dispersive spectroscopy was achieved in a field-emission gun TEM. Energy filtered imaging was carried out at Oak Ridge National Laboratory.
Figure 1. A high-resolution TEM image showing the bi-crystal grain structure of a CoCrPtTa alloy media. Arrows point along the c-axis directions.
RESULTS Clearly there are many possible avenues for altering the microstructure to achieve superior recording performance. One primary goal from the magnetic point-of-view is to bring about the sharpest possible "bit transition regions", where the induced magnetization switches from parallel to anti-parallel. Because of the random nature of the cobalt crystal orientations in the plane of the film, this requires very small grain sizes (e.g. 10 nm), with some degree of magnetic decoupling of adjacent grains. Thus the analysis of grain size and the degree of grain separation is of major 4
technological concern at the present time. There are various scientific difficulties associated with such analyses by TEM, and this presents a major emphasis of the present article. Grain Sizes The determination of average grain size has long been a task for metallography. But when the dimensions involved are in the 10-20 nm range, this is not a straightforward venture at all. In order to utilize a computer analysis such as the NIH program, all of the grain boundaries in the area of interest must be identified [3]. In conventional bright and dark field images only a small fraction of the grains are diffracting strongly at any one time. Moreover, when adjacent grains also diffract to a similar extent, the presence of a grain boundary may not be detected in a single image. Figure 2 illustrates this problem by showing the subtle change of appearance with very small specimen tilts. Therefore in order to achieve a somewhat reliable analysis, a large number of complementary images from the same area is required, with very detailed documentation of each grain, which requires significant investment of time. Furthe
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