Optical Storage Disk Technology
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g to the example of storing text information, it might be necessary to change some stored information to correct errors or make other revisions. With a WORM optical disk, since data already written cannot be erased, the only option is to write the new data on the disk with the corrections. The System software, called an Optical Disk File System, will compare the old and the new data so that only the changed portions need to be rewritten. The software also records the location of the updated data in a separate part of the disk reserved for directory information. This procédure is designed to reduce the cost in both storage space and access time associated with WORM-type optical disks. The impact on access time cornes about because the system must read each cancellation message to find the location of the most recently stored version of the text. 4 Furthermore, having a digital copy of earlier drafts is of no particular advantage for word processing tasks. (A counter example is financial records where the inability to erase information could be a distinct advantage for auditing purposes.) In any case, the computer industry has grown u p with fully erasable magnetic storage so only erasable optical disks can compete in many applications where magnetic storage is presently used. This is particularly true in large Systems where access time may be as important as density and the longer access time of optical disks is c o m p o u n d e d by the search t h r o u g h the locations of old records in a WORM file. Although the CD-ROM and WORM formats hâve been successfully developed and are well suited for particular applications, the computer industry is focusing on erasable média for optical storage. There are two types of erasable optical média: phase-change (PC) and m a g n e t o - o p t i c ( M O ) . Both will b e detailed in this issue of the MRS BULLE-
TIN in "Magneto-Optical Storage Materials" by Frans Greidanus and Bas Zeper and in "Multilayered Thin-Film Materials for Phase-Change Erasable Storage" by Matthew Libéra (also Guest Editor for this issue) and Martin Chen. Magneto-Optic Media In MO storage, a bit of information is stored as a ~l/xm diameter magnetic domain, which has its magnetization either up or down. The information can be read by monitoring the rotation of the plane of polarization of light reflected from the surface of the magnetic film. This rotation, called the Magneto-Optic Kerr Effect (MOKE) is typically less than 0.5 deg. The materials used for MO storage are generally amorphous alloys of the rare earths (RE) and the transition metals (TM), Fe or Co. A m o r p h o u s materials hâve a distinct advantage for this purpose because they do not suffer from "grain noise," spurious variations in the plane of polarization of reflected light caused by randomness in the orientation of grains in a polycrystalline film.5 It is also possible to modify the properries of thèse materials over a wide range by alloying or by varying processing conditions. 6 Because of thèse advantages amorphous alloys are used as t
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