598 



Subsurface Geologic Methods 



have residual or permanent magnetism that can be measured when the core 

 is effectively shielded from all magnetic fields.^ 

 To be ferromagnetic a material must: 



1. Be composed of atoms having permanent magnetic moments. The 

 magnetic moment of an atom is caused by uncompensated spin of certain 

 of its electrons. This theory is diagrammatically illustrated in figure 310. 



2. Have strong interatomic (exchange) forces that maintain the 

 magnetic moments of many atoms parallel to one another. 



The atoms of only a few related elements meet these rigid require- 

 ments. In addition to the foregoing, a ferromagnetic substance must have 

 a favorable ratio of R (the distance of atomic separation) over r (the 



Figure 310. Iron (Fe) atom, where in a the M shell is the uncompensated spin 

 5+ to 1—) giving rise to ferromagnetism ; and b represents free electrons of the 

 metal. Atomic number = 26; protons to nucleus = 26; electrons in shells = 26. 

 (After Bozarth, "Reviews of Modern Physics," 1947.) 



diameter of the shell of uncompensated spin electrons). To illustrate this, 

 manganese, which is normally not ferromagnetic, may become so when 

 the atoms are separated by abnormally large distances, as they are in some 

 compounds and alloys. Figure 311 shows this relationship. 



To elaborate somewhat on requirement 1 above we refer to figure 310, 

 which represents a highly magnified structure of an atom of iron. True, 

 modern theory regards the shells not as precise orbits as shown, but 

 rather as zones where the expectancy of finding rapidly moving electrons 

 is greater than in adjacent regions. However, the diagram is regarded as 

 essentially correct and is ideal for explaining such phenomena as emission 

 spectra and to a lesser extent ferromagnetism. 



* Newer instrumentation eliminates the susceptibility effects caused by the presence of the astatic 

 magnetic balance and thus measures the polarity of the ferromagnetic minerals more accurately. 



