magneto-optic storage

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magneto-optic storage (M-O storage) A storage method, used in rewritable optical disk drives, that combines magnetic and optical recording techniques. The disk is coated with film that initially is uniformly magnetized. A laser beam is used to demagnetize a small spot on the film by heating it above a critical temperature (the Curie point or compensation point), and a local magnetic field determines the direction in which the spot is magnetized when it cools. To read the information, the disk is scanned by polarized light from a low-power laser. The plane of polarization of the light reflected from a magnetized surface is rotated according to the direction of the magnetic field – the Kerr effect. This rotation, though small, can be detected and the original binary signal can be reproduced. In early M-O disk drives data had to be erased separately (usually during the previous revolution of the disk) before it could be rewritten, but direct rewriting is now possible.

The M-O technique achieves recording densities similar to those of other optical stores and much higher than has been achieved by magnetic recording. M-O media compete with dye-polymer media in the rewritable disk field. Dye-polymer media are in principle cheaper to make and need a simpler drive, but successful M-O media were produced some years before comparable dye-polymer media.