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Revised Pages18.11 Magnetic Storage • 749(b)(a)Figure 18.25 (a) A high-
resolution transmission
electron micrograph showing
the microstructure of a
cobalt–chromium–platinum
thin film that is used as a
high-density magnetic storage
medium. The arrow at the top
indicates the motion direction
of the medium. 500,000×.
(b) A representation of the
grain structure for the
electron micrograph in (a);
the arrows in some of the
grains indicate the texture, or
the direction of easy
magnetization. (From M. R.
Kim, S. Guruswamy, and
K. E. Johnson,J. Appl. Phys.,
Vol. 74, No. 7, p. 4646, 1993.
Reprinted with permission.)form, as 1’s and 0’s. In one system,a1isrepresented by a reversal in the magnetic
field direction from one small area of the storage medium to another as the numerous
acicular particles of each such region pass by the head. A lack of reversal between
adjacent regions is indicated by a 0.
The thin-film storage technology is relatively new and provides higher storage
capacities at lower costs. It is employed mainly on rigid disk drives and consists of a
multilayered structure. A magnetic thin-film layer is the actual storage component
(see Figure 18.25). This film is normally either a CoPtCr or CoCrTa alloy, with a
thickness of between 10 and 50 nm. A substrate layer below and upon which the thin
film resides is pure chromium or a chromium alloy. The thin film itself is polycrys-
talline, having an average grain size that is typically between 10 and 30 nm. Each
grain within the thin film is a single magnetic domain, and it is highly desirable that
grain shape and size be relatively uniform. For magnetic storage disks that employ
these thin films, the crystallographic direction of easy magnetization for each grain is
aligned in the direction of disk motion (or the direction opposite) (see Figure 18.25).