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
Magnetic recording and other electronic applications
are a growth area for cobalt. Although quantities may
not be large in each film, the amount of recording material
now being produced will ensure cobalt’s future.
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