MOTION OF INDIVIDI Al. DOMAIN" WALLS 



1037 



5 300 



O 200 

 O 



0.6 0. 



HpRI 



8 1.0 1.2 1.4. 1.6 1.8 



N OERSTEDS 



Fig. 9 — Plot of wall velocity as a function of applied field at 77°K. Note that 

 at the higher fields the velocity' is no longer a linear function of the applied field. 



air once, gave the same value of vfijl — H^ as before. Samples 1 and 2, 

 howe^'er, were cooled several times, and after the later runs they no 

 longer did tliis. In both cases the value of v/{H — He) as deduced from 

 the ideal domain pattern and (1) was lower by about one-half; we inter- 

 pret this to mean that the domain pattern in these samples was changed 

 by the repeated thermal shock. Direct confirmation of this interpreta- 

 tion by observation was not possible, however, for reasons mdicated 

 above in connection \\\t\i domain pattern observations on these samples. 



In view of the fact that only one point is plotted beyond the knee of 

 the curve in Fig. 8, it should be emphasized that continuous qualitative 

 observations made while the sample was cooling showed that the change 

 was continuous and monotonic. On Sample 1, furthermore, the data at 

 201°K was somewhat doi\xn from the knee of the curve {v/{H — He) = 

 18000 cm/sec/oe] because of the fact that the knee occurred at higher 

 temperature as mentioned above. 



Another feature of the data is indicated by Figs. 9 and 10. Data dis- 

 cussed thus far have been taken at the lowest convenient velocities in 

 order to minimize the possibility of wall distortion. However, when 

 data were taken at higher fields, a non-linearity of the sort shown in 

 Fig. 9 appeared. The average velocity increases more rapidly at the 

 higher fields than our viscous damping coefficient would lead us to ex- 

 pect. This effect was observed at all temperatures, but as comparison 

 of Figs. 7 and 9 Anil show, it set in at lower velocities at low temperatures 

 where the enlarged viscous damping appeared. Simultaneously with the 

 appearance of this non-linearity in the apparent average velocity of the 



