Background on magneto-optical technology for use with backups

The ideal backup solution should do away with the restore operation required by tapes.

The need for an erasable optical disk has stimulated the development of the magneto-optical (MO) disk. The MO disk has a recording density of 200-800M Bytes for 12cm. disks, but also has an additional advantage of being erasable. Recently, however, densities as high as 108 bits/sq. cm. have become possible. MO disks are quoted with a thin layer of magnetic material on which data can be written (by optical means) and erased over and over again.

The active layers of the magneto-optical disk consist of two 10-nm. thick layers of magneto-optical film (one for each side of the disk). The magneto-optic materials that have received widespread attention are rate earth, transitional metal alloys such as TbFe, GdCo, GdTbFe, Tb-Fe-Co or TbFeCo+Gd, etc.

The layers are deposited on polycarbonate disks. Thin films of Rare Earth Transitional (RE-TM) alloy, when prepared in the proper compositional range, exhibit magnetic anisotropy perpendicular to the plane and magnetic hardness (large coercivity) which are the essential features for magneto-optic recording. Di-electric material and adhesives separate and bond the layers. This disk is encased in a rigid plastic cartridge with a metal shutter. Its finished dimensions are typically 5.3" x 6" x 4", and it can hold 640 to 1300M Bytes of data on two sides. The optical disk has to be flipped to access data on the second surface.

Digital information is recorded on the disk in the form of magnetic flux reversals. The magnetic field of each bit is either in the north-pole-down (digital 0) or a north-pole-up ( digital 1) orientation. A blank or pre-erased disk has all its magnetic bits in the north-pole-down orientation.

The magnetic field required to alter the bit direction varies greatly with temperature. At room temperature, the magnetic field required is far too high for any common magnet (including the drive?s writing magnet) to have any effect.

So, to change the bit direction, a laser beam heats the surface over the bit for about 800ns, to 150 degrees Celsius. At this temperature (i.e. around the Curie point), the film's magnetic properties change, allowing the drive magnet to alter the bit's magnetic polarity. Thus the writing is basically a thermo-magnetic process. In operation, the laser beam turns on and off as a disk rotates.

When the magnet is polarized north side up and the laser is on, the drive writes digital 1's. Digital 0's are written when the magnet's polarity is reversed. However, nothing will be changed while the laser is off. To write over a previously written disk, the drive performs two passes -- an erase and a write. During the erase pause the polarity of the magnet is reversed, and the laser is turned on for one disk operation. This writes all 0's on the areas designated for the new data. During the second pause, 1's are returned where needed to represent the data. Once the laser beams turns off, the heated area quickly cools, freezing the bits in their new magnetic direction.

The writing magnet is too large for its magnetic field to be reversed in time for each bit that passes by; its polarity changes once per disk rotation. Therefore, random writes require tow disk rotations to complete. Writes on new or erased disks, which are known by the host computer to contain all 0's, can be automatically handled by the disk drive controller. For data safety, the disk can be write-protected by setting up the tab on either side of the disk cartridge.

Due to the Kerr effect, the reflected laser beam rotates either clockwise or counterclockwise, depending on the magnetic direction of the bit. Bit direction, representing 1's and 0's is read by detecting the reflected beam's rotation. This reading method differs from CD-ROM and WROM and that the MO drive detects light polarization rather than the differences in the beam's brightness where etched pits and lands affect it. Because of the disk's extreme stability to room temperature, it is almost impossible to alter or erase data on an MO disk accidentally.

To increase reliability, information on an MO disk is written in performed spiral or concentric grooves approximately 1-nm. wide and 0.8-nm. deep. The optical head senses the groove using light-diffraction techniques and keeps the focused beam on the center of the groove with servomechanisms. This eliminates the physical misalignment of the head, a common problem with magnetic drives.

This was first published in February 2001
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