Products that use magneto-optic (MO) recording technology have been available since 1989. The leading manufacturers are Fujitsu, Sony, Konica and Olympus. Capacities started at 0.325 GB per side for a 5.25" disk and are about to become 2.6 GB. The 5.25" drives have a low volume market for archival storage, especially within automated libraries. The 3.5" disk drives have achieved a moderate volume market at just under one million drives per year. Most of the sales for 3.5" drives are in Japan with little acceptance in either Europe or the United States. Finally, the Sony minidisk for recording music has been on the market for many years, but is recently starting to gain acceptance and achieve significant volumes.

MO technology has several advantages over phase change. It offers more rewrite cycles, longer archival lifetime and faster write and erase times. In addition, magnetic field modulation allows bits to written that are shorter than the optical spot size (up to 5x). Finally, various super resolution techniques have been demonstrated to extend the areal density towards 100 Gb/in2.

MO also has several disadvantages compared with phase change. Magnetic field modulation requires a flying head and close spacing between the head and the medium. This requires a clean surface and careful control of the environment. It may impair the reliability, if the disk is removable. If optical modulation is used, then MO and phase change can only write bits that are about the size of the optical spot. Finally, MO senses a 1% change in the polarization of the reflected light. Although the signal to noise is adequate, the polarization sensing optics are a little more complicated than the optics for phase change signal detection.

The MO companies have formed the Advanced Storage MO group (ASMO) to create a standard to challenge phase change. They have selected a 12 cm disk that is compatible with the DVD standard. They are planning to achieve a single-sided capacity of 6 GB using magnetic field modulation. They hope to offer faster access time (30 msec vs. 180 msec) and higher transfer rates (3 Mb/s vs. 1.4 Mb/s) than the phase change can offer. Even though the phase change optics are a little more complicated, and even though higher performance usually requires more costly components, the ASMO companies hope to be able to offer these advantages at the same price that can be achieved with phase change.

Recently TeraStor, a startup company in the United States, introduced a major breakthrough in magneto-optic technology. It is using an immersion lens that focuses the light inside of a high-index medium rather than through a significant air gap. As a result, TeraStor's optical spot size is 2-3 times smaller in diameter and areal density is 4-9 times greater than previously possible. TeraStor must use a flying optical head with a very small air gap (much smaller than a wavelength of light) between the head and the disk. Key issues are the reliability of the wear layers on the head and disk, plus the control of the environment to avoid a buildup of contaminants on the head or the disk. Whether a head-disk system can be developed to be reliable for removable medium is also a key issue. If not, then the immersion lens might be used for non-removable disks that would compete with magnetic disk drives.

Published: June 1999; WTEC Hyper-Librarian