No matter which substrate is used, the platters are covered with a thin layer of a magnetically retentive substance, called the medium, on which magnetic information is stored. Three popular types of magnetic media are used on hard disk platters:

• Oxide media

• Thin-film media

• AFC (antiferromagnetically coupled) media

Oxide Media

The oxide medium is made of various compounds, containing iron oxide as the active ingredient. The magnetic layer is created on the disk by coating the aluminum platter with a syrup containing iron-oxide particles.

This syrup is spread across the disk by spinning the platters at high speed; centrifugal force causes the material to flow from the center of the platter to the outside, creating an even coating of the material on the platter. The surface is then cured and polished.

Finally, a layer of material that protects and lubricates the surface is added and burnished smooth. The oxide coating is usually about 30 millionths of an inch thick. If you could peer into a drive with oxide-coated platters, you would see that the platters are brownish or amber.

As drive density increases, the magnetic medium needs to be thinner and more perfectly formed. The capabilities of oxide coatings have been exceeded by most higher-capacity drives. Because the oxide medium is very soft, disks that use it are subject to head-crash damage if the drive is jolted during operation.

Most older drives, especially those sold as low-end models, use oxide media on the drive platters. Oxide media, which have been used since 1955, remained popular because of their relatively low cost and ease of application. Today, however, very few drives use oxide media.

Thin-Film Media

The thin-film medium is thinner, harder, and more perfectly formed than oxide medium. Thin film was developed as a high-performance medium that enabled a new generation of drives to have lower head-floating heights, which in turn made increases in drive density possible.

Originally, thin-film media were used only in higher-capacity or higher-quality drive systems, but today, virtually all drives use thin-film media.

The thin-film medium is aptly named. The coating is much thinner than can be achieved by the oxide-coating method. Thin-film media are also known as plated, or sputtered, media because of the various processes used to deposit the thin film on the platters.

Thin-film plated media are manufactured by depositing the magnetic medium on the disk with an electroplating mechanism, in much the same way that chrome plating is deposited on the bumper of a car.

The aluminum/magnesium or glass platter is immersed in a series of chemical baths that coat the platter with several layers of metallic film. The magnetic medium layer itself is a cobalt alloy about 1 µ-inch thick.

Thin-film sputtered media are created by first coating the aluminum platters with a layer of nickel phosphorus and then applying the cobalt-alloy magnetic material in a continuous vacuum-deposition process called sputtering.

This process deposits magnetic layers as thin as 1 µ-inch or less on the disk, in a fashion similar to the way that silicon wafers are coated with metallic films in the semiconductor industry. The same sputtering technique is again used to lay down an extremely hard, 1 µ-inch protective carbon coating.

The need for a near-perfect vacuum makes sputtering the most expensive of the processes described here. The surface of a sputtered platter contains magnetic layers as thin as 1 µ-inch. Because this surface also is very smooth, the head can float more closely to the disk surface than was previously possible.

Floating heights as small as 10nm (nanometers, or about 0.4 µ-inch) above the surface are possible. When the head is closer to the platter, the density of the magnetic flux transitions can be increased to provide greater storage capacity.

Additionally, the increased intensity of the magnetic field during a closer-proximity read provides the higher signal amplitudes necessary for good signal-to-noise performance. Both the sputtering and plating processes result in a very thin, hard film of magnetic medium on the platters.

Because the thin-film medium is so hard, it has a better chance of surviving contact with the heads at high speed. In fact, modern thin-film media are virtually uncrashable. If you could open a drive to peek at the platters, you would see that platters coated with the thin-film medium look like mirrors.

AFC Media

The latest advancement in drive media is called antiferromagnetically coupled (AFC) media and is designed to allow densities to be pushed beyond previous limits. Anytime density is increased, the magnetic layer on the platters must be made thinner and thinner.

Areal density (tracks per inch times bits per inch) has increased in hard drives to the point where the grains in the magnetic layer used to store data are becoming so small that they become unstable over time, causing data storage to become unreliable.

This is referred to as the superparamagnetic limit, and it has been determined to be between 30Gb/sq. in. and 50Gb/sq. in. Drives today have already reached 35Gb/sq. in., which means the superparamagnetic limit is now becoming a factor in drive designs.

AFC media consists of two magnetic layers separated by a very thin 3-atom (6 angstrom) film layer of the element ruthenium. IBM has coined the term "pixie dust" to refer to this ultra-thin ruthenium layer.

This sandwich produces an antiferromagnetic coupling of the top and bottom magnetic layers, which causes the apparent magnetic thickness of the entire structure to be the difference between the top and bottom magnetic layers.

This allows the use of physically thicker magnetic layers with more stable larger grains, so they can function as if they were really a single layer that was much thinner overall.

IBM has introduced AFC media into several drives, starting with the 2 1/2'' Travelstar 30GN series of notebook drives introduced in 2001; they were the first drives on the market to use AFC media. In addition, IBM has introduced AFC media in desktop 3 1/2'' drives starting with the Deskstar 120 GXP.

AFC media is also used by Hitachi Global Storage Technologies, which owns the former IBM hard drive lines. I expect other manufacturers to introduce AFC media into their drives as well. The use of AFC media is expected to allow areal densities to be extended to 100Gb/sq. in. and beyond.