Memory systems using "hard disks" to store information in annular tracks concentrically arranged in magnetic material formed on a disk surface are well known in the prior art. The information (often called "data") is stored in magnetic domains and each domain typically stores an encoded data bit. A major advantage of this type of memory system is that information can be easily stored on a selected track on a disk surface as the disk rotates at high speed and can be randomly accessed at high speed to store information or to read out information stored on a given track. A read-write head or transducer having a small coil and a magnetic core is moved across the disk surface to the track on which the desired information is to be stored, or from which the desired information is to be retrieved while the disk is spinning. The read-write head stores or detects data in a selected data track on the disk surface. Ideally, the head rides on a thin layer of air generated by the spinning disk. It is then said to fly over the disk surface. However, due to imperfections in the disk surface or to other operating factors such as system vibrations, the head may come into undesired contact with the magnetic film during flight while the disk is spinning, causing instant friction between the head and the magnetic film which may ultimately lead to the destruction of the film and to a head crash. Also, during start/stop there is intimate contact between the head and the disk surface with associated friction which may lead to excessive wear, head crash, or stiction. Stiction may be defined as head/disk adhesion strong enough to require manual removal of the head, resulting in severe marks on the disk surface. Stiction is often encountered when using relatively thick liquid lubricant or when the memory system is operating under conditions of high relative humidity.
In the prior art various techniques have been used to reduce the friction between the head and the magnetic film and the associated damage caused by it.
In memory systems which utilize "flexible" media such as magnetic tape or floppy disks, the space between the moving medium and the read/write head is much smaller than in hard disk systems, and there is continuous intimate contact between the medium and the read/write head. In both hard and flexible media, a magnetic film is formed on a substrate. For a hard disk, the substrate is typically an aluminum alloy with a relatively thick and hard undercoat formed thereon. The magnetic film is formed on the undercoat and is typically a cobalt phosphorus alloy or particulate oxide. The technique used to reduce friction or contact damage depends on the composition and quality of the magnetic film.
One method of forming a magnetic film on a disk substrate involves suspending iron-oxide power in a liquid media containing resins and coating the disk surface with the liquid. The disk is then cured in a strong magnetic field to provide the desired anisotrophy of magnetization. Such particulate magnetic films are typically formed to a thickness of between 0.7 and 5.0 microns, although recently particulate films having a thickness approaching 0.5 micron have been employed. These particulate iron-oxide magnetic films have a porous surface, and for these surfaces, one antifriction coating technique consists in applying a thin film of an organic lubricant, i.e., an organic liquid or wax such as Fomblin or Bareco. These organic lubricants are not very satisfactory because they tend to spin off the disk when the disk is rotated at operating speeds, e.g., 3600 rpm.
The trend in magnetic films has been to thinner films, because generally speaking, the thinner the magnetic film, the greater the information storage density, and to metal alloy film, as opposed to particulate oxide films. Particulate oxide films are porous and have low remanance due to their relatively small volume fraction of magnetic material in the resin. Since the magnitude of the read back signal is directly related to the remanance of the film, particlate oxide films are necessarily thick relative to alloy films of the same remanence.
The quest for thin magnetic films has led to several deposition techniques. Thin films typically comprise magnetic alloys of cobalt with such elements as phosphorus, nickel, chromium, or tungsten. Methods of deposition of thin magnetic alloy films include electroless, electroplating, sputtering, evaporation and other vapor deposition techniques. Thin magnetic films may also be formed by vapor deposition or sputtering or a magnetic metal oxide such as iron oxide in the form of Fe.sub.3 O.sub.4 or Fe.sub.2 O.sub.3 or by oxidation of metallic iron or by thermal decomposition of iron salts. Thin magnetic films typically have a thickness of 600-2000 .ANG., although even thinner films are extant, some having a thickness in the range of 300 .ANG.. These thin magnetic films generally have a smooth, nonporous surface, and the prior art liquid or waxy lubricants that were marginally satisfactory for particulate films, are less satisfactory for smooth, thin films since a liquid or waxy lubricant spins off the disk after a short period of operation.
Powdered non-metallic solid lubricants, such as graphite and molybdenum disulfide have been applied to plated thin magnetic films by rubbing under pressure, but have proven to be unsatisfactory. (See Yanagisawa, U.S. Pat. No. 4,398,562, issued June 28, 1983). In general, these lubricants are made of loose particles which often detach and break, leading to a head crash. The particles are too large, e.g. 3 microns, average diameter, for an adequate thin layer of lubricant, and they present poor adhesion to metallic magnetic films.
Since no satisfactory antifriction lubricants have been devised for thin magnetic films, the trend has been to use an extremely hard protective coating, such as plated rhodium and nickel-tin, which have a hardness in the range of 500-1000 Kg/mm.sup.2, and sputtered carbon, which has a hardness in the range of 3,000-5,000 Kg/mm.sup.2, as shown by R. J. Gambino and J. A. Thompson. Solid State Communications, Vol. 34 pp. 15-18 (1980). While these hard coatings tend to protect the magnetic medium from the head riding above the disk they have several drawbacks: one, they are generally expensive to implement; two, since they are so extremely hard, they tend to cause wear of the head riding on their surface. This tends merely to shift the wear problem from the disk to the head (and increasing the hardness of the head tends to shift the problem back to the disk).
In the case of sputtered carbon, the sputter deposition equipment is every expensive, the throughput low, and it is difficult to control its adhesion to the magnetic film and its crystalline character.
Rhodium, which is often taught, is still unsatisfactory in its performance. To assure effective protection, a thick layer of rhodium, typically on the order of about ten micro inches, has been employed. See U.S. Pat. No. 3,767,369, "Duplex Metallic Overcoating", issued Oct. 23, 1973, which is incorporated herein by reference. The U.S. Pat. No. 3,767,369 proposes an alternative to rhodium comprising a duplex of an at least 2 micro inches thick layer of an alloy of nickel and tin overlaid by a layer of rhodium at least 2 micro inches thick. The nickel-tin layer contains a proportion of nickel of about 35% by weight. Again, this nickel-tin alloy undercoat has a Vickers hardness of 500 to 900. More recently, a layer of rhodium 750 .ANG. thick has been suggested as a protective layer in combination with a thin layer of liquid lubricant, Dupont 804. See Rossi, et al, "Vacuum Deposited Thin Metal Film Disk", J. Appl. Phys., 55 (No. 6) 2254 (1984), which is incorporated herein by reference. Such a combination ignores the long-term reliability problem due to the spin-off of the lubricant. Also, the total thickness of the overcoat, including chromium, rhodium, and liquid lubricant is about 1,000 .ANG..