Chapter 9: Hard Disks and Floppy Disks^177
area of the platters, where the read/write head can safely “crash” when the disk is
powered off. Virtually all disk drives made in the past 20 years have included automatic
head parking, which moves the read/write heads to the landing zone. Some even include
a locking feature that holds the heads in the landing zone until power is turned on.
Read/Write Head Operation
The space between the spinning disk platter and the read/write head is called itsfloating
heightor itshead gap. The size or distance of the head ga pis a function of the disk drive’s
design and the type of read/write head technology in use. The size of the gap is very
important, because the head must be exactly the right height to properly sense flux transi-
tions on the disk without banging into the disk surface. Most disk drives have a head gap
of five millionths of an inch or shorter.
The read/write heads in a disk drive are U-shaped and made from electrically
conductive materials. Wire through which an electric current can flow is coiled around
each of the heads—the U-shaped objects. By running a DC current through the wire in
one direction or the other, a magnetic field with one of two polarities is created. These
two polarities, if you haven’t already guessed, are used to store electrical values representing
binary 1s and 0s.
There are four types of read/write heads used in hard disk drives:
Ferrite heads This is the oldest of the magnetic head designs, and as such, they
are bigger and heavier than any of the thin film heads and use a larger floating
height to guard against contact with the disk. Ferrite heads use an iron-oxide
core that is wrapped with electromagnetic coils. The coils are energized to create
a magnetic field. During the 1980s, a composite ferrite head was popular that
incorporated glass to reduce its weight and size and improve its operation.
Ferrite heads have largely been replaced by TF and MIG head technologies.
Metal-in-Gap (MIG) A MIG head is an enhanced version of the composite
ferrite head. Metal is added to the trailing edge of the head gap to help it ignore
nearby magnetic fields and focus on the cells beneath the head.Single-sided
MIG headshave a layer of magnetic alloy on the trailing edge of the gap. A
double-sided MIG headadds a layer of metal alloy to both the leading and
trailing sides of the gap. For a while, MIG heads were the most popular type
in use, but demands for higher capacity disks have resulted in the TF head
becoming more popular.
Thin film (TF) TF heads, which are manufactured much like a semiconductor
(see Chapter 3), are used in small form factor high-capacity drives. TF heads are
the most common type of disk drive head in use. They are light and much more
accurate than the ferrite heads and operate much closer to the disk surface.
Magneto-resistive (MR) MR heads are found in most 3.5-inch disk drives
that have 1GB or higher capacity. Instead of signaling a flux transition, an MR head
merely changes the resistance on an electrical line. MR heads are power-read heads
only. Disk drives with MR heads typically also have a TF head for writing.