142 CHAPTER 13. HIGH-DENSITY RECORDING MATERIALSThe minimum magnetization transition length L for a Co–Cr film is assumed to be of the
order of a column diameter, which is roughly one-tenth to one-twentieth of the film thick
ness, and is independent of the saturation magnetization and coercivity of the film.
A possible magnetic-transition model for this film is shown in Fig. 13.3.2a.
In longitudinal recording, if conventional so-called particulate media are used, which
consist of an assembly of coated magnetic particles (for instance,
persed in a binder, one expects a rather wide transition region as shown in Fig. 13.3.2b.
The magnetization transition is composed of an assembly of particles in this case, and the
transition width L is independent of the particle size. It can be shown that L is given by the
expression (see, for instance, Mee and Daniel, 1987)
dis-where is the remanence, the coercivity, and the film thickness. If one also takes into
account the demagnetization in the write process, one finds a somewhat different value:
It follows from these expressions that these media must be made very thin if one wishes to
obtain a high bit density. For particulate media, this requirement is difficult to achieve.
A better approach to high-density longitudinal recording employs ultrathin metallic
films (thinner than 100 nm) to prevent the circular magnetization mode. In this case, how
ever, a sawtooth magnetization mode is frequently obtained at the transition, even in very
thin and highly coercive films. The effective transition length is given by the sawtooth
amplitude and is approximately equal to which usually amounts to one half
to one third of the thickness for typical film parameters. It should be noted that the minimum
transition length depends on as well as on for all types of longitudinal recording
media. This is a distinct disadvantage, because it is difficult to optimize both quantities
simultaneously with respect to the transition width. We recall that this problem is absent in
perpendicular recording media.
We will conclude this section by briefly discussing the most important magnetic-
recording materials currently employed. More details can be found in the surveys of Hibst
and Schwab (1994) and Richter (1993). Particulate recording media are most widely used.
In these media, magnetic particles are dispersed in an organic binder system. A survey of
some important materials used for these magnetic particles is given in Table 13.3.1. The
requirement of high bit density on the ultimate tape or rigid disk dictates that the particle
size be small. It was mentioned already that, for avoiding the circular mode, it is desirable
to have sufficient anisotropy that keeps the magnetization in the film plane of longitudinal
recording media.
Not all of the materials listed have a sufficiently high magnetocrystalline anisotropy so
that additional shape anisotropy of the particles is required. For this reason, considerable
attention is paid in the manufacturing process of the particles to give them an elongated
shape. The presence of anisotropy is also needed for the attainment of coercivity. The
exact value of the coercivity needed depends on the specific recording system and has to