Hardware Reference
In-Depth Information
PMR is covered in more detail at the end of this chapter.
Ferrite
Ferriteheads,thetraditionaltypeofmagnetic-headdesign,evolvedfromtheoriginalIBM
30-30 Winchester drive. These heads have an iron-oxide core wrapped with electromag-
netic coils. The drive produces a magnetic field by energizing the coils or passing a mag-
netic field near them. This gives the heads full read/write capability. Ferrite heads are lar-
ger and heavier than thin-film heads and therefore require a larger floating height to pre-
vent contact with the disk while it is spinning.
Manufacturers have made many refinements to the original (monolithic) ferrite head
design. One type of ferrite head, called a
composite ferrite head
, has a smaller ferrite core
bonded with glass in a ceramic housing. This design permits a smaller head gap, which
enables higher track densities. These heads are less susceptible to stray magnetic fields
than the older monolithic design heads.
During the 1980s, composite ferrite heads were popular in many low-end drives, such as
the Seagate ST-225. As density demands grew, the competing MIG and thin-film head
designs came to be used in place of ferrite heads, which are virtually obsolete today.
Ferrite heads can't write to the higher coercivity media necessary for high-density disk
designs and have poor frequency response with higher noise levels. The main advantage
of ferrite heads is that they are the cheapest type available.
Metal-In-Gap
MIG heads are a specially enhanced version of the composite ferrite design. In MIG
heads, a metal substance is applied to the head's recording gap. Two versions of MIG
heads are available: single sided and double sided. Single-sided MIG heads are designed
with a layer of magnetic alloy placed along the trailing edge of the gap. Double-sided
MIG designs apply the layer to both sides of the gap. The metal alloy is applied through a
vacuum-deposition process called
sputtering
.
This magnetic alloy has twice the magnetization capability of raw ferrite and enables the
head to write to the higher coercivity thin-film media needed at higher densities. MIG
heads also produce a sharper gradient in the magnetic field for a better-defined magnetic
pulse. Double-sided MIG heads offer even higher coercivity capability than the single-
sided designs.
Because of these increases in capabilities through improved designs, MIG heads were for
a time the most popular head design and were used in many hard disk drives in the late
1980s and early 1990s, and most recently in LS-120 (SuperDisk) drives.
