Hardware Reference
In-Depth Information
heads generate a pulsed current when passed over magnetic flux transitions. A newer type of head
design pioneered by IBM instead relies on the fact that the resistance in the head wires also changes.
Rather than use the head to generate tiny currents, which must then be filtered, amplified, and
decoded, an MR head uses the head as a resistor. A circuit passes a voltage through the head and
watches for the voltage to change, which occurs when the resistance of the head changes as it passes
through the flux reversals on the media. This mechanism for using the head results in a much stronger
and clearer signal of what was on the media and enables the density to be increased.
MR heads rely on the fact that the resistance of a conductor changes slightly when an external
magnetic field is present. Rather than put out a voltage by passing through a magnetic-field flux
reversal—as a normal head would—the MR head senses the flux reversal and changes resistance. A
small current flows through the heads, and this sense current measures the change in resistance. This
design provides an output that is three or more times more powerful than a TF head during a read. In
effect, MR heads are power-read heads, acting more like sensors than generators.
MR heads were more costly and complex to manufacture than older TF heads because of the
additional components and manufacturing steps required:
• Additional wires must be run to and from the head to carry the sense current.
• Four to six more masking steps are required.
• Because MR heads are so sensitive, they are susceptible to stray magnetic fields and require
additional shielding.
Because the MR principle can only read data and is not used for writing, MR heads are really two
heads in one. The assembly includes a standard inductive TF head for writing data and an MR head
for reading. Because two separate heads are built in to one assembly, each head can be optimized for
its task. Ferrite, MIG, and TF heads are known as single-gap heads because the same gap is used for
both reading and writing, whereas the MR head uses a separate gap for each operation.
The problem with single-gap heads is that the gap length is always a compromise between what is
best for reading and what is best for writing. The read function needs a thinner gap for higher
resolution; the write function needs a thicker gap for deeper flux penetration to switch the medium. In
a dual-gap MR head, the read and write gaps can be optimized for both functions independently. The
write (TF) gap writes a wider track than the read (MR) gap reads. Thus, the read head is less likely
to pick up stray magnetic information from adjacent tracks.
A typical IBM-designed MR head is shown in
Figure 8.5
.
This figure first shows the complete MR
head-and-slider assembly on the end of an actuator arm. This is the part you would see if you opened
up a drive. The slider is the block device on the end of the triangular-shaped arm that carries the
head. The actual head is the tiny piece shown magnified at the end of the slider, and then the MR read
sensor in the head is shown further magnified.
