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Giant
magnetoresistive (GMR) heads represent a logical progression in the
storage industry's endless quest for a way to increase areal density while
reducing the price per megabyte. Driven by new and expanded applications
and the quest for lower storage costs, areal density rates have risen at
about 50 percent a year. The Barracuda ATA is Seagate's first disc drive
family to incorporate GMR heads.
Seagate's GMR heads have demonstrated the capability to read areal
densities in excess of 15 Gbits per square inch, more than three times
that of the existing read/write technology. GMR sensors represent the most
sensitive device to date for reading computer data from hard drives.
Areal density - bits per inch on a track multiplied by tracks per inch -
is the key method of increasing disc capacity. During the past decade,
areal density technology moved in three distinct segments to quench the
public's thirst for more storage:
- Thin-film
inductive (TFI)
- Anisotropic
magnetoresistive (AMR)
- Giant
magnetoresistive (GMR)
A Historical Context
To understand the full impact of GMR heads, it is important to view
changes that have taken place during the past 10 years. Between 1990 and
1995, disc drives
used a read/write technology called TFI. These heads were made of
wire-wrapped magnetic cores. Voltage generated from the disc (a permanent
magnet) moved past the wired core. TFI read heads reached their limit
because when magnetic sensitivity was increased, their ability to write
decreased.
Seagate began researching anisotropic magnetoresistive heads (AMR), in
which a magnet resides in the head. In 1982. This program was initiated in
anticipation of the time when inductive thin- film head technology would
no longer be able to meet the high areal density demands of the disc drive
industry. By the mid-90s, Seagate began introducing drives that used AMR
heads.
An AMR head uses a TFI head to write. However, the read element is
composed of a thin stripe of magnetic material. The essence of AMR heads
is their resistance-transducing capability. A strong signal is generated
when the stripe's resistance changes in the presence of a magnetic field.
The disc drive senses and decodes the resistance changes caused by
reversing magnetic polarities. This heightened sensitivity provides a
higher signal output per unit of recording track width on the disc
surface. The results: more information can be placed on each disc, and
fewer components are necessary.
There are, however, challenges within AMR that have led to the development
of GMR - the main problem is the maximum change in resistance in AMR films
and, therefore, its sensitivity limitations.
The GMR Advancement
GMR heads build on existing read/write technology found in TFI and AMR.
GMR read heads, however, exhibit a higher sensitivity to changing
magnetization on the disc and work on spin-dependent electron scattering.
With AMR sensors, a single film alters resistance in response to a change
in the magnetic field on the disc. GMR sensors have greater than three
times AMR sensitivity, which provides the heads with increased areal
density and performance.
A GMR read film structure is composed of free, pinned, and
antiferromagnetic layers:
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