38 CHAPTER 4. THE MAGNETICALLY ORDERED STATE
the effective molecular field at the A sites is stronger than at the B sites. In this case, the
spontaneous magnetization exhibits sign reversal. The temperature range in which this
occurs is indicated by the dashed line. However, since the quantity measured in practice is
the curve plotted as the full line is actually observed. The temperature
at which the resultant magnetization is zero is commonly called the compensation
point or compensation temperature.
Various other possible curves are shown in Fig. 4.4.3. In practice, these different
types of curves are observed when the composition of the compounds investigated is varied.
For instance, there are various compounds in which rare earths (R) are combined with
3d metals (T), represented by the formula There are several possibilities for choosing
the T element (T = Ni, Co, Fe, Mn) and 15 possibilities for choosing the R element (see
Table 2.2.1). An example of how the compensation temperature can be shifted to lower
temperatures by reducing the R-sublattice magnetization via substitution of non-magnetic
Y is shown in Fig. 4.4.4.
It follows from the discussion given above that the temperature dependence of the
magnetization in ferrimagnetic compounds is determined by the magnitude and sign of
the intrasublattice-coupling contants and the intersublattice-coupling constant
appearing in Eqs. (4.4.8) and (4.4.9). If the sublattice moments and are
known, these constants can be determined by fitting experimental curves of the temperature
dependence of the total magnetization M(T). The determination of three constants by fitting
a simple M(T) curve can, however, not always be accomplished in an unambiguous way.
This is true, in particular when the M(T) curve has not much structure. This is generally the
case when it does not exhibit the singular point at which the two sublattice
moments become equal (Fig. 4.4.2b).
A most elegant and simple method, the high-field free-powder (HFFP) method, for
determining the intersublattice-coupling constant has been provided by Verhoef et al. (1988).
In this method, the molecular-field constant that determines the moment coupling