At the heart of personal computers in current use is the hard magnetic disk drive, which provides the software programmes and data with long term storage. The increasing use of Windows, the graphically based computer interface which enables users to run several programmes simultaneously, has meant that an ever-increasing amount of data storage space is required. With Windows itself requiring some 15 megabytes (Mb) of storage space and individual Windows programmes typically taking up 10 to 15 Mb each; the average Windows personal computer (PC) now needs at least 100 Mb of storage space to contain a useful number of programmes and data. The trend for more long term storage will inevitably accelerate with the growth in multimedia applications, which combine photographs, video clips and sound into PC applications. With a single high quality photograph requiring up to 30 Mb of storage space, future PCs are expected to need hundreds, and possibly thousands of megabytes of storage space on the desktop.

At present the mainstream of magnetic hard disk drives can provide up to about 1000 Mb locally. Optical data storage, on the other hand, provides an accessible route to larger data storage densities since the use of a non-contacting laser to access the data allows for data bit sizes of around 1 micron from the focused beam and enables disks to be easily removed so that several disks can be used with a single drive. Three types of optical data storage media are in common use, CD-ROMs (Compact Disk Read Only Memory) being the most widespread at present. These disks are prepared using the same technology as audio compact discs and enable data to be read from but not written to the disk. CD-ROM disks can hold 650 Mb on a single 5.25 inch diameter disk. Their main uses are in database or software distribution. WORM (Write Once Read Many times) technology has become established as a niche market for archival data storage. The advantage of WORM disks is that once thay have been written to, they cannot be changed and so provide a permanent record.

Several optical data storage methods have been developed where the data can be written and subsequently erased. These include Phase Change Erasable and dye polymer systems, but the one which has become the most widely established to date is magneto-optic data storage. Magneto-optic disks of diameter 5.25 inches currently available provide a removable, overwritable medium with 650 Mb of storage. During 1993, new agreed standards are likely to result in the commercial availability of 1200 Mb 5.25 inch disks and 256 Mb 3.5 inch disks. The technology for these disks still uses a magnetic thin film on which to store the data, but instead of a magnetic read/write head, a finely focused laser beam is used to write, read and erase the magnetic domains, as is shown in Figure. 1

The three main elements of the magneto-optic storage system are: the magnetic film, which has a high perpendicular magnetic anisotropy, that is the domains are oriented perpendicularly to the plane of the film; the laser beam, which can be used at high or low power; and the bias coil which can provide an external biasing magnetic field in either the “up ” or “down” directions. Figure 1a shows the erase process. Here the external magnetic field is pointing downwards and the laser beam, at high power, heats up each domain in turn to above its Curie temperature, while the magneto-optic disk spins round its vertical axis. The Curie temperature of a magnetic material is the temperature above which it loses its coercive magnetic field and is typically around 200°C in these rare earth films. As the domains cool, they take on the orientation of the external bias field, and all the domains point downwards. This is equivalent to all the binary information bits being set to “zero ”. The write process is now shown in Figure lb. Here the external bias field is reversed and the high power laser beam heats selective domains to above their Curie temperature, which as they cool take on a reversed magnetic orientation, thus setting these information bits to “one”. At room temperature the magneto-optic films have high coercivities, typically of several thousand oersteds, which make the domains very stable and much less susceptible to magnetic change than conventional magnetic storage media. The high laser power reduces this coercivity to a figure approaching zero near its Curie temperature, thus allowing the domain to be reversed by the relatively low strength external magnetic field. Finally, Figure 1c shows the read process. Here a low power laser beam is used so that the magnetic properties of the film remain unchanged. The different magnetic orientations are read by means of the polar Kerr effect, in which the plane of polarisation of the reflected laser beam is rotated to the left or to the right, depending on the magnetic pole from which it is reflected. A detector system is used to record the polarisation shift and thus the magnetic orientation of the domain.

Magneto-optic disks and drives have only been available in quantity in the marketplace since 1991, but already research and development into disks and drives of the next generation is well advanced, see Figure 2. The technology of the current generation reads, writes and erases using infrared semiconductor lasers which form domains of about 1.0 micron in diameter. The use of shorter wavelength blue lasers will enable much smaller bits to be written and permit a fourfold increase in data storage density to be achieved. The development of blue lasers small enough to be incorporated into disk drives is likely to be achieved within the next few years by doubling the frequency of current generation semiconductor lasers, although a significant worldwide research effort on new materials systems for blue semiconductor lasers is also underway.

The magnetic films used in present day magneto-optic disks are based around amorphous rare earth-transition metal alloys, typically TbFeCo or GdTbCo, which have a good perpendicular magnetic anisotropy, high coercivities and relatively large polar Kerr rotations, θ_k.

However, as can be seen from Figure 3, the key magneto-optic property, the polar Kerr rotation, diminishes rapidly for such materials as the wavelength of the light is reduced from infrared to blue (1). Of the new materials systems that have been considered as future replacements for the rare earth-transition metals, the most promising is that of platinum/cobalt multilayer thin films. Figure 3 shows that this material possesses an increasing polar Kerr rotation as the wavelength of the incident laser beam is reduced and should give a superior performance to rare earth films at short wavelengths. These multilayers are prepared by alternately sputtering ultra-thin films of platinum and cobalt to give a structure of the type shown in Figure 4, where each metal layer is just a few ångströms thick.

The use of platinum/cobalt multilayers can provide other benefits as well. Although extremely thin, they are remarkably stable on exposure to air, unlike the equivalent rare earth-transition metal films and should provide for an inherently stable disk structure. This may enable simpler disk structures to be used.