A type of computer storage medium which combines the principles of the magnetic disk and the optical disc. Essentially, an MO disc consists of four layers of material: a very thin (i.e., only a few atoms thick) crystalline metal layer and a thin aluminum layer, which are sandwiched between two layers of transparent plastic. The thin crystalline layer can have the polarity of its crystals altered by exposure to a magnetic field. Thus, some crystals have their polarities adjusted to a configuration that signifies a "1," while unaltered crystal polarities signify "0"s. (Computer data can only ever be some combination of 1s and 0s; see Digital.) Data written to the disc is erased by heating it with a high-energy laser to a certain critical level (i.e., the temperature at which the crystal polarity is "reset" to all 0s). To record new data, the magnetic head again switches the polarity of certain crystals to 1s. When the disc is read, a low-energy, polarized laser beam is reflected from the aluminum layer beneath the crystalline layer. As the light is reflected back up, the polarization of the light is altered with respect to the polarization of the magnetic crystals. Thus, unaltered crystal polarity (0s) and altered crystal polarity (1s) can be distinguished by the laser, and the proper bit pattern transferred to the computer.
Magneto-optical discs, unlike optical discs such as CD-ROMs or WORMs, can be erased and rewritten ad nauseam. MO discs are increasing in speed and storage capacity and decreasing in expense. However, many imaging and prepress service bureaus are discontinuing support for them.