A type of storage mechanism that combines hard drive and compact disc technologies. A magneto-optical (MO) disk consists of an optical disc coated in a special dye that can only becomes magnetic above a specific temperature called the Curie point. When writing, a laser is used to heat the material to this temperature and a magnetic head is used to change the polarity of the material to write the bit, like a conventional computer hard drive. This causes the light to be polarised against the flux of the write field (the Kerr effect). When reading, the drive simply uses the laser1 to read the material. The incoming light passes through a polariser, causing a "1" to be read when no light is detected2. NRZ coding is used for clock recovery and bit reassembly3. A feedback loop also exists between the spindle motor and the speed of the disk4. The disk itself is contained in a damn near indestructible plastic casing similar in design to those on floppy disks and MiniDiscs.

MO disks are 3.5in in size and come in capacities of 128Mb, 320Mb, 540Mb, 640Mb and 1.3Gb. They can be used for any application that requires large storage capacities such as photo/graphics, system backup, video/audio storage and medical imaging. In fact, one of the most popular applications of MO technology has been in the MiniDisc recording system, using MO disks of approximately 140-160Mb capacity. MO disks have an average seek time of 23ms and a transfer rate of up to 5.9Mb/s.

1. The disk is read by the laser, not by the magnetic head as I previously stated. Mad props to pokey for setting the record straight there.
2. Cheers to pokey for giving some more detailed technical info on the operation of the drive during the writing and reading phases.
3. Thanks again to pokey for that bit of info.
4. Once again, three cheers for pokey on that one.

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