402 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



less than a wavelength of visible light. This means that with an 

 optical microscope we cannot resolve any structure within the wall. 

 We might be able to see the wall, and determine whether the mag- 

 netization is up or down, but it is unreasonable to expect to be able 

 to see anything meaningful about how the magnetization is distributed 

 within it or, for example, how thick the wall is. Anything which looks 

 like structure within the wall or a clear-cut thickness in our photo- 

 graphs is probably an artifact. 



The manner in which the magnetization varies across the wall 

 depicted in figure 9 is like a right-hand screw. It could equally 

 well have been like a left-hand screw. However, we can visually dis- 

 tinguish between these two cases. Note that at the center of the wall 

 in figure 9 the magnetization is directed forward. If it were a left- 

 handed wall, the magnetization would be back along the middle of 

 the wall. By setting the analyzer off the original extinction setting, 

 we can make these two wall segments appear light and dark. There 

 are several examples of this in the photographs shown in this paper. 

 Plate 3 has a 180° domain wall between domains in the plane which 

 alternates from right to left handed. The boundaries between right- 

 and left-handed wall segments may be regarded as entities in them- 

 selves. They are known as Bloch lines, and have an energy per unit 

 length associated with them. Plate 8 contains a very simple example 

 of a wall in two segments of opposite sense separated by a Bloch 

 line. 



If there is an energy per unit area associated with a domain wall, 

 the wall energy can always be reduced by decreasing the area of the 

 wall. Thus it tends to stretch tight if possible. This "surface ten- 

 sion" is exactly comparable with the surface tension observed in a 

 soap bubble. Similarly in the case of the Bloch line, the line acts 

 like an elastic string under tension always trying to decrease its length. 



CONCLUSION 



The domain theory view of the magnetic properties of ferromag- 

 netic materials is by no means the most sophisticated. However, it is 

 a great deal more fundamental than that in which the magnetic in- 

 duction versus field relationship is merely tabulated and characterized 

 for a range of technologically interesting materials. Unfortunately, 

 in many cases this latter has been tlie basis of teaching ferromagnetism. 

 The macroscopic magnetic properties of most specimens may be con- 

 sidered as arising from the domain structure that prevails and from 

 the hindrances to its alteration [15]. A soft magnetic material shows 

 a high permeability at low fields, and its entire induction can be re- 

 versed fairly easily. These are the materials for transformer cores, 

 inductors, and so on. An example of this is the part of plate 3 in 



