MOTION OF INDIVIDT^VL DOMAIN WALLS 



1035 



Ferrer. ^^ We have extrapolalcd tlicir data to got the variation up to our 

 highest temperatures. 



Ill general, a plot of the data turns out to have the form shown in Fig. 

 7. This is the data taken on Sample 4 at 201°K. The wall does not move 

 until the field exceeds the eoerci\-e force re(}uired to get it past various 

 imperfections in the crystal. Its motion in fields higher than this is 

 viscously damped. The wall velocity, v, therefore follows the relation: 



V = G{H - He), 



(2) 



where H^ is the coercive field and (r is the slope of the line drawn through 

 the data. The \'alue of G is high if the losses are low, and \'iee versa, of 

 course. 



RESULTS 



Data on Sample 4 of the sort shown in Fig. 7 have been taken at 

 various temperatures. We show in Fig. 8 a plot of v/{H — He) as a 

 function of temperature for this sample. Clearly, the outstanding fea- 

 ture of the data is a tremendous increase in the viscous damping of the 

 domain wall at low temperatures. 



Since the other samples were not as satisfactory, for reasons given 

 above, we do not reproduce the data on them explicitly. Similar data 

 ha\'e been taken on Samples 1 and 2, however, and they show the same 

 l)eha^•ior within their accuracy except that the very sharp decrease in 

 v/(H — He) seemed to occur at a somewhat higher temperature. This 

 difference may be due to slight variations in composition among the 



a. 6 



UJ 2 



0.05 0.10 0.15 0.20 0.25 0.30 0.35 



HpRi IN OERSTEDS 



applied field. 



T\])ical plot of actual data for doiiiaiii wall velocity as a I'uiictioii of 



