The newest type of technology commonly used in read/write heads is much more of a
radical change to the way the read/write head works internally than the earlier advances,
which were much more evolutionary and more related to how the head was made than how it
worked. While conventional ferrite or thin film heads work on the basis of inducing a
current in the wire of the read head in the presence of a magnetic field, magnetoresistive
(MR) heads use a different principle entirely to read the disk.
Note: The correct
technical name for first-generation MR heads is anisotropic magnetoresistive
(AMR), but traditionally they have just been called "magnetoresistive" (MR).
With GMR heads now on the market, there is the
potential for confusion between the terms "magnetoresistive" and "giant
magnetoresistive". Therefore, some companies have now gone back to calling the older
MR heads "AMR" heads to distinguish them from GMR ones. Normally though, if you
are told a drive has "MR heads", this means the older technology described here.
An MR head employs a special conductive material that changes its resistance in the presence of a magnetic
field. As the head passes over the surface of the disk, this material changes resistance
as the magnetic fields change corresponding to the stored patterns on the disk. A sensor
is used to detect these changes in resistance, which allows the bits on the platter to be
read.
The use of MR heads allows much higher areal densities to be used on the platters than
is possible with older designs, greatly increasing the storage capacity and (to a lesser
extent) the speed of the drive. Because the MR head is not generating a current directly
the way standard heads do, it is several times more sensitive to magnetic flux changes in
the media. This allows the use of weaker written signals, which lets the bits be spaced
closer together without interfering with each other, improving capacity by a large amount.
MR technology is used for reading the disk only. For writing, a separate standard
thin-film head is used. This splitting of chores into one head for reading and another for
writing has additional advantages. Traditional heads that do both reading and writing are
an exercise in tradeoffs, because many of the improvements that would make the head read
more efficiently would make it write less efficiently, and vice-versa. For example, if you
increase the number of windings of wire around the core of a standard read/write head, you
increase the sensitivity of the head when reading, but you make it much more difficult to
write at high speed. Also, for best results we want to write a wider data track (to ensure
the media is properly magnetized) but read a narrower one (to make sure we don't
accidentally pick up signals from adjacent bits). In an MR design the MR head does the
reading, so the thin film write head can be optimized solely for writing without worrying
about these sorts of compromises.
|
Closeup view of an MR head assembly. Note that the
separate
copper lead wire of older head designs is gone, replaced by thin
circuit-board-like traces. The slider is smaller and has a distinctive shape.
The actual head is too small to be seen without a microscope.
If you look at the page discussing GMR heads you
will see a diagram
showing in detail where on the assembly the head is found. |
First introduced in 1991 by IBM--who else--but not used widely until several years
later, MR heads were one of the key inventions that led to the creation of hard disks over
1 GB in size, and the subsequent explosive growth in size since then. Despite the
increased cost of MR heads, they have now totally replaced thin film heads, which just are
not up to the challenge of hard disks in the tens of gigabytes. MR heads are commonly
found in hard disks from about 1 GB to about 30 GB in size.
Even MR heads however have a limit in terms of how much areal
density they can handle. Successive generations of MR heads were reduced in size to
allow still greater areal density. Sometimes these more advanced designs were dubbed MRX
for Extended Magnetoresistive heads. The successor to MR now appears to be GMR
heads, named for the giant magnetoresistive effect. They are similar in basic
concept to MR heads but are more advanced; GMR heads
are discussed here.
Next: Giant Magnetoresistive (GMR) Heads
