1328 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



The term cos d requires further explanation. The magnetization vector 

 is constrained by energy considerations to ahgn with the easy direction 

 of magnetization. The angle between the applied field and the easy 

 direction of magnetization is called 6. Equation (14) is valid for any 

 direction of applied field. The angles ^i and di used in deriving (9) and 

 (11) respectively are each 45 degrees for the helical pitch angle assumed 

 above. 



Equation (14) indicates that for maximum switching speed a material 

 with low saturation flux density and high resistivity is required. The 

 lower limit on s^ will be determined by internal loss mechanisms not 

 treated here. Experimentally, this lower bound is found to be approxi- 

 mately 0.2 oe-/isec. 



3.12 Reversal by Single Wall 



The switching time of a twistor when operating in a memory array 

 under coincident current conditions will depend upon the low^-drive 

 switching coefficient. Experimentally, it is observed that the low drive 

 Sy, is several times the high-drive value. In this section, following the 

 method of Williams, Shockley, and Kittel,^ flux reversal by the move- 

 ment of a single wall will be treated. Only the circular flux case will be 

 considered. 



The technique used to obtain Sw is identical to that used in Section 

 3.1.1 except it is postulated that a single wall concentric to the wire 

 moves either from the wire surface inward, or from the wire axis outward. 

 The result is independent of the direction in which the wall moves. As- 

 sume the wall moves from r = to r = ro , as indicated in Fig. 8. In- 



V{0) 



Fig. 8 — Flux reversal by expanding wall instantaneously located at radial 

 position aro moving with velocity v. 



