SECTION 4.4. FERRIMAGNETISM 39between the rare-earth (R) sublattice and transition-metal (T) sublattice in ferrimagnetic
intermetallic compounds is derived from magnetic measurements made on powder particles
in high fields at low temperatures. The powder particles have to be sufficiently small in size
so that they can be regarded as an assembly of small single crystals, able to rotate freely
and orient their magnetization in the direction of the external field.
In many types of R–T compounds, the anisotropy of the R sublattice exceeds that of
the T sublattice by at least one order of magnitude at 4.2 K. By minimizing the free-energy,
it can easily be shown that under such circumstances the low-temperature magnetization
curve consists of three regions, as illustrated in Fig. 4.4.5. Below there is a strictly
antiparallel alignment between the (heavy)-R moments and the T moments, so that M =
For sufficiently high values of the applied field, the R and T
moments are parallel and In the intermediate field range,
there exists a canted-moment configuration, the R-and T-sublattice moments
bending toward each other with increasing H. In this region, the field dependence of the
total moment is given by
The slope of the M(H) curve in the intermediate regime can therefore straightforwardly
be used to determine the experimental value of
can be obtained via Eq. (4.4.9). A prerequisite for this method is that the two sublattice
moments
from which the coupling constantand do not differ too much in absolute value. The reason for this is that
the first critical field
has to be sufficiently low so that the linear magnetization region given by Eq. (4.4.20) falls
within the experimentally accessible field range.