Physics of Magnetism

22 CHAPTER 4. THE MAGNETICALLY ORDERED STATE

This does not lend credence to the notion that short Fe–Fe distances favor

antiferromagnetic interactions. Equally illustrative in this respect is the magnetic moment

arrangement in the compound FeGe shown in Fig. 4.1.2. The shortest Fe–Fe distance

(2.50 Å) occurring in the horizontal planes gives rise to ferromagnetic rather than antiferro­

magnetic interaction. Antiferromagnetic interaction occurs between Fe moments separated

by much larger distances (4.05 Å) along the vertical direction. This is a behavior opposite

to that expected on the basis of the Bethe–Slater curve, showing that its validity is rather

limited.

4.2. FERROMAGNETISM

The total field experienced by the magnetic moments comprises the applied field H

and the molecular field or Weiss field

We will first investigate the effect of the presence of the Weiss field on the magnetic
behavior of a ferromagnetic material above In this case, the magnetic moments are no
longer ferromagnetically ordered and the system is paramagnetic. Therefore, we may use
again the high-temperature approximation by means of which we have derived Eq. (3.2.2)

We have to bear in mind, however, that the splitting of the (2J + 1)-manifold used to
calculate the statistical average is larger owing to the presence of the Weiss field. For
a ferromagnet above we therefore have to use instead of H when going through