Physics of Magnetism

SECTION 13.3. MATERIALS FOR HIGH-DENSITY MAGNETIC RECORDING 143

be optimized in the manufacturing process. As a rule, higher recording densities require

higher coercivities in order to avoid demagnetizing effects when the written bits are closely

spaced. However, the switching field provided by the head during writing is limited so

coercivities in the range

that the coercivities must not be too high. Satisfactory results are generally obtained with

An important property for obtaining a high signal-to-noise ratio is also the remanence
of the recording layer. One of the criteria for selecting recording particles is therefore a
high specific magnetization and the capability of the particles to be loaded at high volume
fractions into the polymeric binder system. Volume fractions close to 40 vol.% should
be possible. Higher volume fractions are less desirable because of the high demands in
mechanical properties required for the polymer/particle composite medium. Schematic
representations of the microstructure in Metal Particle (MP) tapes and Barium Ferrite (BaFe)
tapes are displayed in the top part of Fig. 13.3.4.
Magnetic oxides have the advantage of being chemically fairly stable. Their disadvan­
tage is their comparatively low specific magnetization. Much higher specific magnetizations
would be obtained when using pure-metal particles. However, the small metal particles are
pyrophoric and have to be protected by a passivation layer. The latter is usually obtained
during the manufacturing process by means of controlled particle oxidation. This leads to
a stable oxide shell when the thickness is about 4 nm, meaning that roughly half of the
particle consists of oxide. This is the main reason why the range of specific saturation mag­
netization values listed in Table 13.3.1 for the MP materials are far below the values of the
pure metals. Figure 13.3.4 illustrates that the saturation magnetization of the tape, due to
particle passivation and the low volume fraction, has dropped by a factor of about six with
respect to the value for pure iron.
Magnetic thin-film media are free of organic binder materials and principally can have
much higher remanences than particulate media. Generally, they have thicknesses of only a
few hundred nanometers. Even in magnetic thin films prepared by metal evaporation (ME),
only a part of the volume is magnetic. This can be seen in the lower part of Fig. 13.3.4.
Roughly half of the volume consists of voids, which is a consequence of the vapor-deposition
process. However, the amount of oxygen in the film is much lower than in metal-particle
films, giving them a substantially higher remanence. A further advantage is the very uniform
orientation of the particles, which is hardly achieved with particulate media and which
generates favorable switching characteristics.