Read/Write Head Designs: Ferrite, Metal-In-Gap, And Thin-Film
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:
- 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 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.
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.
Thin-film heads are manufactured much the same way as a semiconductor chip—through a photolithographic process. This process creates many thousands of heads on a single circular wafer and produces a small, high-quality product.
TF heads have an extremely narrow and controlled head gap that is created by sputtering a hard aluminum material. Because this material completely encloses the gap, the area is well protected, minimizing the chance of damage from contact with the spinning disk. The core is a combination of iron and nickel alloy that has two to four times more magnetic power than a ferrite head core.
TF heads produce a sharply defined magnetic pulse that enables them to write at extremely high densities. Because they do not have a conventional coil, TF heads are more immune to variations in coil impedance. These small, lightweight heads can float at a much lower height than the ferrite and MIG heads; in some designs, the floating height is 2 micro-inches or less. Because the reduced height enables the heads to pick up and transmit a much stronger signal from the platters, the signal-to-noise ratio increases and improves accuracy. At the high track and linear densities of some drives, a standard ferrite head would not be capable of picking out the data signal from the background noise. Another advantage of TF heads is that their small size enables the platters to be stacked closer together, enabling more platters to fit into the same space.
Many of the drives in the 100 MB–2 GB range used TF heads, especially in the smaller form factors. TF heads displaced MIG heads as the most popular head design, but they have now themselves been displaced by newer magneto-resistive heads.