magnetic disk A rotatable storage medium usually in the form of a circular nonmagnetic rigid plate coated on both sides with magnetic material, followed by some form of lubricating layer(s). The disks themselves (the substrate) are made of aluminum alloy or of some glass–ceramic composite material. The magnetic coating on early disks was a ferric oxide in a binder. Current disks have a thin metallic film, such as cobalt/nickel or cobalt/chrome, which is created by vacuum deposition (i.e. sputtering). The lubricating layer is a coat of carbon a few angstroms thick, sometimes followed by a proprietary lubricant. Metallic coatings have advantages in being homogeneous, having a better hysteresis loop shape, and allowing storage densities 10 times that achievable with conventional ferric oxide coatings. Flexible magnetic disks (i.e. floppy disks) with oxide coating provide low-cost lightweight media that can be handled in a normal office environment.

Data is stored on and retrieved from magnetic disks by means of a disk drive. See also access time, fixed disk drive, Winchester technology, memory hierarchy.

magnetic encoding The method by which binary data is recorded on magnetic media. In horizontal recording on magnetic disks, tapes, and cards, magnetic domains in the media are aligned along the direction of the applied magnetic field with either north or south pole leading: each domain is a tiny bounded region in which the magnetic moments of the component atoms are aligned, and it therefore behaves like a magnet. The domains are arranged end to end along a track, which may be either a concentric ring on a disk or run the length of a tape or card. There may not be a one-to-one relationship between the binary information of the data and the orientation of the magnetic domains. See disk format, tape format.

In 1975 Shun-ichi Iwasaki published his work on vertical recording methods. The magnetic domains are oriented through the thickness of the magnetic film and have either a north or a south pole at the exposed surface. The magnetic material is usually a vacuum-deposited film of metal such as an alloy or combination of cobalt and chromium over a layer of permalloy. Linear densities as high as 200 000 bits per inch have been demonstrated. Vertical recording can thus yield an increase of at least 25 times and possibly 100 times the bit density achievable by current horizontal recording techniques.

magnetic disk Plastic disk coated with magnetic material and used for storing computer programs and data (information) as a series of magnetic spots. Most computers contain a hard disk unit for general storage. Hard magnetic disks can store larger amounts of data and come in cartridges that slot into a special drive unit. Computers also often have a disk drive for inserting portable, lower-capacity compact discs or floppy disks. Data is stored magnetically on both sides of a floppy disk, and is read by magnetic heads in the computer as the disk rotates. See also CD-ROM

magnetic drum The earliest form of rotating magnetic storage device, used in some of the first computers at a time when random-access store was volatile, bulky, and expensive. The drum therefore formed the main memory of some of these machines, the random-access stores being used only as registers. Although random-access store developed rapidly it was still relatively expensive and the drum was retained as a local backing store on some computers. Magnetic disk, when introduced, took over a large part of the backing store function. Drums remained in use however on certain systems that required faster access than was generally provided by disk, but today they are obsolete apart from a few special applications.

A magnetic drum consists of a cylinder whose curved surface is coated with a suitable recording medium, either metal or iron oxide. On the head-per-track drum the drum rotates past a number of fixed read/write heads, one for each track of recorded information. On the moving-head drum the drum rotates past a single head or small group of heads that can be moved axially to access any track. The latter was rapidly superseded by disk stores but the head-per-track drum survived: track selection requires only electronic switching between heads rather than movement of the head so that such drums have much shorter access times than disk stores.

magnetic head See head.