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BELL SYSTEM TECHNICAL JOURNAL 



only by some other forces such as those imposed by an electric field. 



The expression for the energy of interaction in a boundary layer has 



been derived by Bloch,^^ and found to be inversely proportional to the 



thickness of the layer, 



kd 1 



a 



per unit area of boundary. Here k is Boltzmann's constant, 6 the 

 Curie temperature, a the distance between atoms and 5 the thickness 

 of the layer; since the layer has no sharp limit, 5 is measured between 



Fig. 17- 



-The nature of the domain boundary. The transition region between two 

 domains is believed to be about 1000 atom diameters thick. 



points at which the spins are inclined at a certain small angle (a almost 

 0° or 180° as shown) to the spins in the middle of the domains. 



The forces of interaction are opposed by forces (e.g. of crystal 

 anisotropy or strain) which correspond to fixed values of energy per 

 unit volume. This opposing energy is thus directly proportional to the 

 thickness of the boundary, 



7i = Cb. 



The minimum energy occurs when 



dh ^^° + ^^) = 



i» F. Bloch, Z.f. Physik, 74, 295-335 (1932). See also the more recent article by 

 H. Kersten in "Probleme der Technischen Magnetizurungskurve" (R. Becker, ed.) 

 42-72, Springer, BerHn (1938). 



