Database Reference
In-Depth Information
1.2.3 Magnetoresistive Reading
Another aspect that can limit the density of recorded data is the sensitiv-
ity, size, and consequent flying height of the read heads. In early devices, the
read operation employed an inductive head that depended on the magnetized
medium inducing a current in coiled wire on the read head as the medium
moves under the read head. Magnetoresistive materials change their resis-
tance in response to the magnetization of the substrate. Magnetoresistive read
heads offered order of magnitude improvements in sensitivity compared with
inductive heads, enabling increased recording densities. The performance of a
magnetoresistive head is independent of the rate of movement of the underly-
ing medium, which is particularly useful for tape heads where the rate of the
medium is very low or subject to velocity changes. The magnetoresistive effect
cannot be used to induce magnetization in the medium, so the conventional
inductive heads must remain, but magnetoresistive heads are substantially
smaller than the typical inductive heads and can often be embedded within
the structure of a typical thin-film recording head. The magnetoresistive heads
are typically narrower than the recorded track in the medium, which can dra-
matically reduce the cross-talk between neighboring tracks during the read
operation.
1.2.4 Giant Magnetoresistance (GMR)
The “Giant Magneto-Resistive” (GMR) effect was discovered in the 1980s
by Peter Gruenberg and Albert Fert, and led to their 2007 Nobel Prize in
physics. In their research on materials that are comprised of alternating lay-
ers of magnetic and nonmagnetic materials, they observed that sensitivity to
magnetization improved between 6% and 50% in comparison to conventional
magnetoresistive materials. Further refinement of the GMR technology led
to read heads that were an order of magnitude more sensitive than its con-
ventional magnetoresistive approach. The improvements in read sensitivity
enabled further reductions in bit sizes and enabled continued annual doubling
of storage densities from 1997 to approximately 2005.
The GMR effect also gave birth to a new form of magnetism-based computer
logic referred to as “spintronics,”
3
that refers to the underlying mechanism of
the effect, which is spin-dependent scattering of electrons. The GMR effect
is also being used for new solid-state devices such as MRAM, which we will
discuss in the Emerging Technologies section.
1.2.5 Perpendicular Recording
Superparamagnetic effects limit recording density to 150 gigabits/mm
2
for
conventional longitudinal recording where the polarization is parallel to the
magnetic storage medium. In such an arrangement, the areas between oppo-
sitely polarized regions tend to demagnetize their neighbors. As shown in
Figure 1.2, perpendicular recording technology induces the magnetic field
