Hardware Reference
In-Depth Information
Another magnetic effect that is well known today is being used in modern drives. When a
wire is passed through a magnetic field, not only does the wire generate a small current,
but the resistance of the wire also changes. Standard read heads use the head as a tiny
generator, relying on the fact that the 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 us-
ing 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 re-
versal 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
dataandanMRheadforreading.Becausetwoseparateheadsarebuiltintooneassembly,
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.
Theproblemwithsingle-gapheadsisthatthegaplengthisalwaysacompromisebetween
whatisbestforreadingandwhatisbestforwriting.Thereadfunctionneedsathinnergap
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)
