SECTION 13.2. MAGNETO-OPTICAL RECORDING MATERIALS 133
13.2. MAGNETO-OPTICAL RECORDING MATERIALSAn important group of magneto-optical recording media is based on amorphous alloys
of Gd–Fe or Gd–Co with some alloying additives to optimize the magnetic and magneto-
optical properties.
The relative concentrations of rare-earth (R) and 3d elements are chosen in such a way
that the R-sublattice magnetization exceeds the 3d-sublattice magnetization at low tempera
tures. The exchange-coupling constants responsible for the magnetic coupling
between the R moments and the 3d moments are negative, as in the case of crystalline
materials. The absolute values of these intersublattice-coupling constants are much smaller
than the 3d-intrasublattice-coupling constant The R-intrasublattice-coupling
constant is comparatively small and can be neglected in most cases. Using Eqs. (4.4.7)
and (4.4.9) and the fact that one then finds that the 3d-sublattice moment is
coupled antiparallel to the R-sublattice moment if the R component belongs to the heavy-R
elements see Table 2.2.1). The temperature dependence of the magnetic polar
ization can then be calculated by means of Eqs. (4.4.15–4.4.19) and behaves as shown in
Fig. 13.2.1.
It is essential for the application of amorphous R-3d alloys that their easy magnetization
direction be perpendicular to the film plane, that is, Here, we have used the symbol
instead of to indicate the difference from crystalline materials with uniaxial lattice
symmetry.
Various models have been proposed in the literature that describe the origin of
the positive anisotropy constant found in some of the R-3d films. The model of