PRESENT STATUS OF FERROMAGNETIC THEORY 



75 



lines determined by the crystal axes, indicating that stray magnetic 

 fields go in and out of the surface just as if some sections were mag- 

 netized differently from their neighbors. This occurs even when the 

 iron is unmagnetized, but never occurs with materials that are not 

 ferromagnetic. 



Now consider in more detail by what processes changes in magneti- 

 zation occur. Most changes are attributable to the reorientation of 

 electron spins in domains, from one direction of easy magnetization 

 to another (Fig. 10). These are the changes that take place over the 



CRYSTAL 

 AXES 



DEMAGNETIZED 



MAGNETIC 

 FIELD 



SUDDEN REVERSALS COMPLETE 

 (knee of MAGNETIZATION CURVE) 



SATURATED, DOMAINS ROTATED 

 IN HIGH FIELD 



Fig. 10 — As the magnetic field strength increases, domains first change direction 

 suddenly, then rotate smoothly. 



large central portion of the magnetization curve. In general, how- 

 ever, it is obvious that this process is complete before the material is 

 saturated. When all the domains are magnetized parallel to that 

 direction of easy magnetization which is nearest to the direction of the 

 applied field, the only way in which the magnetization can be increased 

 further is by rotating the electron spins in each domain out of the 

 stable position toward the field direction. Such a process is described 

 loosely as the "rotation of the domain." This is the process that 



