Miscellaneous Subsurface Methods 



599 



In the study of metals and alloys the next-larger unit of ferroraag- 

 netism is the domain. A domain is of small size (approximately 10~^ 

 cubic millimeters) and contains many ferromagnetic atoms, all of which 

 are aligned parallel so that their magnetic moments are additive. Adjacent 

 domains have different orientations. The real existence of domains has 

 been shown by powder patterns as well as by the Barkhausen effect, which 

 is caused by a boundary shift of unit domains (the size of one domain 

 increases at the expense of its neighbors) in response to an increasing 

 directionally applied magnetic field. 



Numerous magnetic measurements of both intrusive and extrusive 

 igneous rocks indicate that a considerable amount of residual magnetism 

 is acquired upon crystallization and cooling and thereafter tenaciously 

 retained by the ferromagnetic minerals. The same is true of those ferro- 

 magnetic minerals that are deposited from water solutions. 



Figure 311. Bethe's curves, where Em — energy magnetization; R = atomic separation; 

 r = diameter of uncompensated electron shell {3d) of the atom. (After Bozarth, 

 "Reviews of Modern Physics," 1947.) 



Crystals display the property of anisotropy, that is, they exhibit 

 preferred crystallographic directions of magnetism. Thus, when a pre- 

 ferred or easy direction of magnetism in an incipient crystal approaches 

 or parallels the direction of the earth's field we may expect the growing 

 crystal to acquire strong directional ferromagnetism. The very powerful 

 interatomic "exchange" forces hold the atoms in the positions of crystal- 

 lization, and it is only when high temperature or applied magnetic fields 

 of appreciable strength (much greater than the earth's field) are intro- 



