Hardware Reference
In-Depth Information
As disk drive technology has evolved, so has the design of the read/write head. The earliest heads
were simple iron cores with coil windings (electromagnets). By today's standards, the original head
designs were enormous in physical size and operated at low recording densities. Over the years, head
designs have evolved from the first simple ferrite core designs into the several types and technologies
available today. This section discusses the various types of heads found in PC hard disk drives,
including the applications and relative strengths and weaknesses of each.
Several types of heads have been used in hard disk drives over the years:
• Ferrite
• Metal-In-Gap (MIG)
• Thin-film (TF)
• Magneto-resistive (MR)
• Giant magneto-resistive (GMR)
• Perpendicular magnetic recording (PMR)
Note
By the end of 2005, hard drives based on PMR were being used in devices such as portable
music players and laptop PCs. Desktop PC hard drives based on the technology became
available in 2006.
PMR is covered in more detail at the end of this chapter.
Ferrite
Ferrite heads, the traditional type of magnetic-head design, evolved from the original IBM 30-30
Winchester drive. These heads have an iron-oxide core wrapped with electromagnetic coils. The
drive produces a magnetic field by energizing the coils or passing a magnetic field near them. This
gives the heads full read/write capability. Ferrite heads are larger and heavier than thin-film heads
and therefore require a larger floating height to prevent 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
.
