An MO disk is constructed by "spattering" a number of films onto a high-strength polycarbonate resin substrate base - the same material as used in "bullet-proof" glass - and coating the entire disk with an ultra-violet hardened protective resin. The recording film itself is made of alloy including a number of different metal elements, such as Tb (terbium), Fe (iron) and Co(cobalt). This is sandwiched between protective dielectric films which, as well as providing thermal insulation, also enhance the rotation of polarisation angle so that the sensor is better able to detect the "Kerr Effect". Beneath the disk's protective resin on its upper side is a reflective film whose role is to improve read efficiency. It is this that gives the MO disk its distinctive "rainbow" appearance.
The transparent substrate on top of the recording layer arrangement is referred to as substrate incident recording - the laser light passing through the substrate to reach the recording layer. Whilst the substrate is effective in protecting the recording layer from contamination and oxidation, its thickness is a limiting factor on the numerical aperture that can be used in the objective lens which, in turn, is a primary limiting factor to the capacity and performance of ISO type MO drives.
MO disks are available in several different capacities and at per megabyte storage costs that are very competitive with other removable storage media. Furthermore, unlike the removable hard disk technologies, MO disks are not proprietary, being available from many different storage media manufacturers.
The current 3.5-inch cartridges - the same size as two 3.5in floppy disks stacked on top of each other - are rated at 640MB and Fujitsu is expected to introduce a 1.3GB 3.5in drive in early 1999. Standard 5.25in double-sided media provide up to 2.6GB of storage - disks having to be ejected and flipped to allow access to their full capacity. Higher capacities are planned and indeed already available in proprietary formats.
Most MO vendors have agreed to keep newer drives backward-compatible within at least two previous generations of MO capacity points. For example, a 3.5in 640MB MO drive can accept the older 530MB, 230MB, and 128MB cartridges. Also, most capacity levels follow ISO file-format standards, so cartridges can be exchanged between drives of different manufacturers.
MO media are extremely robust and durable. Because the bits are written and erased optically, MO disks are not susceptible to magnetic fields. With no physical contact between disk surface and drive head, there is no possibility of data loss through a head crash and vendors claim data can be rewritten at least a million times, and read at least 10 million times. Also, since the disks are permanently fixed in rugged cartridge shells that manufacturers have made to demanding shock-tolerance standards, a useful life of the data stored on the media in excess of 30 years is claimed - some manufacturers quoting media life of 50 or even 100 years.
