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PostSubject: Did yo know   Thu Jul 31, 2008 5:27 pm


HDDs record data by magnetizing ferromagnetic material directionally, to
represent either a 0 or a 1 binary digit. They read the data back by detecting
the magnetization of the material. A typical HDD design consists of a spindle
which holds one or more flat circular disks called platters, onto which the data are recorded.
The platters are made from a non-magnetic material, usually aluminum alloy or
glass, and are coated with a thin layer of magnetic material. Older disks used
iron(III) oxide
as the magnetic material, but current disks use a cobalt-based alloy.[citation needed]
A cross
section of the magnetic surface in action. In this case the binary data is
encoded using frequency modulation.

The platters are spun at very high speeds (details follow). Information is
written to a platter as it rotates past devices called read-and-write heads that operate very
close (tens of nanometers in new drives) over the magnetic surface. The
read-and-write head is used to detect and modify the magnetization of the
material immediately under it. There is one head for each magnetic platter
surface on the spindle, mounted on a common arm. An actuator arm (or access arm)
moves the heads on an arc (roughly radially) across the platters as they spin,
allowing each head to access almost the entire surface of the platter as it
spins. The arm is moved using a voice coil actuator or (in older designs) a stepper motor. Stepper
motors were outside the head-disk chamber, and preceded voice-coil drives. The
latter, for a while, had a structure similar to that of a loudspeaker; the coil
and heads moved in a straight line, along a radius of the platters. The
present-day structure differs in several respects from that of the earlier
voice-coil drives, but the same interaction between the coil and magnetic field
still applies, and the term is still used.
Older drives read the data on the platter by sensing the rate of change of
the magnetism in the head; these heads had small coils, and worked (in
principle) much like magnetic-tape playback heads, although not in contact with
the recording surface. As data density increased, read heads using magnetoresistance
(MR) came into use; the electrical resistance of the head changed according to
the strength of the magnetism from the platter. Later development made use of spintronics; in these heads, the
magnetoresistive effect was much greater that in earlier types, and was dubbed
magnetoresistance (GMR). This refers to the degree of effect, not the
physical size, of the head the heads themselves are extremely tiny, and are
too small to be seen without a microscope. GMR read heads are now
commonplace.[citation needed]
HD heads are kept from contacting the platter surface by the air that is
extremely close to the platter; that air moves at, or close to, the platter
speed.[citation needed] The
record and playback head are mounted on a block called a slider, and the surface
next to the platter is shaped to keep it just barely out of contact. It's a type
of air bearing.
The magnetic surface of each platter is conceptually divided into many small
sub-micrometre-sized magnetic
regions, each of which is used to encode a single binary unit of information. In
today's HDDs, each of these magnetic regions is composed of a few hundred
magnetic grains. Each magnetic region forms a magnetic dipole which
generates a highly localized magnetic field nearby. The write head magnetizes
a region by generating a strong local magnetic field. Early HDDs used an electromagnet both to
generate this field and to read the data by using electromagnetic induction. Later
versions of inductive heads included metal in Gap (MIG) heads and thin film heads. In today's heads,
the read and write elements are separate, but in close proximity, on the head
portion of an actuator arm. The read element is typically magneto-resistive while the write
element is typically thin-film inductive.[5]
In modern drives, the small size of the magnetic regions creates the danger
that their magnetic state might be lost because of thermal effects. To counter
this, the platters are coated with two parallel magnetic layers, separated by a
3-atom-thick layer of the non-magnetic element ruthenium, and the two layers are magnetized in
opposite orientation, thus reinforcing each other.[6] Another technology
used to overcome thermal effects to allow greater recording densities is perpendicular recording, first shipped
in 2005[7], as of 2007 the technology was used in many
See File
System for how operating systems access data on HDDs and other storage
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