MOTION OF INDIViniAL DOMAIN W AT.LS 101' 



side ol' CM)) with I'cxcrscd siji;ii (Miual lo: 



' (In 



L 



'-00 (II 



as moiitionod after (15) to get the relation between the velocity of the 

 domain wall and the applied field. As we shall see later, the right hand 

 side of (30) is positive if we use a g^^ associated with a positive contribu- 

 tion to the anisotrojn' (>nerg3\ We find: 



Je 



1 2M^ 



d^dd,/''- (31) 



hi dd'^ dz 



The derivatix'es of d with respect to z in (29), (30), and (31) are to l)e 

 e\"aluated by means of (8) and (10). In using these equations, of course, 

 we are assuming that the wall is mo\'ing slowly enough so that its shape 

 remahis that of the wall at rest. 



Equation (31) shows that v -^ H/t as we mentioned earlier. Inspec- 

 tion of Fig. 8 shows that this relationship explains very satisfactorily 

 the sharp drop in v,'(H — H,) at low temperatures if we remember that 

 T depends on temperature as follows: 



(32) 



where e is an activation energy. 



Finally, the right hand side of (30) may be set ecjual to: 



/ {H-dM/dt)dz, 



J—r^ 



as calculated from the Landau-Lifshitz equation, see (11), to obtain a 

 value for X. We find: 



X = r ^ ^'^ ^^" ^^ . (33) 



[ [g{d) - g{do)f''dd 

 Je, 



DISC rs.siox 



It is clear that (4) fits our experimental data at each temperature. 

 By fitting our data to this expression we obtain values for 13, the para- 

 meter characteristic of the material which measures the damping of the 

 wall. \'alues of 13 ol)taiiH'd in this way are gixcii in Table I. .\. more 



