Magneto-Optical Storage Systems

Erasability always implies that the recording media can undergo an unlimited (or very large) number of write/erase operations without any loss in recording / reading quality. There are two main media designs for rewritable optical systems: MO (magneto-optical) media and phase-change media (known as CD-RW).

The MO systems include basic principles of both magnetic and optical storage systems: MO systems write magnetically (with thermal assist) and read optically. Presently, there are two standard form-factors used for MO systems: 5.5-inch and 3.5-inch, which are protected by hard envelops. The larger form-factor MO disks are capable of holding about as much as the standard CD-ROM. Under pressure from the inexpensive and relatively fast CD-R and CD-RW, MO drives seems to be losing ground. On the other hand, some of the principles of the MO technology (thermally-assisted magnetic recording) may find their way into the most advanced magnetic storage devices of the future.

Basics of MO recording

All magnetic materials have a characteristic temperature, called the Curie temperature, above which they lose magnetization due to a complete disordering of their magnetic domains. Therefore, they lose all the data they had stored before. More importantly, the material's coercivity, which is the measure of material's resistance to magnetization by the applied magnetic field, decreases as the temperature approaches the Curie point, and reaches zero when this temperature is exceeded. For the modern magnetic materials used in MO systems, this Curie temperature is on the order of 200oC. It is important (since this is a multiply-erasable system) that the only change to the material when it is heated and cooled is the change in magnetization, with no damage to the material itself. This fact that the material's coercivity drops at higher temperatures allows thermally-assisted magnetic recording with relatively weak magnetic fields, which simplifies the drive's design. Even a relatively weak laser can generate high local temperatures when focused at a small spot (about 1 micron in case of MO systems). When the material is heated, and its coercivity is low, a magnetization of the media can be changed by applying a magnetic field from the magnet. When the material is cooled to room temperature, its coercivity rises back to such a high level that the magnetic data can not be easily affected by the magnetic fields we encounter in our regular daily activity. The basic schematic of this recording process is illustrated by the next figure.

When the disk is inserted into the drive, the label side will face the magnet, and the transparent side will face the laser. The direction of magnetization in the thin magnetic films (on magnetic rigid disks, for example) can be parallel to the surface (longitudinal recording) or perpendicular to the surface (perpendicular recording). The latter has potential for higher density of magnetic recording. Most of the magnetic hard drives nowadays utilize longitudinal recording, while the MO systems use the perpendicular direction of magnetization.

Basics of MO reading

Unlike traditional magnetic recording systems, which use currents induced in the magnetic heads by the changing magnetic fluxes on the disk surface to read the data, MO systems use polarized light to read the data from the disk. The changes in light polarization occur due to the presence of a magnetic field on the surface of the disk (the Kerr effect) . If a beam of polarized light is shined on the surface, the light polarization of the reflected beam will change slightly (typically less than 0.5o) if it is reflected from a magnetized surface. If the magnetization is reversed, the change in polarization (the Kerr angle) is reversed too. The magnetized areas - pits - can not be seen in regular light, but only in polarized light. The change is direction of magnetization could be associated with numbers 0 or 1, making this tecnique useful for binary data storage.

Basic design of MO disk

The next figure shows basic design of the quadrilayer magneto-optical disk.

The thermal properties of the optical disk can be changed if the active magneto-optical layer is combined with thin optimizing layers, which may also improve the protection and signal-to-noise characteristics of the medium. The multi-layers are usually designed in such a way that they increase light absorption by the active layer, and thus are called 'anti-reflection layers'. For example, a typical smooth metal surface reflects back about 50% of the light. With the help of anti-reflecting structure, the light reflection can be reduced down to 20% and less, thus decreasing the required laser power.

So-called quadrilayers are usually used in the design of MO disks. The aluminum layer can play the roles of both light reflector and heat sink (to minimize lateral heating of the active layer).

Amorphous rare earth-transition metal alloys are typically used for the MO media. The general structure of this alloy is (Tby Gd1-y)x (Fez Co1-z)1-x, where iron and cobalt are transition metals and terbium and gadolinium are the rare earth elements, which make about 80% of the alloy. The rare earth elements have poor corrosive resistance and thus require protective layers. The layers are typically deposited by sputtering from one alloy target or from several single-element targets simultaneously.

Finally, the materials for MO recording should meet the following major criteria:

Have amorphous structure (smooth surface and domain's boundaries to decrease system's noise)
Low thermal conductivity (to limit lateral heating to the recording layer itself)
High melting point at about 200o - 300oC (media stability, accidental data loss prevention)
Rapid drop of coercivity near the Curie temperature (sharp recording threshold)
High coercivity at room temperature (media stability, accidental data loss prevention)
Vertical anisotropy (perpendicular magnetic recording)
Chemical stability (constant material's properties under repeated heating-cooling)

Read White Papers on MO technology.