Device Implications of the Theory of Cylindrical Magnetic Domains

The application of cylindrical magnetic domains or "bubbles" to memory and logic devices has recently received considerable attention. 2_n Such domain devices may operate in a continuum of modes ranging from the wall motion coercivity dominated mode to the "hard bubble" mode. In the coercivity dominated mode, applied fields determine the domain configuration which is then maintained by coercivity. In the hard bubble mode the coercivity must be sufficiently low that the domains have a well-defined size and shape permitting the move* Portions of this article were presented at the "Fifteenth Annual Conference on Magnetism and Magnetic Materials" Philadelphia, Pennsylvania, on Nov. 20. 1969. (See Ref. 2.) 725 726 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1971 ment of individual domains as distinct entities. A previous paper developed the theory of static stability of cylindrical domains in materials having zero coercivity. 1 The present work applies this theory to the determination of the preferred conditions for construction of devices operating in the hard bubble mode. It is found that specifying an operating domain diameter determines preferred values for the plate thickness, magnetization, anisotropy constant and the maximum allowable value of the wall motion coercivity. Figure 1 shows the model for the domain structure from which the static stability theory was developed. The coordinate system and symbols used here are the same as in Ref. 1 except for the addition of a few symbols such as /j.w, the wall mobility; Hc, the wall motion coercivity; vrf, the domain velocity; and Ao-1P the variation in wall energy.