Hardware Reference
In-Depth Information
contrast, in perpendicular recording, the bits are aligned vertically, perpendic-
ular to the disk, which allows additional room on a disk to pack more data,
thus, enabling higher recording densities. In March 2005, Hitachi Global Stor-
age Technologies demonstrated an areal density of 230 gigabits per square inch
(Gb/in
2
) on perpendicular recording technology. This accomplishment repre-
sents a doubling of todays highest data densities on longitudinal recording
technology. Such products with perpendicular recording is expected to greet
the market as early as in 2007. Projection made by Hitachi suggests the avail-
ability of 1-inch micro drive with 20 gigabytes capacity and 3
2
inch products
with terabyte capacity.
Heat Assisted Magnetic Recording (HAMR):
HAMR shows the promises to be the key enabling technology that will increase
the areal density to a level breaking through the so-called super paramagnetic
limit of magnetic recording. This technology is expected to deliver storage
densities as high as 50 terabits per square inch. If disk drives are produced
to have such a great areal density, one can store the entire printed contents of
the Library of Congress on a single disk drive.
If the phenomenal growth rate of bit density continues, the size of an indi-
vidual bit will soon reach such a small dimension that the bits become mag-
netically unstable. At that stage, though the bits can be written very tiny,
they may not be suitable for information storage as some of them may
fl
ip into
different polarization. This phenomenon is known as super paramagnetism.
This problem can be overcome by heating the medium with a laser beam at
the precise spot where a data bit is being recorded and subsequently cooling
the spot rapidly to stabilize the written bit. Heating makes it easier to write
on the medium. This heat assisted recording can increase the recorded density
dramatically.
Patterned Media:
Another promising approach to circumvent the density limitations imposed by
the super paramagnetic effect is the use of patterned media. Conventionally,
the disk is coated with a thin layer of magnetic alloy. If the disk surface is
examined at high magni
fi
cation, it becomes apparent that within each bit cell
there are many tiny magnetic grains. These grains are randomly created during
the deposition of the magnetic
fi
lm. Each grain behaves like an independent
magnet whose magnetization can be
fl
ipped by the write head during the data
writing process. In patterned media, the magnetic alloy is not coated on the
entire disk surface. The layer is created as an ordered array of highly uniform
tiny islands, each island capable of storing an individual bit. Each bit is stored
in a single deliberately formed magnetic switching volume. This may be one
grain, or several exchange coupled grains, rather than a collection of random
