Physics of Magnetism

SECTION 13.3. MATERIALS FOR HIGH-DENSITY MAGNETIC RECORDING 139

is due to the Fe sublattice. It can also be seen in the figure that there is a reversal of the

hysteresis loops when going from the Gd-dominated range (x < 0.79) to the Fe-dominated

range (x > 0.79).

13.3. MATERIALS FOR HIGH-DENSITY MAGNETIC RECORDING

Magnetic recording has been a subject of interest already for a long time. It has received
additional impetus with the advent of computer systems and the associated demand for high-
density recording devices. In most of such devices, digital magnetic recording is used in
which a transducing head (write/read head) magnetizes small areas on a magnetic-recording
medium so as to record digital data and scan the magnetized areas to read the data. The only
commercially useful systems employed in the past were so-called longitudinal magnetic-
recording materials having an easy axis of magnetization parallel to a major surface of the
material.
For longitudinal magnetic recording, a head of the granular type is used. It comprises
a core of a magnetically highly permeable material (see also Chapter 14), provided with a
narrow air gap. The gap is placed transversely to the direction of movement of the magnetic-
recording medium in such a way that flux coupling is possible. A current pulse applied
to a coil wound around the core generates magnetic flux lines in the core which close
along a path that comprises one edge of the gap, the part of the magnetic tape adjoining
the gap, and the other edge of the gap. The flux passing through the magnetic layer in
this manner causes data to be recorded. The data are read as the magnetized area on the
medium moves past the gap, thereby closing the flux through the core. As a result, flux
lines pass through the coil and induce an electric signal which is representative of the stored
information.
The disadvantage of conventional longitudinal recording is that the system can handle
only a rather restricted linear bit density. This restriction occurs because the magnetized
areas in the magnetic layer are magnetically oriented in the longitudinal direction of the
medium, that is, in the plane of the tape or the rigid disk. In conventional longitudinal
recording methods, there is a certain maximum tolerable demagnetization field at the bit
boundary, as a result of which the number of bits that can be stored per centimeter of the
information track is limited.
A further problem arises when high recording currents are used. In that case, the mag­
netization pattern recorded will have a shape such that the magnetic-flux lines close inside
the medium, which reduces the flux available for read out. Such a circular magnetization
mode is schematically represented in Fig. 13.3.1. In order to obtain high densities, it is
essential to avoid the nucleation of such magnetization modes. There are two methods to
accomplish this. One is the use of longitudinal recording materials that have an enhanced
longitudinal magnetization component. This can be achieved when the recording medium
is made extremely thin so that the magnetization is forced to he in the medium plane. The
use of thin magnetic films is equivalent to media having a strong-shape anisotropy so that
the magnetization is within the film plane. The thinner the film, the narrower the transition
region will become. Such high-density longitudinal recording media can be made from
films consisting of chemically deposited Co–Ni–P or Co–P