by August 11, 2001 0 comments

There are three ways to increase the capacity of a hard disk: increase the area of a platter, increase the number of platters, or increase the density at which data (and more directly magnetic material) is packed on the recording surface. The first two increase the bulk of the drive, hence should be avoided. Till recently, the fineness, and thus the density of the magnetic material on the disk, could not be increased because of what is called the super magnetic effect.

AFC Origins

In 1990, IBM researchers discovered that a thin layer of ruthenium atoms created the strongest anti-parallel coupling between adjacent ferromagnetic layers of any nonmagnetic spacer-layer element. The structure was used in the first giant magnetoresistive read element for disk drives, which was introduced in 1997. GMR heads are now used in virtually all disk drives. 

The superparamagnetic effect originates from the shrinking volume of magnetic grains that compose the hard-disk media, in which data bits are stored as alternating magnetic orientations. To increase data-storage densities, while maintaining acceptable performance, designers have shrunk the media’s grain diameters and decreased the thickness of the media. The resulting smaller grain volume makes them increasingly susceptible to thermal fluctuations, which decreases the signal sensed by the drive’s read/write head. If the signal reduction is great enough, data could be lost in time because of this superparamagnetic effect.

Historically, disk drive designers have had only two ways to maintain thermal stability as the media’s grain volume decreases with increasing areal density. The first was to improve the signal processing and ECC (Error-correction Codes) so that fewer grains are needed per data bit, while the other was to develop new magnetic materials that resist more strongly any change to their magnetization, known technically as having higher coercivity. But higher coercivity alloys are also more difficult to write on. What the new AFC (antiferromagnetically coupled) media does is make it easy to write at very high areal densities, without sacrificing stability.

How AFC media works

Conventional disk media stores data in only one magnetic layer, typically of a complex magnetic alloy (such as cobalt-platinum-chromium-boron). AFC media is a multi-layer structure in which two magnetic layers are separated 
by an extraordinarily thin–just three atoms thick–layer of the nonmagnetic metal, ruthenium. This precise thickness of the ruthenium causes the magnetization in each of the magnetic layers to be coupled in opposite
directions–anti-parallel which constitutes an antiferromagnetic coupling.

When reading data as it flies over the rapidly rotating disk, a disk drive’s recording head senses the magnetic transitions in the magnetic media that coats the disk. The amplitude of this signal is proportional to the media’s magnetic thickness–the product of the media’s remanent magnetic moment density (Mr) and its physical thickness (t). As data densities increase, the media’s magnetic thickness (known technically as Mrt) must be decreased proportionately so the closely packed transitions will be sharp enough to be read clearly. For conventional media, this means a decrease in the physical thickness of the media. 

The key to AFC media is the anti-parallel alignment of the two magnetic layers across each magnetic transition between two bits. As it flies over a transition, the recording head senses an effective Mrt of the composite structure ( ), that is the difference in Mrt values for the two magnetic layers

Close looks

In the picture are transmission electron micrographs for two different disk media illustrating how the grain structure has changed over time. The TEM on the left is a magnetic media that supports a data density of about 10 gigabits per square inch with an average grain diameter of about 13 nanometers. The magnetic media on the right supports a data density of 25 gigabits per square inch with an average grain diameter of about 8.5 nanometers.

This property of the AFC media permits its overall Mrt to be reduced–and its data density increased–independent of its overall physical thickness. Thus for a given areal density, the Mrt of the top magnetic layer of AFC media can be relatively large compared with single-layer media, permitting more thermally stable, larger grain volumes. AFC media has the thermal stability of conventional media having about twice its magnetic thickness. 

Two additional advantages of AFC media are that it can be made using existing production equipment at little or no additional cost, and that its writing and readback characteristics are similar to conventional longitudinal media. The output pulse sensed by the recording head is a superposition of the fields from transitions in both the top and bottom magnetic layers. As with conventional media, this output is detected as a single pulse, so no changes to the disk drive’s recording head or electronic data channel components are required.

In the future, AFC media structures are expected to enable thermally stable data storage at densities of 100 gigabits per square inch and possibly beyond. 

Adapted from IBM’s white paper on AFC media

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