Figure: 3-10 Ferromagnetic domain structures: a) single crystal, b) polycrystalline specimen.

Arrows represent the direction of magnetization.

Figure: 3-11 Magnetization of a ferromatic material: a) unmagnetized, b) magnetization by

domain wall motion, c) magnetization by domain rotation.

In soils and rocks, magnetic behavior is dictated by the amount of magnetite (or similar minerals such as meghemite or ilmenite). The graph (from Grant & West (1965)) in Figure 3-14 shows how susceptibility varies with magnetite volume fraction. The simple approximate formula is

=3.8 （3-14）

where q is the volume fraction of magnetite in the material. To put this result in perspective, 1% by volume magnetite (which is very high in most cases) content

yields Km=1.038. Only in rare cases will Kmbe significantly different from unity.