Recording Materials

Magnetic Recording

This is really where the main use of cobalt in electronics lies at present.

Magnetic recording materials use the magnetic properties of solids to store and retrieve information. Cobalt modified iron oxide (Fe2O3) particles are now the predominant material used in video tapes. Cobalt-containing magnetic recording media can be divided into two categories: i) oxides and metal particles or ii) metal films. It is perhaps worthwhile outlining the principles of recording (Figure 1).

A magnetic head (transducer) and the recording medium move in relation to one another. During recording, a varying current applied to the transducer coil, induces a magnetic field which magnetises a small region of the ferromagnetic recording medium. On play back, the varying magnetic fields on the tape pass the coil of the transducer and reverse the process by inducing varying voltages in the coil. Quality in this area is of course measured by how close the output is to the original input (i.e. high fidelity).

The recording head needs to be a soft magnetic material which is easily magnetised to a high saturation level and then easily reversed, i.e. low coercivity. The recording media obviously needs to be a permanent magnet and to possess not too low a coercivity (otherwise strong fields would destroy the recording) but not too high or the recorder could never erase and reuse it.

Currently, tapes have coercivities of 250-600 and up to 1,200 Oersteds (similar to Alnico magnets). Ideally, each particle will be one magnetic domain. In practice, recording media consist of fine single domain particles immersed in a plastic binder or of continuously deposited films. In both cases, a strong substrate (terylene) provides mechanical strength.

The earliest tapes simply used iron oxide and this is still the most widely used material. Substituting cobalt into the iron oxide improves its properties. The cobalt doping can be done by coating with CoCl2 in solution, drying and decomposing (245°C). The level of substitution in fact controls the coercivity from 400 to 800 Oersteds at 0 to 3% cobalt content.

The doped variety of iron oxide powders are now standard for video tapes and are becoming so for flexible discs.

Thin Films

Newer processes have also employed cobalt. Thin metallic films with high coercivities have been deposited by various means – electroless plating, evaporating, sputtering, etc.

The films can be Co-P, Co-Ni, etc., but mainly Co-Ni for video recording. The alloy of 20 at %Ni is deposited at an oblique angle to the tape by evaporation and the properties vary with the angle of deposition. Floppy discs can also be made this way, although this is more difficult. Coercivities vary with the Co/Ni ratio from 400 to1,000 Oe at high cobalt levels.

Cobalt/chromium alloys have more recently been used in what is termed perpendicular recording. This is a complex subject but is illustrated by Figure 2.

The Co/Cr alloys used are sputtered onto a film and produce columnar structures. The Co/Cr alloys are mutually soluble and a range of values of coercivity from 100-2,000 Oe can be obtained making them a very versatile medium.

Future