Hardware Reference
In-Depth Information
foranear-perfectvacuummakessputteringthemostexpensiveoftheprocessesdescribed
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. Addition-
ally, 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
ThelatestadvancementindrivemediaiscalledAFCmediaandisdesignedtoallowdens-
ities 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 tech-
nology 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 dif-
ference between the top and bottom magnetic layers. This allows the use of physically
thickermagneticlayerswithmorestablelargergrains,sotheycanfunctionasiftheywere
really a single layer that was much thinner overall.
IBM introduced AFC media starting with the 2 1/2-inch Travelstar 30GN series of note-
bookdrivesintroducedin2001;theywerethefirstdrivesonthemarkettouseAFCmedia.
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 manufactur-
ers. 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.
