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Magnetic Materials

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Cambridge University Press978-1-107-41335-1 - Materials Research Society Symposium Proceedings: Volume 589:Advances in Materials Problem Solving with the Electron MicroscopeEditors: Jim Bentley, Charles Allen, Uli Dahmen and Ivan PetrovExcerptMore information

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MICROSTRUCTURAL CHARACTERIZATION OF LONGITUDINAL MAGNETICRECORDING MEDIA

ROBERT SINCLAIR, DONG-WON PARK, CLAUS HABERMEIER and KAI MADepartment of Materials Science and Engineering, Stanford University, Stanford, CA 94305-2205, [email protected]

ABSTRACT

The optimization of disc manufacturing conditions is required to increase the storage capaci-ties of magnetic recording media, which is strongly related to both magnetic properties and micro-structural features. Analyzing the microstructure requires transmission electron microscopy(TEM), since the small grain sizes of the media prevent other tools from characterizing them. Thispaper discusses several fascinating characteristics of TEM in understanding and analyzing theproperties of the recording media.

INTRODUCTION

One of the most remarkable high-technology industries at the present time concerns com-puter hard disc manufacturing. For the last several years, increases in the areal density of mag-netic recording have occurred at a rate of 60% or more per annum, which is achieved by advancesboth in the media and in the recording heads. At the time of writing, products with 36 Gbits/in2

have been demonstrated, and the industry goal of 100 Gbits/in2 is clearly in sight. This articlefocuses on the magnetic medium itself.

The important magnetic properties usually comprise coercivity, squareness of the magnetiza-tion hysteresis loop and the signal-to-noise ratio of the recording. All are manipulated by the discprocessing conditions, which at the materials level influence the underlying microstructure. Struc-tural parameters which are thought to play a role in determining properties include crystal sizeand orientation, defect density, phase identity, grain boundary segregation or separation etc. Asthe grain sizes are typically in the range 10-20 nm, only transmission electron microscopy (TEM)has the capability to analyze the microstructure in a detailed fashion. In this paper, we highlighthow TEM can be applied to establish the structure-property relationships and illustrate the diffi-culties associated with addressing this critical problem.

BACKGROUND

The magnetically active material currently used in hard disc technology is a thin film of acobalt-chromium-X alloy (X being one or more minor additional elements), in the hexagonalclose packed (HCP) crystal structure. For longitudinal magnetic recording, the magnetically"easy" c-axis is induced to lie in the plane of the film (i.e. the basal planes are standing proud withrespect to the thin film surface). This is achieved by suitable epitaxial growth, the most commonarrangement utilizing a body-centered cubic (BCC) underlayer of chromium or a chromium alloy.These films are grown sequentially by sputtering onto either nickel-phosphorus plated aluminumor glass substrate discs. A final carbon overcoat is deposited to protect the film during use.

The most commonly employed processing conditions bring about (200) oriented Cr poly-crystalline films, with the cobalt alloy growing with (1120) planes parallel to the surface (i.e. thec-axis must crystallographically lie in the plane of the film). Loss of this epitaxy, for instance byan interfacial reaction, results in a vertical c-axis with concomitant degradation in the longitudinalrecording performance. Two orientations of the cobalt crystals are possible, with the c-axis paral-

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Mat. Res. Soc. Symp. Proc. Vol. 589 © 2001 Materials Research Society






lei to either the [011] or [011] Cr directions. This results in a cobalt grain size usually smaller thanthat of the chromium, with orthogonal orientations growing on the same underlying Cr grains (theso-called "bicrystal structure") as shown in Fig.l. Further explanations of this detail can be foundelsewhere [1,2]. Discs described here were manufactured in a standard fashion at HMT Technol-ogy, Seagate Technology and Komag Corporation.

Specimens for TEM analysis are prepared by conventional means. 3 mm diameter discs maybe cut from the (larger) hard discs, mechanically dimpled from one side to less than 10 [im thick-ness and finally ion-beam milled to perforation. The final ion-milling step can also be refined sothat either the cobalt or the chromium film could be examined preferentially. Bright field, darkfield, high-resolution images and their associated diffraction patterns were obtained in regularTEM's (Philips EM430 or CM20), and nanoprobe analysis for X-ray energy dispersive spectros-copy was achieved in a field-emission gun TEM. Energy filtered imaging was carried out at OakRidge National Laboratory.

Figure 1. A high-resolution TEM image showing the bi-crystal grain structure of a CoCrPtTaalloy media. Arrows point along the c-axis directions.

RESULTS

Clearly there are many possible avenues for altering the microstructure to achieve superiorrecording performance. One primary goal from the magnetic point-of-view is to bring about thesharpest possible "bit transition regions", where the induced magnetization switches from parallelto anti-parallel. Because of the random nature of the cobalt crystal orientations in the plane of thefilm, this requires very small grain sizes (e.g. 10 nm), with some degree of magnetic decouplingof adjacent grains. Thus the analysis of grain size and the degree of grain separation is of major






technological concern at the present time. There are various scientific difficulties associated withsuch 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 thedimensions involved are in the 10-20 nm range, this is not a straightforward venture at all. Inorder to utilize a computer analysis such as the NIH program, all of the grain boundaries in thearea of interest must be identified [3], In conventional bright and dark field images only a smallfraction of the grains are diffracting strongly at any one time. Moreover, when adjacent grainsalso diffract to a similar extent, the presence of a grain boundary may not be detected in a singleimage. Figure 2 illustrates this problem by showing the subtle change of appearance with verysmall specimen tilts. Therefore in order to achieve a somewhat reliable analysis, a large number ofcomplementary images from the same area is required, with very detailed documentation of eachgrain, which requires significant investment of time.

Furthermore, when a bicrystal structure is present, it is not possible to distinguish the twovariants by conventional imaging. High-resolution pictures are required. As the latter involvesuperior specimens, more careful microscope adjustment, higher magnifications, and so fewergrains in the image, developing sufficient statistics becomes a key issue.

Measurement of the grain size itself is not so obvious. The NIH program, when it can beused, converts the grain area into its equivalent representative dimension. But when individualmeasuring is necessary, the length across the grain is to be determined. We generally use theapproach to estimate the longer axis and its orthogonal dimension, and take the average. An alter-native which has also been employed is to determine the grain size parallel and perpendicular tothe c-axis (from high-resolution images), and to quote both numbers [4]. Note that the linear inter-cept method commonly employed in optical microscopy cannot be used here because very fewgrain boundaries can be seen in the images. In reality any sensible approach is acceptable, but per-haps it is useful to specify the method in each case.

Of course, one of the major concerns with TEM analyses concerns the statistical nature of thedata, especially compared to bulk techniques such as X-ray diffraction. In one study comparingdata from high-resolution images with those from grazing incidence X-ray line broadening [5],both the absolute values from each technique and their trend as a function of underlayer materialwere in reasonable agreement. However, in our case, the microstructure is at a very small scale.Clearly the magnetic properties are reproducible across the disk (otherwise the manufacturingprocess would be suspect!), and the written bit sizes are not that much larger than the grains them-selves (e.g. 63 nm at 400 kfci). Therefore, it is commonly found that samples taken from differentareas on the same, or even an alternative, disc yield similar data.

In our experience, it is generally not necessary to analyze a large number of samples for reli-able results. This issue has been considered in some detail by Carpenter et al. [6]. One perhapssurprising result is that the TEM grain size average for 100 grains in poly crystalline Al is withintwo percent of the value for 1000 or even 2500 grains (see Fig. 10 in Ref. 6). We obtain a similarfinding (e.g. Fig. 3). Accordingly, a quantitative analysis does not require as many grains as mightbe expected - we generally use about 200 grains to allow same safety level.

One interesting point is associated with the subjectivity of the analysis. Different researchers,even those with significant experience, may adopt slightly different criteria in deciding what con-stitutes a grain, or in evaluating the dimension of "grain size". Both the grain size distribution andaverage may vary from one person to another (e.g. Fig. 4). We recommend, therefore, that oneresearcher only determines the grain sizes in a particular comparative set, and that the individualcriteria and approach be clearly stated.






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Figure 4. Grain size distributions obtained from the same micrographs by two differentresearchers. Note that the averages were determined to be 15.3 and 16.5 nm, respectively.

Grain Size Distributions

In addition to the grain size itself, the variation in grain size, or rather the "grain size distribu-tion", is an important microstructural parameter. Technologists aim to achieve as narrow a distri-bution as possible, but manipulation of this feature by processing is naturally limited.

Display of the data is often achieved using a standard histogram (e.g. Fig. 5) [7,8], but itsnature is altered by the "bin size" populations. We find that a cumulative percentage curve is supe-rior, plotted either linearly with increasing grain size, or as a function of the logarithm of grainsize. The latter allows simple distinction of mathematical descriptions of the distribution. Forinstance, for the former a Gaussian distribution is a linear plot and a log-normal is curved, andvice versa for the log-log graph (e.g. Fig. 6). In all our data, on Co-Cr-Ta and Co-Cr-Pt alloys and






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on Cr and Cr alloy underlayers, we find that a log-normal distribution is the best fit, althoughresearchers at Hitachi have developed a "modified" Gaussian for their data on the Co-Cr-Pt alloymedia [7]. Of some surprise here is that again we would anticipate that a very large samplingwould be required to establish the distribution. This turns out not to be the case, as shown by Fig.7. On the logarithmic plot the distribution from a small, randomly chosen number of grains (22)remains rather constant even up to a sampling an order-of-magnitude larger. This indicates that an






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analysis of a few hundred grains is more than adequate to establish the nature of the grain size dis-tribution when displayed this way, although of course "more is better".

However, while the graphical data yield useful numbers, it is worthwhile remembering thatthe original images still remain useful in evaluating the microstructure, as shown in Fig. 8 for a Crunderlayer analysis.

Defects

One of the central paradigms of materials research is that "defects influence properties".Therefore it should be no surprise that this same statement should be applied to recording media,not necessarily always discriminatively. Dislocations appear to play no major role as the grainsthemselves are so small. However, with the very low stacking fault energy of cobalt and its alloys,significant stacking fault densities are present which vary with, at the least, alloy content. At thefaults, the stacking sequence is changed locally to an FCC arrangement. As the FCC cobalt phasehas different magnetic properties from those of the HCP phase, some effect on recording might beanticipated.

A detailed study of the effects of stacking faults is not easy. Really, only high resolutionimaging is capable of unequivocally identifying the faults, and so the development of reasonablestatistics becomes an issue. One such attempt was made by Ishikawa et al. [9]. By varying thealloy content of Co-Cr-Pt and Co-Cr-Ta alloys (15% Cr, with up to 8% Pt and 6% Ta), it wasfound that the stacking fault density increased (from about 0.3 nm"1 up to 0.5 nm'1 for the former,






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Figure 8. A comparison of the microstructure and cumulative percentage curves for Cr under-lay ers deposited with different film thickness. The images reveal the microstructural differ-ences just as well as the graphical data.

and close to 0.4 nm for the latter). However the coercivity and anisotropy field approximatelydoubled for the Co-Cr-Pt alloys, but rather decreased by about 20% for the Co-Cr-Ta alloys. Thusthere was no clear concomitant changes of magnetic properties with stacking fault density or alloycontent in general. One can only conclude at this stage that the role of stacking faults remainsunclear, and that some general trend (e.g. either increasing or decreasing stacking fault popula-tions) does not necessarily influence the magnetic performance one way or the other.

Grain Boundaries and Segregation

The grain boundaries themselves represent perhaps the most interesting microstructural vari-able. Not only are there random high angle and low angle boundaries (where cobalt grains, grownon adjacent, random chromium grains, meet), but also the 90° boundaries in bicrystal domains

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