The best way to increase areal density is to reduce the head/media separation. Typical Winchester-type disk read/write heads “fly” above the surface of the media passing below. Air flows between the head and the platter surface to keep the head out of direct contact with the media surface. Design improvements have reduced the amount of air bearing to incredibly small distances; the standard flying height is about 3.5 micro-inches, or just over 1/1000 the diameter of a human hair.
Improved Media
With such low clearance, the surface over which the head flies must be smooth indeed. Until the mid-’80s, disk media was formed with magnetic particulate matter suspended in a nonmagnetic, organic material bonded to the disk substrate, similar to the way asphalt or macadam is bonded to the road bed beneath it. Also like a road surface, particulate media is irregular, at least in comparison with thin-film surfaces. The physical surface of thin-film media is electroplated, or spattered, on the substrate beneath, and the result is an extremely flat, thin, uniform media with much higher coercivity properties than particulate media. Another benefit of the thin-film deposit process is that the media’s properties can be determined accurately and controlled.
Thin-film technology also has been adapted to drive heads, creating significant reductions in head size and similar improvements in areal density. These heads are made by photolithographic processes, similar to those used for integrated circuits, and offer much stronger write fields than the standard ferrite heads. Typically, the material used in the fabrication of thin-film heads is Permalloy–a nickel and iron compound–which has a higher magnetic saturation moment than the ferrites used in previous generations. A standard bar/coil electromagnet can handle only a certain current level before it reaches the saturation level. At this point, sending more current through the coil does not result in any further magnetic flux. The same principle holds true for drive heads: An upper limit caps their ability to read or write data. However, contemporary thin-film heads are dual-function, which means they perform both write and read activities.
The implementation of these heads has made it possible to easily miniaturize and inexpensively manufacture these drives using mass-production techniques. Conner Peripherals, which produces much of its own high-coercivity media (1,900 oersteds), has taken thin-film media-process technology to the zenith without moving to a different substrate material. Conner offers several products in this category, including two drives we tested here: the 1.2GB Filepro CFA1275A and the 2GB Filepro CFP2107S.
New Evolution
Two other methods are the catalysts to thrust areal density technology to the next level: semicontact recording and magneto-resistive head technology.
Semicontact recording moves the head even closer to the media surface. Magnetic field force is inversely proportional to the square of its distance from its source, which means that as the two elements come closer together, the magnetic field becomes stronger. Because of this, even a relatively small move can have a significant effect on the head’s ability to read and write densely packed data. Another possibility is to separate the drive head into discreet read and write elements so that each head component can be optimized in regard to function.
While incredibly smooth, the thin-film media is scored to keep the slider and media from forming gauge block, which occurs when two slick surfaces bind together. The scoring creates a somewhat uneven surface, with peaks and valleys separated by about 1.5 micro-inches. There is also a layer of lubrication on the media surface itself, which further reduces the problem of binding. The head then makes “pseudo-contact” with the media, hence the term semicontact recording. This means that the head/slider actually comes in contact with the media surface, but only for a moment. In fact, it “skips” off the high points in the media surface, providing very close-order operation.
Western Digital’s latest Caviar series of hard disk designs, including the tested 1.6GB Caviar AC31600, also uses the Tri-pad head, a type of air bearing, not a head design. While most current drive heads fly over the media surface at the standard 3.5 micro-inches, Tri-pad systems reduce the height to approximately 1.3 micro-inches. The Tri-pad is so named because the slider has three points of contact when the system is at rest. The pads are arranged as two side rails with inclined front ends and a single central pad that carries the sensor head. When rotation begins, the two front-end pads lift off and create the fluid air bearing, while the trailing pad is the actual contact point. The head gimbal assembly (HGA) is designed to allow only a slight touchdown on the head, minimizing the possibility of damage to either the head or media surface.
By moving the head even closer to the media, Western Digital is able to increase its drives’ capacities, in terms of areal density, by approximately 80 percent. The result is single-platter HDAs that have about the same capacity as previous generations of dual-platter drives. According to Western Digital, this technology offers the most cost-effective solution at all capacity points.
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