SECTION 4.1. THE HEISENBERG EXCHANGE INTERACTION AND THE WEISS FIELD 21ferromagnetic 3d elements like Ni, Co, and Fe (parallel moment arrangements) from the
antiferromagnetic elements Mn and Cr (antiparallel moment arrangements).
The validity of the Bethe–Slater curve has seriously been criticized by several authors.
As discussed by Herring (1966), this curve lacks a sound theoretical basis. In the form of
a semi-empirical curve, it is still widely used to explain changes in the magnetic moment
coupling when the interatomic distance between the corresponding atoms is increased or
decreased. Even though this curve may be helpful in some cases to explain and predict
trends, it should be borne in mind that it might not be generally applicable.
We will investigate this point further by looking at some data collected in Table 4.1.1.
In this table, magnetic-ordering temperatures are listed for ferromagnetic compounds
and antiferromagnetic compounds As will be explained in the following sections,
negative exchange interactions leading to antiparallel moment coupling exist in the latter
compounds. The shortest interatomic Fe–Fe distances occurring in the corresponding crystal
structures have also been included in Table 4.1.1. The shortest Fe–Fe distances, for which
antiferromagnetic couplings are predicted to occur according to Fig. 4.1.1, are seen to adopt
a wide gamut of values on either side of the Fe–Fe distance in Fe metal.