A rewritable optical disk that is used in combination with magnetic technology. Employed in a variety of storage and archival applications, including large disk libraries, magneto-optic (MO) disks are housed in removable cartridges that have a 30-year shelf life. Capable of up to a million rewrites, MO access times are in the sub-25ms range and are faster than pure optical CD-RWs and DVD-RAMs.
Form Factors and Capacities
MO discs come in 3.5" and 5.25" cartridges. The latter are double sided, but must be removed and flipped over. Capacities of 3.5" cartridges are 128MB, 230MB, 640MB, 1.3GB. For 5.25", they are 650MB, 1.3GB, 2.6GB, 5.2GB, 9.1GB.
Laser and Magnetic Writing
Data are written on an MO disk by a laser and a magnet. The laser heats the bit to the Curie point, which is the temperature at which molecules can be realigned when subjected to a magnetic field. Then, a magnet changes the bit's polarity. The laser is focused on one side of the platter, and the magnet is used on the opposite side, which is why double sided media must be flipped over to access the other side.
Laser Reading
Reading is accomplished with a lower-power laser that reflects light from the bits. The light is rotated differently depending on the polarity of the bit, and the difference in rotation is sensed. Writing takes two passes. The existing bits are set to zero in one pass, and data are written on the second pass. A direct overwrite method (LIMDOW) was later added that erases and writes in one rotation. Many drives support the LIMDOW disks, which is more costly than standard MO media. See UDO, Kerr effect and optical disc.
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