Hardware Reference
In-Depth Information
platter with several layers of metallic film. The magnetic medium layer itself is a cobalt alloy about 1
μ-inch thick.
Thin-film sputtered media are created by first coating the aluminum platters with a layer of nickel
phosphorus and then applying the cobalt-alloy magnetic material in a continuous vacuum-deposition
process called
sputtering
. This process deposits magnetic layers as thin as 1 μ-inch or less on the
disk, in a fashion similar to the way that silicon wafers are coated with metallic films in the
semiconductor industry. The same sputtering technique is again used to lay down an extremely hard, 1
μ-inch protective carbon coating. The need for a near-perfect vacuum makes sputtering the most
expensive of the processes described here.
The surface of a sputtered platter contains magnetic layers as thin as 1 μ-inch. Because this surface
also is smooth, the head can float more closely to the disk surface than was previously possible.
Floating heights as small as 10nm (nanometers, or about 0.4 μ-inch) above the surface are possible.
When the head is closer to the platter, the density of the magnetic flux transitions can be increased to
provide greater storage capacity. Additionally, the increased intensity of the magnetic field during a
closer-proximity read provides the higher signal amplitudes necessary for good signal-to-noise
performance.
AFC Media
The latest advancement in drive media is called AFC media and is designed to allow densities to be
pushed beyond previous limits. Any time density is increased, the magnetic layer on the platters must
be made thinner. Areal density (tracks per inch times bits per inch) has increased in hard drives to the
point where the grains in the magnetic layer used to store data are becoming so small that they
become unstable over time, causing data storage to become unreliable. This is referred to as the
superparamagnetic limit
, and it was originally determined to be between 30Gb/sq. in. and 50Gb/sq.
in. However, as technology has advanced, this so-called limit has been pushed further and further
back, and commercially produced drives have achieved nearly 500Gb/sq. in., with densities of up to
1,000Gb/sq. in. or more expected in the future.
AFC media consists of two magnetic layers separated by a thin, 3-atom (6 angstrom) film layer of the
element ruthenium. IBM has coined the term
pixie dust
to refer to this ultra-thin ruthenium layer. This
sandwich produces an AFC of the top and bottom magnetic layers, which causes the apparent
magnetic thickness of the entire structure to be the difference between the top and bottom magnetic
layers. This allows the use of physically thicker magnetic layers with more stable larger grains, so
they can function as if they were really a single layer that was much thinner overall.
IBM introduced AFC media starting with the 2 1/2-inch Travelstar 30GN series of notebook drives
introduced in 2001; they were the first drives on the market to use AFC media. IBM then introduced
AFC media in desktop 3 1/2-inch drives starting with the Deskstar 120 GXP. AFC media is now used
by Hitachi Global Storage Technologies, which owns the former IBM hard drive division, as well as
virtually all other hard drive manufacturers. The use of AFC media allowed areal densities to be
extended to 100Gb/sq. in. and beyond, and when combined with perpendicular magnetic recording
(PMR), densities can grow to more than twice that.
“
Magnetic Storage Principles
,”
p.
439
.
Read/Write Heads
