SECTION 7.3. STRONG AND WEAK FERROMAGNETISM 69a decrease in the 3d moment. This explains qualitatively the Slater–Pauling curve (Slater,
1937; Pauling, 1938) shown in the bottom part of Fig. 7.3.3. It is useful to bear in mind
that the change from strong to weak ferromagnetism occurs close to Fe metal, which is a
weak ferromagnet whereas the metals Co and Ni are strong ferromagnets. Other important
points are
- The designations strong ferromagnetism and weak ferromagnetism do not imply that
the spontaneous moments per 3d atom or the magnetic ordering temperatures are higher
in the former case than in the latter. - It has been shown in Section 4.2 that the magnetization in the fully ordered ferromag
netic state is given for localized moments by Once this state has been
realized at low temperatures for a sufficiently high field, no further moment increase
can be expected at still higher field strengths. The magnetization has become field-
independent in a plot of M versus H and the high-field susceptibility defined in the
saturated regime by is equal to zero. The reason for this behavior is
the constancy of the localized moments. The situation is different, however, for itin
erant moments. As we have seen above, the application of an external field stabilizes
the majority-electron states with respect to the minority-electron states. This means
that a small amount of electron transfer will be induced by a sufficiently high external
field even in the saturated ferromagnetic state. Consequently, in a plot of M versus H
the magnetization is not completely field independent and the high-field susceptibility
defined in the saturated regime by is nonzero. Generally, the high-field
susceptibility is larger for weak ferromagnets than for strong ferromagnets. Note that
for the band shapes considered in Fig. 7.3.2 the high-field susceptibility for strong