Number Systems and Bases

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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 130 high power binary system review. 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 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 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.

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. Their main uses are in database or software distribution. 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 130 high power binary system review 5. Duringnew agreed standards are likely to result in the commercial availability of Mb 5. The erase, write and read processes occurring in magnetooptic finon exposure to a laser beam and a magnetic field. The read process employs a lower powered laser beam than the erase and write processes.

The three main elements of the magneto-optic storage system are: 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. As the domains cool, they take on the orientation of the external bias field, and all 130 high power binary system review domains point downwards.

The write process is now shown in Figure lb. At room temperature the magneto-optic films have high coercivities, typically of several thousand oersteds, which make the domains very stable 130 high power binary system review 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 130 high power binary system review of the domain. Magneto-optic disks and drives have only been available in quantity in the marketplace sincebut 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, 130 high power binary system review and erases using infrared semiconductor lasers which form domains of about 1.

130 high power binary system review 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.

However, as can be seen from Figure 3the 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. Figure 3 shows that this material possesses an increasing polar Kerr rotation as the wavelength of the 130 high power binary system review laser beam is reduced and should give a superior performance to rare earth films at short wavelengths. 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. Further work from Du Pont and Philips Research resulted in the preparation of high coercivity multilayers of over 3kOe, prepared by sputtering using krypton or xenon, rather than the usual argon, and by using a zinc oxide under-layer 5. At Johnson Matthey, research into platinum-containing magneto-optic media has been underway since This research programme has included collaborative work with Coventry University on both multilayer modelling 6 and on platinum manganese antimonide magneto-optic films 7.

We have found that close 130 high power binary system review over the argon sputtering process has enabled us to control the film coercivity over a wide range of values 8. In each case a square loop is observed, which demonstrates excellent perpendicular magnetic anisotropy while a high polar Kerr rotation has been maintained.

The out-of-plane coercivity defined as the point where the loop crosses the x-axis, is seen to be controllable from levels of 1 kOe or below, up to over 6 kOe. While the ideal coercivity for a magneto-optic disk will also depend on the optimisation of other disk properties, this degree of control provides the means to make large changes in these other properties without 130 high power binary system review the film coercivity to unacceptably low levels.

Tbe x-axis c-ver indicates the film coercivity, H e. The other basic multilayer property that needs to be controlled is that of the Curie temperature. Several workers have now shown that increasing the platinum layer thickness in the multilayer can reduce the Curie temperature.

Philips Research have recently reported that the addition of a few per cent of elements, such as rhenium, osmium, rhodium, iridium and ruthenium, to 130 high power binary system review cobalt layer can be useful in lowering the Curie temperature, with rhenium and osmium in particular maintaining anisotropy and the hysteresis loop shape Sony Research have used nickel as an additive to the cobalt layer which has produced similar results There are, however, several key technical issues that need to be addressed in order to commercialise this new technology.

These include long term stability of the multilayer structure and also the noise levels that can be achieved. Significant advances have been made in optimising the magnetic properties of these multilayers by using standard argon sputtering processes, which makes the commercial production of disks based on this technology feasible. Enter your email address 130 high power binary system review receive our quarterly alerts.

Gurney Johnson Matthey Technology Centre. SI, Magnetics, 285

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