6 BELL SYSTEM TECHNICAL JOURNAL 



The most definite evidence of the existence of domains is the 

 Barkhausen effect. To produce and detect it, a piece of magnetic 

 material is wound with wire the ends of which are connected to a 

 vacuum tube ampUfier. When the magnetization of the material is 

 changed, as e.g. by moving a permanent magnet near it, a rustling 

 sound or a series of clicks may be heard in phones or in a loud speaker 



(a) —DEMAGNETIZED 



(b)-SUDDEN REVERSALS COMPLETE (KNEE OF MAGNETIZATION CURVE 



(C) -SATURATED, DOMAINS ROTATED IN HIGH FIELD 



CRYSTAL 

 AXES 



MAGNETIC 

 FIELD 



Fig. 3 — Domains in a single crystal of iron. As the magnetic field increases in 

 strength the magnetic moments first change suddenly (a to b) by displacement of the 

 boundaries between them, then rotate smoothly {b to c). 



connected to the output end of the amplifier. Every such click is 

 ascribed to the sudden change in direction of magnetization in a single 

 domain, and from measurements of the sizes of the clicks we get our 

 best estimate of the sizes of the domains. Even more direct evidence 

 of the existence of domains and the changes that they undergo has 

 been obtained recently by spreading colloidal iron oxide over the 

 surface of a magnetic material and looking at it under a microscope. 



