Magneto-Optical storage combines both magnetic and optical technology. The drives are written magnetically and read optically, taking the best from both worlds. Magnet materials have a physical property called a Curie temperature above which they lose magnetization due to the complete disordering of their magnetic domains. The magnet's coercivity, its resistance to magnetization by an outside magnetic force, decreases as the temperature approaches the Curie point, and is zero after it. To record data in a magneto-optical device, a laser heats the media, decreasing the coercivity to a level that allows its magnetism to be modified by a relatively weak magnetic field. Once the data is written, the area then cools, and the magnetic data is not subject to modification or erasure by magnetic forces encountered during its everyday contact with magnetic fields.
To read the data from the MO media, the drive uses the Kerr effect. The Kerr effect is the polarization of light changing when it is shined on an area of the disk that is magnetized. Shining a beam of polarized light on a magnetized surface causes the polarization of the light to shift slightly (usually less than 0.5o). If the magnetization is reversed, then the angle of change in polarization (the Kerr angle) is changed as well. The change in direction of represents the two states, 1 and 0. MO media is similar to CD-RW media and has roughly the same capacity, however both the drives and the media are more expensive and so CD-RW is the more successful.
Punch Cards and Valves
The Hard Disk Drive
The Floppy Disk
The Compact Disk